G. S.

Malande

What are the different digital cellular systems available in 2G?

2G Cellular Systems
Second-generation (2G) digital cellular systems constitute the majority of cellular
communication infrastructures deployed today. 2G systems such as GSM, whose rollout started
in 1987, signaled a major shift in the way mobile communications is used worldwide. In part
they helped fuel the transition of a mobile phone from luxury to necessity and helped to drive
subscriber costs down by more efficient utilization of air interface and volume deployment of
infrastructure components and handsets.

Major geographical regions adopted different 2G systems, namely TDMA and CDMA in North
America, GSM in Europe, and Personal Digital Cellular (PDC) in Japan. Figure 3.3 depicts the
worldwide subscriber numbers for major 2G cellular systems. It effectively shows how the GSM
system has been successful and why it is now being adopted in geographical areas other than
Europe (such as North America, China, the Asia-Pacific region, and more recently, South
America). CDMA, which originated in North America, has also proliferated in South America
and later in the Asia-Pacific region. TDMA remains to be widely deployed in North and South
America regions, but it is expected to decline mostly because of the decisions taken by few
major North American carriers to convert their TDMA networks to GSM.

Figure 3.3: 2G technologies worldwide market share in subscribers (2002).

North American TDMA (IS 136)

This second-generation system, widely deployed in the United States, Canada, and South
America, goes by many names, including North American TDMA, IS-136, and D-AMPS
(Digital AMPS). For the sake of clarity, we will refer to it as North American TDMA, as well as
simply TDMA, when the context makes it clear. TDMA has been used in North America since
1992 and was the first digital technology to be commercially deployed there. As its name
indicates, it is based on Time Division Multiple Access. In TDMA the resources are shared in
time, combined with frequency-division multiplexing (that is, when multiple frequencies are
used). As a result, TDMA offers multiple digital channels using different time slots on a shared
frequency carrier. Each mobile station is assigned both a specific frequency and a time slot
during which it can communicate with the base station, as shown in Figure 3.4.
 Figure 3.4: Time Division Multiple Access.

The TDMA transmitter is active during the assigned time slot and inactive during other time slots, which
allows for power-saving terminal designs, among other advantages. North American TDMA supports
three time slots, at 30 kHz each, further divided into three or six channels to maximize air interface
utilization. A sequence of time-division multiplexed time slots in TDMA makes up frames, which are 40
ms long. The TDMA traffic channel total bit rate is 48.6 Kbps. Control overhead and number of users per
channel, which is greater than one, decrease the effective throughput of a channel available for user
traffic to 13 Kbps. TDMA is a dual-band technology, which means it can be deployed in 800-MHz and
1900-MHz frequency bands. In regions where both AMPS and TDMA are deployed, TDMA phones are
often designed to operate in dual mode, analog and digital, in order to offer customers the ability to
utilize coverage of the existing analog infrastructure.

Global System for Mobile Communications (GSM)

There are still some analog cellular systems in operations in Europe, but their number is
declining, and some regional networks are being completely shut down or converted to Global
System for Mobile Communications. The GSM cellular system initiative was initiated in 1982 by
the Conference of European Posts and Telecommunications Administrations (CEPT) and is
currently governed by European Telecommunications Standards Institute (ETSI), which in turn
has delegated GSM specifications maintenance and evolution to 3GPP (reviewed in part in
Chapter 1). The intent behind GSM introduction was to have a common approach to the creation
of digital systems across European countries, to allow—among other advantages of a common
standard—easy international roaming and better economies of scale by decreasing handset and
infrastructure components costs through mass production. In hindsight, this was a smart political
decision, which contributed to the worldwide success of European cellular infrastructure
providers and equipment manufacturers.

Let's look at some details of the GSM air interface technology. The GSM standard, similarly to
North American TDMA, is based on the use of two simultaneous multiplexing technologies,
TDMA and FDMA. Each radio frequency (RF) channel in GSM supports eight time slots
(compared to three for North American TDMA) grouped into TDMA frames, which are in turn
grouped into multiframes consisting of 26 TDMA frames carrying traffic and control channels.
Multiframes are built into superframes and hyperframes. This yields an 8-to-1 capacity increase
over NMT or TACS in the same RF spectrum. The allocation of the time slots is essentially static
on a short-term basis; for instance, the eighth time slot of a given RF channel is assigned to the
same user each time it comes around, whether or not the user has voice or data to send.

The GSM system, emphasizing not only physical properties but also service definitions (unlike
some 1G systems), supports three major types of services: bearer services, tele-services, and
supplementary services. GSM bearer services allow for transparent or acknowledged user data
transfer and define access attributes, information transfer attributes, and general attributes with
specific roles. Access attributes define access channel properties and parameters such as bit rate;
transfer attributes define data transfer mode (bidirectional, unidirectional), information type
(speech or data), and call setup mode; general attributes define network-specific services such as
QoS and internetworking options. Tele-services are what GSM subscribers actually use. They are
based on the foundation provided by bearer services and govern user-to-user communications for
voice or data applications. Examples of tele-services include Group 3 Fax, telephony, Short
Message Service (SMS), and circuit data IP and X.25 communications. GSM supplementary
services provide additional value-added features such as call waiting, call forwarding, call
barring, and conference calling used by wireless operators to further differentiate their offerings.
Further information about GSM can be obtained from a variety of sources such as [Eberspacher
2001].

High-Speed Circuit-Switched Data

High-Speed Circuit-Switched Data (HSCSD) is an option in GSM that allows combining

multiple GSM time slots (traffic channels) each capable of a 14.4-Kbps data rate. The resulting
bit rate made available for a single user might reach as high as 56 Kbps, although probably at a
steep price tag. In fact, owners of the mobiles capable of HSCSD support will have to pay for the
combined GSM time slots being used.

Wireless carriers can achieve the migration to HSCSD by upgrading GSM Mobile Switching
Center (MSC) and Base Transceiver Station (BTS) software. Wireless carriers also have to
distribute handsets capable of receiving HSCSD transmission or firmware upgrades for the GSM
mobiles based on Personal Computer Memory Card International Association (PCMCIA) and
CompactFlash (CF) cards (such as those produced by Nokia). HSCSD can be supported within
the existing GSM mobility management infrastructure, which also enables roaming and other
familiar GSM services at higher data rates.

cdmaOne

Code Division Multiple Access (CDMA) IS-95—or cdmaOne—is one of the popular 2G
technologies being used in the Americas, Asia, and Eastern Europe. CDMA is based on the
technique in which each subscriber is assigned a unique code, also known as pseudorandom code
that is used by the system to distinguish that user from all other users transmitting
simultaneously in the same frequency band. CDMA belongs to the class of systems called spread
spectrum systems, and more specifically to the Direct Sequence Spread Spectrum (DSSS) family.
Physical channels in CDMA are defined in terms of radio frequency of the carrier and a code—
that is, a sequence of bits. The digital signal resulting from the encoding of voice or data, after
the application of appropriate framing (or radio link layers), is digitally scrambled before it
modulates the carrier frequency. This is accomplished by digitally (base 2) adding the signal to
the pseudorandom code that is used to distinguish the user. The entire carrier spectrum is
available to each single user, hence the name spread spectrum.

The receiver, which has a pseudorandom signal decoder, reproduces the original signal by
demodulating the RF and adding (base 2) the same pseudorandom signal used by the transmitter,
thus obtaining the original signal. CDMA is an interference-limited system, meaning that
anytime a user is not transmitting and thereby not interfering with other users sharing the same
spectrum, the effective bandwidth, and hence signal-to-noise ratio, available to other users will
increase to some degree. CDMA properties are as follows:

 Multiple voice channels are available for each radio channel.

 To prevent interference, callers are assigned to different radio frequency channels (or, if
sharing a radio channel, different pseudorandom codes).
 The same radio channel can be used in adjacent cells.
 The number of calls in a sector is "soft" limited, not hard limited.
 Bandwidth usage influences the number of simultaneous users.

To better visualize the CDMA concept, imagine a room filled with pairs of people talking to
each other, each couple in their own language. They would only be able to understand their
counterparts but not the rest of the conversations in the room. As the number of pairs with unique
language increases, the noise level will reach its maximum, after which no conversations will be
possible (not unlike in some trendy restaurants and pubs).

The CDMA cellular technology also comes with soft handoff capability— that is, the system
specifies a receiver (RAKE receiver) capable of receiving up to three signals related to the same
channel, because of multipath effects or to multiple sources transmitting the same signal. The
system allows the mobile station to send and receive simultaneously with three base stations,
which are defined as belonging to the "active set" of base stations. This allows for the avoidance
of handoff Ping-Pong effects and also allows for improved performance against multipath or
adverse radio conditions.

CDMA was originally deployed under the commercial name cdmaOne based on TIA [IS95], a
mobile-to-base-station compatibility standard for wideband spread-spectrum systems. It is a
direct sequence CDMA scheme in which users are differentiated by unique codes known to both
transmitter and receiver. The IS-95A version of the standard allows for circuit-switched data
service up to 14.4 Kbps. The next generation of IS-95, called IS-95B, requires software and
hardware change in CDMA system elements and mobile stations but will support packet data at a
sustained bit rate of 64 to 115 Kbps. This is achieved mostly through the use of advanced
channel and code aggregation techniques and other modifications to IS-95A. In IS-95B, up to
eight CDMA traffic channels can be aggregated for use by a single subscriber—not unlike
HSCSD used in GSM.
 2013MIT002
PIMPLE MANDAKINI H.
Department of Information Technology
SGGSIE&T

What is the diff between 1g and 2g?

The first generation
mobile phones were based on the analogue system. The introduction of cellular
systems in the late 1970s was a quantum leap in mobile communication, especially
in terms of capacity and mobility. Semiconductor technology and microprocessors
made smaller, lighter, and more sophisticated mobile systems a reality. However,
these 1G cellular systems still transmitted only analogue voice information. The
prominent ones among 1G systems were advanced mobile phone system (AMPS),
Nordic mobile telephone (NMT), and total access communication system (TACS).
With the introduction of 1G phones, the mobile market showed annual growth rate
of 30 to 50 per cent, rising to nearly 20 million subscribers by 1990.
The second generation
2G phones using global system for mobile communications (GSM) were first used
in the early 1990s in Europe. GSM provides voice and limited data services, and
uses digital modulation for improved audio quality.
Multiple digital systems.
The development of 2G cellular systems was driven by the need to improve
transmission quality, system capacity, and coverage. Further advances in
semiconductor technology and microwave devices brought digital transmission to
mobile communications.
Speech transmission still dominates the airways, but the demand for fax, short
message, and data transmission is growing rapidly. Supplementary services such as
fraud prevention and encryption of user data have become standard features,
comparable to those in fixed networks. 2G cellular systems include GSM, digital
AMPS (D-AMPS), code-division multiple access (CDMA), and personal digital
communication (PDC). Today, multiple IG and 2G standards are used in
worldwide mobile communications.
Different standards serve different applications (paging, cordless telephony,
wireless local loop, private mobile radio, cellular telephony, and mobile satellite
communication) with different levels of mobility, capability, and service area.
Many standards are used only in one country or region, and are incompatible. GSM
is the most successful family of cellular standards. It includes GSM900, GSM-
railway (GSM-R), GSM1800, GSM1900, and GSM400. GSM supports around 250
million of the world’s 450 subscribers, with international roaming in
approximately 140 countries and 400 networks.
The core network.
This network links together all the cells into a single network, coordinates
resources to hand over your call from one cell to another as you move, discovers
where you are so that you can receive incoming calls, links to the fixed network so
that you can reach fixed-line phones, and communicates with roaming partners.
You can use your phone on other network links to the Internet, so you can reach
Web servers and corporate systems worldwide to control and deliver services
depending on your subscription

The 2G architecture. The existing mobile-network consists of the radio access-

network (comprising cells and backhaul communications) and the core network
(comprising trunks, switches, and servers). Mobile switching centers (MSCs) are
intelligent servers and the whole network is data-driven, using subscription and
authentication information held in the home location register (HLR) and
authentication Centre (AuC). The standard services include circuit-switched voice,
fax, and data, as well as voicemail and voicemail notification. Additional services
include wireless application protocol (WAP), high-speed circuit- switched data
(HSCSD), mobile location services (MLS), and cell broadcast. You can change to
a new operator keeping your old phone number.

What is the function of base station explain the function of msc?

BSC:
Call Control Function
Handles Mobile-to-Land and Land-to-Mobile call setup and release function
for the calls incoming to BSC by interworking with BTS and processes G3 FAX
and nonvoice circuit data call. Also, performs Markov call processing function for
test call setup.
Handoff Control Function
Handles various handoff types generated during call such as softer handoff,
soft handoff and hard handoff. A new physical channel is not allocated for handoff
function,but only the softer handoff generated-upon movement between sectors in
the BTS is applied and soft and hard handoff between BTSs is reserved as user
option.
Transparent Message Transfer
Performs message handling function for transparent message transfer
between BTS-BSC-switching office such as location registration and
supplementary service during call.
Transcoding Function
Performs IS-95 Traffic Data – PCM Data conversion for voice traffic
packet handling of 20ms cycle per call. For voice traffic, provides 14.4K BPS
grade Rate Set 2 vocoder with excellent voice quality as well as 9.6K vocoder.
Selecting Function
Performs transcoding into PCM data for the best packet selected out of the packets
arriving through multipath established up to three channels for soft
handoff handling without call interruption. This function is activated when BTS-
BTS soft handoff
is provided.
DTMF Tone Conversion
Converts DTMF message transmitted from a fixed subscriber unit into DTMF tone
corresponding to the digits when the WLL system needs to provide
terminating digits or digits for supplementary service control in DTMF tone format
other than
message through call channel using V5.2 protocol.
IWF for Data Call
Handles IWF (Interworking Function) for interworking with public data
network for data call service.
Packet Routing 1
Performs prompt routing function for traffic packets and control messages between
several BTS and BSC systems.
Call Resource Management
Selects, vocoder elements required for call setup and CDPA resources for data call
processing by load distribution and takes back the resources upon call
release.Staggered frame control by frame offset value is applied for load
distribution.

Mobile Switching Centre (MSC):

The main function of the mobile switching center is to manage and co-ordinate to
setup calls between GSM mobile and PSTN users. The Mobile Switching Centre or
MSC is a sophisticated telephone exchange which provides circuit-switched
calling, mobility management, and GSM services to the mobile phones roaming
within the area that it serves. This means voice, data and faxservices, as well
asSMSand call divert.In the GSM mobile phone system, in contrast with earlier
analogueservices, fax and data information is sent directly digitally encodedto the
MSC. Only at the MSC is this re-coded into an "analogue"signal (although actually
this will almost certainly mean soundencoded digitally asPCMsignal in a 64-kbit/s
timeslot, known as aDS0in America).There are various different names for MSCs
in different contextswhich reflects their complex role in the network, all of these
termsthough could refer to the same MSC, but doing different things atdifferent
times.A Gateway MSC is the MSC that determines which visited MSC
thesubscriber who is being called is currently located. It also interfaceswith
thePublic Switched Telephone Network. All mobile to mobilecalls and PSTN to
mobile calls are routed through a GMSC. The termis only valid in the context of
one call since any MSC may provideboth the gateway function and the Visited
MSC function; however,some manufacturers design dedicated high capacity MSCs
which donot have anyBSSesconnected to them. These MSCs will then bethe
Gateway MSC for many of the calls they handle.The Visited MSC is the MSC
where a customer is currently located.The VLR associated with this MSC will have
the subscriber's data init.The Anchor MSC is the MSC from which ahandoverhas
beeninitiated. The Target MSC is the MSC toward which a Handovershould take
place
1. Define a cell. What is frequency reuse?
Most of you might be familiar with the concept of frequency reuse. We often come across this
term in Mobile Computing. Quite a straightforward and simple concept, but still it requires a
detailed explanation. This is one of the most common terms used in the world of Cellular
Telephony (Wireless Communication). Most cellular systems use frequency reuse scheme to
improve capacity and coverage. Let us understand what exactly a cell mean and how they are
related to frequencies.
In a cellular system, each mobile station (MS) is connected with its base station (BS) via a radio
link. The BS is responsible for sending the calls to and from the MS, which lie in the coverage
area of that BS. The coverage area of a base station or a sector of a base station is known as a
cell. Each BS consists of a number of frequency channels, which serve as a link between the MS
and the BS. Every time, a call propagates through a channel which is currently idle and receiving
the best signal. As the coverage area of a BS can be termed as a cell, we can also say that a cell
uses the frequency channels for call forwarding. These cells are usually of hexagonal shape (this
explanation is certainly not in the scope of our discussion here). The Fig 1-1 shows a typical
structure of a cell.

Fig 1-1. A cell.

A PCS (Personal Communication System) is a combination of many such cells. So, a cell may
be surrounded by a large number of adjacent cells. This is shown in Fig 1-2.
 Fig 1-2. Cells adjacent to each other (Cluster).

Now, let us look at a more general term used for the above structure- a cluster. A number of cells
are grouped to form a cluster. So, a cluster is a collection of various cells. Now, after
understanding the concepts of cells and cluster, let us move into the actual concept of frequency
reuse.
As we have seen, cells use frequencies. But imagine two or more cells in a single cluster using
the same frequency. Obviously, there is a wide scope of interference. So, it is always a better
option to avoid two cells in a cluster using the same frequencies. That is, inside a cluster, all the
cells must use different frequencies. A 3-cell cluster with all the adjacent cells using different
frequencies (F1, F2 and F3) is shown in Fig 1-3.

Fig 1-3. Cells in a cluster using different frequencies.

But this will definitely lead to a new problem. As the network grows, if every cell in a system
uses different frequencies, the frequency spectrum will be heavily utilized. A large amount of
frequencies will be utilized by these cells. A solution to this problem is the Frequency Reuse. All
the cells in a cluster must still have different frequencies, but these frequencies can be reused by
the cells in other clusters. This is the concept of frequency reuse. That is, if frequencies A, B, C,
D, E, F and G are used by the cells in a 7-cell cluster, these same frequencies A, B, C, D, E, F
and G can be used by the cells in other clusters. See Fig 1-4.
 Fig 1-4. Frequency Reuse.

In the above figure, three different clusters are shown with three different colors. Each of the 7
cells in each clusters use different frequencies (A through G). But, the same frequencies (A
through G) are reused by the seven cells of each of the other clusters. Thus, the problems of
interfering frequencies as well as over-utilization of frequencies are overcome using the concept
of frequency reuse.
Que: Mention the basic propagation mechanisms, which impact propagation in mobile
communication.
What are the various types of wireless network topologies?

Following are the wireless network topologies:

1.star topologies.

2.mesh topolodies.

3.point to point topologies.

4.Ring topologies(rarely using in wireless)

1.star topologies……

Fig:star topologies

Star topology is a point-to-point or line-of-sight architecture where

individual wireless devices or nodes, communicate directly with a gateway
or central “hub.” The gateway transmits the data to a central collection
point, such as a control room, directly, or by connecting to another
network. Star topology is also sometimes described as “point-to-point” or
“line of sight” architecture because each device communicates directly
with the gateway. Star topologies potentially use the least amount of
power of the three architectures because of the simple, direct wireless
connections. But the distance the data can be transmitted from the
wireless device to the gateway is limited to a range of 30 –100 meters.
Communication may be hindered or data lost if something disrupts the
transmission path between a device and gateway. This disruption could be
radio-frequency interference, physical structures, environmental factors,
or temporary obstructions like trucks, construction equipment, or
scaffolding.
A site survey is important in the planning of a wireless network to identify
where devices can be placed to provide line-of-sight transmission, and
appropriate range to the gateway. Site surveys are expensive and cannot
predict future changes, including new construction or other environmental
changes that may interrupt that line-of-sight transmission. Many of these
limitations may be eliminated with a topology that allows for more than
one transmission path between device and gateway.
2. Mesh topology:

The devices in a mesh topology can also communicate with other nodes in
the network (point-to-multipoint) using a capability called multi-hopping.
A message can “hop” from node to node to node until it reaches the
assigned gateway. The advantages of mesh over star topology includes a
longer range distance and a decrease in loss of data or transmission

3.Point-to-point—

Bluetooth products (as well as Wi-Fi products in the ad hoc mode) use the point-to-point topology.
These devices connect directly with each other and require no access point or other hub-like device to
communicate with each other, although shared Internet access does require that all computers connect
to a common wireless gateway. The point-to-point topology is much less expensive per unit than a star
topology. It is, however, best suited for temporary data sharing with another device (Bluetooth) and is
csurrently much slower than 100BASE-TX networks.

What are the principles of Cellular Architecture?

A. Alignment with the Internet architecture

It can be argued that the current cellular data network still employs circuit–like transport for user
traffic, albeit running on packet transports, through the use of constructs such as APN and GTP.
Future network architecture should be a truly packet network that does not involve semi–
permanent per–service tunnels and heavy reliance on network intelligence. Any protocol or
service that is not proven necessary for the operation of a commercially viable IP network should
have to satisfy a very high threshold for its necessity in order to be included in the basic
architecture.

B. Endpoint–centric protocols

With multiple radio interfaces available on user devices and the reality of many different
generations of radio networks under different administrative control, it should be clear that most
network protocols should be controlled primarily by the endpoint devices, since they have the
most accurate view of their radio connectivity and characteristics of applications and services on
them. The current architecture strives very hard to handle this reality from the network side and
is sometimes failing to meet real needs in terms of scale, cost, performance, and manageability.
An example is the prolonged activity around creating network–based selective IP flow mobility
between cellular and Wi–Fi networks, i.e., Local IP Access and Selected IP Traffic Offload .
With the advent of smartphones, the increased complexity of endpoint devices is a foregone
conclusion. Thus the cellular mobile network should follow the principle that was proven
successful in the Internet: most intelligence should be at the network edges and host devices.

C. Simplicity

The current cellular network architecture employs many protocols that involve four or more
parties and numerous round trips. The initial entry and connection establishment involves a
mobile device, base station, RNC, SGSN, GGSN, and HLR, as described in the standards. In
addition, firewalls, NAPTs, accounting systems, and QoS policy servers are often involved.
There are 10s of roundtrips depending on previous states required to complete this process. It is
doubtful that this is an ideal or unavoidable situation, yet some aspects appear inherent in the
current architecture. Also, the messages carried in these signaling flows are complex, overloaded
and/or nested in multiple layers, so that the correctness of any implementation is doubtful at best.
It also presents a rich target for hacking or DoS attacks. Thus, simplicity of protocols should be
one of the prime objectives of any new protocol design, along with modularity of protocols and
soft–failure under errors.

D. Designing for uncertainty

Most of 3GPPs recent activities, such as latency reduction, local traffic off–loading, flattening
the network architecture, and peak rate improvements, have been in reaction to seemingly
unexpected growth of demands from reality: Web centric content, Smartphones, push
applications, etc. It should be obvious that the basic network architecture standards should be
designed for uncertainty and flexibility , rather than specific service scenarios. Also, considering
the long delays in responding to market demands through the standard–setting process, the basic
standards should be independent of the details of features and services, even if such a separation
sacrifices some of the benefits of tight integration. The use of host–centric protocols is also
consistent with this principle, since new services or features can typically be implemented on
host devices and servers, and should not involve changes deep inside networks.

G. Substitute hard–state rigid protocols with soft–state, soft–fail protocols

As mentioned earlier, 3GPP standards rely heavily on hard–state multi–party protocols for most
operations, such as mobility, paging, network entry, sleep/idle modes, and routing. These
protocols often require that three or more parties remain synchronized in their protocol states for
correct operation. Perhaps not surprisingly, these complex protocols are rarely verified for
correctness and race conditions. They fail hard, recover slowly, require large memories and
processing power, and are difficult to interoperate and debug. Most of them could well be
replaced with soft–state, soft–fail protocols with better results.

H. Separation of air interface and mobility core network evolution

Each new generation of the cellular air interface has been accompanied by a large overhaul of the
wired core and RAN. That approach appears no longer sustainable for keeping pace with
different rates of innovation in wired networking, mobile computing, and wireless link
technologies. Arguably, innovations and changes in the network and application layers are more
rapid and unpredictable, as evidenced by the rapid rise of the smartphone ecosystem, compared
to wireless link layer evolution that proceeds more slowly and requires industry–wide
coordination. This separation also widens the market for air interface technology beyond cellular
mobile technologies into fixed wireless access, indoor private networks, etc. without being
burdened by the necessary network elements dictated by the current standards. The current tight
integration of standards for the air interface and the wired network should be separated, with
clear interfaces to allow their separate evolution.
What is co-channel interference? What is adjacent channel
intereference?

Co-channel interference or CCI is crosstalk from two different radio transmitters using the
same frequency. There can be several causes of co-channel radio interference; four examples
are listed here.

 Cellular Mobile Networks: In cellular mobile communication (GSM & LTE Systems, for
instance), frequency spectrum is a precious resource which is divided into non-overlapping
spectrum bands which are assigned to different cells (In cellular communications, a cell
refers to the hexagonal/circular area around the base station antenna). However, after
certain geographical distance, the frequency bands are re-used, i.e. the same spectrum
bands are re-assigned to other distant cells. The co-channel interference arises in the
cellular mobile networks owing to this phenomenon of Frequency reuse. Thus, besides the
intended signal from within the cell, signals at the same frequencies (co-channel signals)
arrive at the receiver from the undesired transmitters located (far away) in some other cells
and lead to deterioration in receiver performance.

 Adverse weather conditions: During periods of uniquely high-

pressure weather, VHF signals which would normally exit through the atmosphere can
instead be reflected by the troposphere. This tropospheric ducting will cause the signal to
travel much further than intended; often causing interference to local transmitters in the
areas affected by the increased range of the distant transmitter.

 Poor frequency planning: Poor planning of frequencies by broadcasters can cause CCI,
although this is rare. A very localised example is Listowel in the south-west of Ireland.
The 2RN UHF television transmitter systems in Listowel and Knockmoyle (near Tralee) are
on the same frequencies but with opposite polarisation. However in some outskirts of
Listowel town, both transmitters can be picked up causing heavy CCI. This problem forces
residents in these areas to use alternative transmitters to receive RTÉ programming.
Another example is the surrounding area of viewers that can receive mainly Gunung
Ledang transmitter in Malaysia. Examples are TV1 and TV2 from Bukit Tinggi and Bukit
Tampin, who are using both Channel 6 and Channel 9. Moreover, Negeri FM from Bukit
 Tampin and Asyik FM from Gunung Ledang, which frequencies are 95.7MHz and 95.6MHz
respectively. TV3 and TV1 from Gunung Ledang and Suria and Channel U from Singapore
also using same frequency, who are using both Channel 12 and Channel 28. That Channel
12 and 28 from Ledang transmits in high TX power, causing interference in most of Johor
area, but viewers are still able to receive TV station from Gunung Pulai (Johor Bahru) well.
This causing difficulty for viewers who watching using analogue TV, and needs to using
alternate transmitter or even using better antenna for better reception, or force them to
subscribe Astro/buying NJOI satellite or Unifi Fibre service. In addition, Minnal FM from
Gunung Kledang, Ipoh which transmits to Central Perak and 988FM from Gunung Ulu Kali
which transmits to Klang Valley and South Perak, which are on 98.9 MHz and 98.8 MHz
respectively. The 98.9MHz can transmits to most of Perak area, causing interference in
most of Perak area, but listeners are still able to tune to Minnal FM on 96.3 MHz from
Gunung Ulu Kali (Klang Valley, South Perak and Tapah) or 107.9 MHz from Maxwell Hill,
Taiping, and 988 on 99.8 MHz from Gunung Kledang, Ipoh.

 Overly-crowded radio spectrum: In many populated areas, there just isn't much room in
the radio spectrum. Stations will be jam-packed in, sometimes to the point that one can hear
loud and clear two, three, or more stations on the same frequency, at once. In the USA, the
FCC propagation models used to space stations on the same frequency are not always
accurate in prediction of signals and interference. An example of this situation is in some
parts of Fayetteville, Arkansas the local 99.5 FM KAKS is displaced by KXBL 99.5 FM in
Tulsa, particularly on the west side of significant hills. Another example would be
of Cleveland's WKKY 104.7 having interference from Toledo's WIOT 104.7 FM on
the Ontario shore of Lake Erie, as well as Woodstock's CIHR-FM (on rare occasions), which
is also on 104.7 FM, due to the signals travelling very far across Lake Erie. The interference
to WIOT from the operation of W284BQ, translator, has been resolved by the FCC. Effective
October 18, 2011 it must cease operation.

 Daytime vs Nighttime: In the Medium frequency portion of the radio spectrum where
most AM broadcasting is allocated, signals propagate full-time via groundwave and, at
nighttime, via skywave as well. This means that during the nighttime hours, co-channel
interference exists on many AM radio frequencies due to the medium waves reflecting off
the ionosphere and being bounced back down to earth. In the United
States, Canada, Mexico, and the Bahamas, there are international agreements on certain
frequencies which allocate "clear-channel" broadcasting for certain stations to either have
 their respective frequencies to themselves at night, or to share their respective frequencies
with other stations located over hundreds or even thousands of miles away. On other
frequencies, there are "Regional Channels" where most stations on these frequencies either
reduce power or change to a directional antenna system at nighttime to help reduce co-
channel interference to each other's signals. In the United States, there are six "Local
Channel" frequencies, also known as "graveyarders" where nearly every station on those
frequencies has the same power and antenna pattern both day and night and, as a result of
skywave propagation, there is normally massive co-channel interference in rural areas on
these frequencies, often making it difficult, if not impossible, to understand what's being said
on the nearest local station on the respective channel, or the other distant stations which
are bouncing on the same channel, during the nighttime hours. Skywave has been used for
long distance AM radio reception since radio's inception and should not be construed as a
negative aspect of AM radio. FCC deregulation allowed many new AM radio stations on the
former clear and regional channel designations; this is the principal cause of overcrowding
on the AM band at night. A new source of interference on the AM broadcast band is the new
digital broadcast system called HD, any AM station that broadcasts HD superimposes digital
"hash" on its adjacent channels. This is especially apparent at night as some stations, for
example WBZ transmits its 30 kHz wide signal for hundreds of miles at night causing
documented interference and covering another station on an adjoining frequency (WYSL
1040) as far as 400 miles away, The FCC refuses to do anything about interference to the
AM band ignoring HD and many other man made causes of interference to AM radio
including many electronic devices. Although there are FCC rules against interference they
routinely ignore them.
 Cancellation of signal: In addition, many AM stations, including but not limited to the clear
channel stations, often experience cancellation of their own signals within the inner and
outer fringes of their normal groundwave coverage areas at nighttime due to the stations'
individual skywave signals reaching the listeners' receivers at or near equal strength to the
stations' individual groundwave signals; this phenomenon is very similar to the multipath
interference experienced on FM Radio in the VHF band within mountainous regions and
urban areas due to signals bouncing off of mountains, buildings, and other structures,
except that the groundwave-skywave cancellation occurs almost exclusively at nighttime
when skywave propagation is present.
Adjacent-channel interference (ACI) is interference caused by extraneous power from
a signal in an adjacent channel. ACI may be caused by inadequate filtering (such as incomplete
filtering of unwanted modulation products in FM systems), improper tuning or poor frequency
control (in the reference channel, the interfering channel or both).

ACI is distinguished from crosstalk.[1]

Broadcast regulators frequently manage the broadcast spectrum in order to minimize adjacent-
channel interference. For example, in North America, FM radio stations in a single region cannot
be licensed on adjacent frequencies — that is, if a station is licensed on 99.5 MHz in a city, the
frequencies of 99.3 MHz and 99.7 MHz cannot be used anywhere within a certain distance of
that station's transmitter, and the second-adjacent frequencies of 99.1 MHz and 99.9 MHz are
restricted to specialized usages such as low-power stations. Similar restrictions formerly applied
to third-adjacent frequencies as well (i.e. 98.9 MHz and 100.1 MHz in the example above), but
these are no longer observed.

The adjacent-channel interference which receiver A experiences from a transmitter B is the sum
of the power that B emits into A's channel—known as the "unwanted emission", and
represented by the ACLR (Adjacent Channel Leakage Ratio)—and the power that A picks up
from B's channel, which is represented by the ACS (Adjacent Channel Selectivity). B emitting
power into A's channel is called adjacent-channel leakage (unwanted emissions). It occurs
because RF filters require a roll-off, and do not eliminate a signal completely. Therefore, B emits
some power in the adjacent channel which is picked up by A. A receives some emissions from
B's channel due to the roll off of the selectivity filters. Selectivity filters are designed to "select" a
channel.

What is PHP? Write down the applications of PHP? What are

the features of PHP?

PHP is a server-side scripting language designed for web development but also used as
a general-purpose programming language. PHP is now installed on more than 244
million websites and 2.1 million web servers.[2] Originally created byRasmus Lerdorf in 1995,
the reference implementation of PHP is now produced by The PHP Group.[3] While PHP
originally stood for Personal Home Page,[4] it now stands for PHP: Hypertext Preprocessor,
a recursive acronym.[5]

PHP code is interpreted by a web server with a PHP processor module, which generates the
resulting web page: PHP commands can be embedded directly into an HTML source document
rather than calling an external file to process data. It has also evolved to include a command-
line interface capability and can be used in standalone graphical applications.[6]

PHP is free software released under the PHP License, which is incompatible with the GNU
General Public License (GPL) due to restrictions on the usage of the term PHP.[7] PHP can be
deployed on most web servers and also as a standaloneshell on almost every operating
system and platform, free of charge.[8]

PHP development began in 1994 when the developer Rasmus Lerdorf wrote a series
of Common Gateway Interface (CGI) Perl scripts, which he used to maintain his personal
homepage. The tools performed tasks such as displaying his résumé and recording his web
traffic.[3][9][10] He rewrote these scripts in C for performance reasons, extending them to add the
ability to work with web forms and to communicate with databases, and called this
implementation "Personal Home Page/Forms Interpreter" or PHP/FI. PHP/FI could be used to
build simple, dynamic web applications. Lerdorf initially announced the release of PHP/FI as
"Personal Home Page Tools (PHP Tools) version 1.0" publicly to accelerate bug location and
improve the code, on the comp.infosystems.www.authoring.cgi Usenet discussion group on
June 8, 1995.[11][12] This release already had the basic functionality that PHP has as of 2013.
This included Perl-like variables, form handling, and the ability to embed HTML.
The syntax resembled that of Perl but was more limited and simpler, although less
consistent.[3] A development team began to form and, after months of work and beta testing,
officially released PHP/FI 2 in November 1997.

Zeev Suraski and Andi Gutmans rewrote the parser in 1997 and formed the base of PHP 3,
changing the language's name to the recursive acronym PHP: Hypertext
[3]
Preprocessor. Afterward, public testing of PHP 3 began, and the official launch came in June
1998. Suraski and Gutmans then started a new rewrite of PHP's core, producing the Zend
Engine in 1999.[13] They also founded Zend Technologies in Ramat Gan, Israel.[3]

On May 22, 2000, PHP 4, powered by the Zend Engine 1.0, was released.[3] As of August 2008
this branch reached version 4.4.9. PHP 4 is no longer under development nor will any security
updates be released.[14][15]
On July 13, 2004, PHP 5 was released, powered by the new Zend Engine II.[3] PHP 5 included
new features such as improved support for object-oriented programming, the PHP Data Objects
(PDO) extension (which defines a lightweight and consistent interface for accessing databases),
and numerous performance enhancements.[16] In 2008 PHP 5 became the only stable version
under development. Late static binding had been missing from PHP and was added in version
5.3.[17][18]

A new major version has been under development alongside PHP 5 for several years. This
version was originally planned to be released as PHP 6 as a result of its significant changes,
which included plans for full Unicode support. However, Unicode support took developers much
longer to implement than originally thought, and the decision was made in March 2010[19] to
move the project to a branch, with features still under development moved to trunk.

Changes in the new code include the removal of register_globals,[20] magic quotes, and safe
mode.[14][21] The reason for the removals was that register_globals had opened security holes by
intentionally allowing runtime data injection, and the use of magic quotes had an unpredictable
nature. Instead, to escape characters, magic quotes may be replaced with
the addslashes() function, or more appropriately an escape mechanism specific to the database
vendor itself like mysql_real_escape_string() for MySQL. Functions that will be removed in
future versions and have been deprecated in PHP 5.3 will produce a warning if used.[22]

Many high-profile open-source projects ceased to support PHP 4 in new code as of February 5,
2008, because of the GoPHP5 initiative,[23] provided by a consortium of PHP developers
promoting the transition from PHP 4 to PHP 5.[24][25]

PHP interpreters are available on both 32-bit and 64-bit operating systems, but on Microsoft
Windows the only official distribution is a 32-bit implementation, requiring Windows 32-bit
compatibility mode while using Internet Information Services (IIS) on a 64-bit Windows platform.
Experimental 64-bit versions of PHP 5.3.0 were briefly available for Microsoft Windows, but
have since been removed.[26]

PHP can preform calculations - PHP can preform all types of calculations. From figuring
outwhat day it is, or what day of the week March 18, 2046 is, to preforming all different types
ofmathematical equations.
PHP can collect user information - By this I mean, you can let your user directly interact with
the script. This can be something really simple, like collecting the temperature from the user that
they want to convert from degrees to another format, or it can be more extensive information,
like adding their information to an address book, or letting them post on a forum.
PHP can interact with MySQL databases - And in doing this, the possibilities are endless. You
canwrite users information to the database and you can retrieve information from the database.
This allows you to create pages on the fly using the contents of the database. You can even do
more complex things like setting up a login system, creating a website search feature, or keep
your store's product catalogue and inventory online.
PHP and GD Library can create graphics - You can use PHP to create simple graphics on the
fly. You can also use it to edit existing graphics. You might want to do this to resize images,
rotate them, or greyscale them. Some practical applications for this are allowing users to edit
their avatars or creating CAPTCHA verifications. You can also create dynamic graphics that are
always changing, my favorite example being dynamic twitter signatures.
This list of what PHP can do could go on for pages and pages, but if you're new to PHP, hopeful
this gives you a taste of the type of things this dynamic language can bring to your website.
Want to learn? Start with the beginners tutorial!

Introduction

Almost all large PHP applications, as well as many small ones, have a notion of user accounts,
and, whether we like it or not, they typically use passwords (or at best passphrases) to
authenticate the users. How do they store the passwords (to authenticate against)? Reasonable
applications don't. Instead, they store password hashes. There have been many short articles,
blog posts, even book chapters that try/claim to show you how to properly compute and use
password hashes. Older ones will tell you to use the md5() function. Newer ones will tell you to
use sha1() or hash() (SHA-256, etc.), add salting (but "forget" to add stretching, which is equally
important), and use mysql_real_escape_string() on the username. Unfortunately, while some of
these recommendations are steps in the right direction (although not all are), none of the articles
on password security in PHP that I saw were "quite it".

Finally, some of the more recent blog posts, forum comments, and the like have started to
recommend phpass, the password/passphrase hashing framework for PHP that I wrote, and
which has already been integrated into many popular "web applications" including phpBB3,
WordPress, and Drupal 7. Obviously, I fully agree with this recommendation. However, I was
not aware of an existing step-by-step guide on integrating phpass into a PHP application, and
password security is not only about password hashing anyway.
In this article/tutorial, I will guide you through the steps needed to introduce proper (in my
opinion at least) user/password management into a new PHP application. I will start by briefly
explaining password/passphrase hashing and how to access the database safely. Then we will
proceed through several revisions of the sample program. We'll start with a very simple PHP
program capable of creating new users only and having some subtle issues. We will gradually
improve this program adding functionality (logging in to existing user accounts, changing user
passwords, and enforcing a password policy) and "discovering" and dealing with the issues.

We will also briefly touch many related topics. Sub-headings have been chosen such that
you may skip or skim over the topics you think you're already familiar with... or better
read those sections anyway. Let's get started.

Password/passphrase hashing

Decent systems/applications do not actually store users' passwords. Instead, they transform
new passwords being set/changed into password hashes with cryptographic (one-way) hash
functions, and they store those hashes. They should preferably use hash functions intended for
password hashing. Direct/naive use of other cryptographic hash functions, such as PHP's
md5(), sha1(), or hash('sha256', ...) for that matter, has dire consequences.

When a user authenticates to the application with a username and a previously-set password,
the application looks up some auxiliary information (such as the hash type, the salt, and the
iteration count - all of which are described below) for the provided username, transforms the
provided password into its hash, and compares this hash against the one stored for the user. If
the two hashes match, authentication succeeds (otherwise it fails).

- "Why bother with password hashing when I use (or don't use) SSL (https URLs) anyway?"
(Surprisingly, this question is really being asked both ways. More often, people would make an
incorrect statement that you don't need password hashing, or don't need to do it right, because
you do or because you don't use SSL.)
- Password hashing, if done right, reduces the risk impact of having the hashes stolen or
leaked - an attacker will recover fewer plaintext passwords from the hashes. Also, the cost of
recovery from an incident like this may be reduced - rather than change all passwords at once,
which may be costly or prohibitive to do, a system's administrator may audit the password
hashes with a tool such as John the Ripper and only have the weak passwords changed. With
proper password hashing and password policy enforcement in place, the majority of the
passwords could be considered "strong enough" and would not need to be changed
immediately even after a known and otherwise-resolved security compromise. The use of SSL
mitigates the risk of having some plaintext passwords captured while in transit. Clearly,
these risks are different. An attacker capable of capturing some of the network traffic is not
necessarily capable of getting a copy of the database, and vice versa. Thus, it makes perfect
sense to use one of these countermeasures - password hashing and SSL - without the other
(which does not address "the other" risk then), and it also makes sense to use both of them
together.

Salting

Salts are likely-unique values that are entered into a password hashing method along with the
password, which results in the same password hashing into completely different hash values
given different salts. Proper use of salts may defeat a number of attacks, including:

 Ability to try candidate passwords against multiple hashes at the price of one
 Use of pre-hashed lists (or the smarter "rainbow tables") of candidate passwords
 Ability to determine whether two users (or two accounts of one user) have the same or
different passwords without actually having to guess one of the passwords

Salts are normally stored along with the hashes. They are not secret.

Stretching

Offline password cracking (given stolen or leaked password hashes) involves computing hashes
of large numbers of candidate passwords. Thus, in order to slow those attacks down, the
computational complexity of a good password hashing method must be high - but of course not
too high as to render it impractical.

Typical cryptographic hash functions not intended for password hashing were designed for
speed. If these are directly misused for password hashing, then offline password cracking
attacks may run at speeds of many million of candidate passwords per second.

These cryptographic hash functions (or even block ciphers) - let's call them "cryptographic
primitives" - may be used as building blocks to construct a decent password hashing method,
which would use thousands or millions of iterations of the underlying cryptographic primitive.
This is called password (or key) stretching (or strengthening). Preferably, the number of
iterations should not be hard-coded, but rather it should be configurable by an administrator for
use when a new password is set (hashed), and it should be getting saved along with the hash
(to allow the administrator to change the iteration count for newly set/changed passwords, yet
not break support for previously-generated password hashes).

- "My web application must be fast. I can't afford to use a slow hash function!"
- Actually, you can. No one said it should be taking an entire second to compute a password
hash. Is 10 milliseconds fast enough for you? Perhaps it is, but if not you can make it 1 ms or
less (which is likely way below other per-request "overhead" that your application incurs
anyway) and still benefit from password stretching a lot. Please note that without any stretching
a cryptographic primitive could be taking as little as some microseconds or even nanoseconds
to compute (at least during an offline attack, which would use an optimal implementation) . If
you go from one microsecond to one millisecond, which is clearly affordable, you make offline
attacks (against stolen or leaked hashes) run 1000 times slower, or you effectively stretch your
users' passwords or passphrases by about 10 bits of entropy each. That's significant - it is
roughly equivalent to each passphrase containing one additional word, without actually adding
that extra word and having the users memorize it. Besides, the password hash is typically only
computed when a user logs in (or when a new user is registered or a password is changed),
which occurs relatively infrequently (compared to the frequency of other requests). Subsequent
requests by the logged in user will use a session ID instead.

Choice of the underlying cryptographic primitive

The choice of the underlying cryptographic primitive - such as MD5, SHA-1, SHA-256, or even
Blowfish or DES (which are block ciphers, yet they may be used to construct one-way hashes) -
does not matter all that much. It's the higher-level password hashing method, employing
salting and stretching, that makes a difference.

- "I heard that MD5 has been "broken". Shouldn't we use SHA-1 instead?"
- It is true that MD5 has been broken as it relates to certain attacks (practical). SHA-1 has also
been broken in certain other ways (mostly theoretical). However, neither break has anything to
do with the uses of these functions for password hashing, especially not as building blocks in a
higher-level hashing method. Thus, any possible reasons to move off MD5 or SHA-1 as
underlying cryptographic primitives for password hashing "because of the break" are purely
"political" rather than technical. (It may be easier to just phase out MD5 and SHA-1 rather than
differentiate their affected vs. unaffected uses.)

phpass - the password/passphrase hashing framework for PHP applications

phpass provides an easy to use abstraction layer on top of PHP's cryptographic hash functions
suitable for password hashing. As of this writing, it supports three password hashing methods,
including two via PHP's crypt() function - these are known in PHP as CRYPT_BLOWFISH and
CRYPT_EXT_DES - and one implemented in phpass itself on top of MD5. All three employ
salting, stretching, and variable iteration counts (configurable by an administrator,
encoded/stored along with the hashes).

PHP 5.3.0 and above is guaranteed to support all three of these hashing methods due to
code included into the PHP interpreter itself. Specific builds/installs of older versions of PHP
may or may not support the CRYPT_BLOWFISH and CRYPT_EXT_DES methods - this is
system-specific. For example, the Suhosin PHP security hardening patch, included into many
distributions' packages of PHP, has been adding support for CRYPT_BLOWFISH for years,
many operating systems - such as *BSD's, Solaris 10, SUSE Linux, ALT Linux, and indeed
Openwall GNU/*/Linux - are also providing support for CRYPT_BLOWFISH via the system
libraries (which PHP uses), and some operating systems - *BSD's, Openwall GNU/*/Linux - also
provide support for CRYPT_EXT_DES.

The MD5-based salted and stretched hashing implemented in phpass itself is supported
on all systems - starting with the ancient PHP 3. phpass provides a way for you (the
application developer or administrator) to force the use of these "portable" hashes - this is a
Boolean parameter to the PasswordHash constructor function.

Unless you force the use of "portable" hashes, phpass' preferred hashing method is
CRYPT_BLOWFISH, with a fallback to CRYPT_EXT_DES, and then a final fallback to the
"portable" hashes. CRYPT_BLOWFISH and CRYPT_EXT_DES are preferred primarily for the
efficiency of the underlying implementations (in C and on some systems in assembly),
compared to phpass' own code around MD5 (in PHP, even though the underlying MD5 code is
in C). This greater code efficiency allows for more extensive and thus more effective use of
password stretching (higher iteration counts). (It is assumed that an attacker would have a near-
optimal implementation of any of these hashing methods anyway.)

Besides the actual hashing, phpass transparently generates random salts when a new
password or passphrase is hashed, and it encodes the hash type, the salt, and the
password stretching iteration count into the "hash encoding string" that it returns. When
phpass authenticates a password or passphrase against a stored hash, it similarly
transparently extracts and uses the hash type identifier, the salt, and the iteration count
out of the "hash encoding string". Thus, you do not need to bother with salting and
stretching on your own - phpass takes care of these for you.

- "What source of randomness does phpass use? Does it work on Windows?"

- You might have noticed that phpass uses /dev/urandom, which is a decent supply of
randomness on modern Unix-like systems. However, phpass will transparently fallback to its
own pseudo-random byte stream generator (which is based primarily on multiple measurements
of the current time with up to microsecond precision) when /dev/urandom is unavailable or when
it fails. Thus, yes, phpass works on Windows (as well as on Unix-like systems indeed),

Naturally, we'll use phpass for our sample program.

The database (and how to access it safely)

SQL injections

What SQL injections are

In many cases, we will need to pass pieces of untrusted user input into SQL queries. Even with
our trivial database and the initial revision of our user management program (which we'll create
soon), there will be untrusted user input: the username and password (or passphrase), at least
before we've verified them. If we blindly embed the target username string obtained via a
website form into an SQL query string, we might alter the SQL query. Since the username is
under a potential attacker's control, the attacker may be able to alter our SQL query in a way
such that another valid SQL query of the attacker's choice is formed. This may allow not only to
circumvent our program's intended behavior (e.g., have it change another user's password with
that altered query), but also to mount all sorts of attacks on the SQL server, as well as on our
program (such as via query results that would suddenly become fully untrusted input as well).
How to deal with SQL injections

"- Can't we just enclose the user inputs in single quotes when embedding them in an SQL query
string? Wouldn't that do the trick?"
- No. One of the input values can simply close the quotes, braces, etc., do its dirty deed, then
provide additional SQL statements (or whatever) to make the rest of the original query
"complete" (avoiding a syntax error). Thus, this naive approach alone does not work at all.

There are several real ways to combat SQL injections, of varying effectiveness and with
different pros and cons. Most of these can be used together for greater assurance.

 Filtering - sanitize the input values rejecting or modifying "bad" ones (preferably using a
whitelist of known-safe input values rather than a blacklist of known-unsafe ones)
 Escaping - prefix any special characters (most notably the single quote character) with
an escape character (preferably using the API functions specific to the target SQL server
type)
 Encoding - turn any input strings into other strings consisting of safe characters only -
e.g., an application may introduce '%' as its own escape character, then URL-encode all
characters not from a known-safe set (the '%' character has a special meaning in certain
contexts, though, so you might choose another or you might only use this technique
along with escaping)
 Prepared statements - rather than form SQL query strings with inputs embedded into
them (in one way or another), an application may use advanced APIs to pass SQL
queries with placeholders to the SQL server and then pass the input values to the SQL
server "separately"

In the sample program that we'll be writing during the rest of this article, we'll use filtering (the
"rejection" kind of it) and prepared statements in such a way that if any one of these
techniques fails to provide its security, the application will nevertheless remain secure.

Prepared statements with PHP and MySQL

As of this writing, PHP offers three main interfaces to MySQL: PHP's MySQL Extension
(obsolete, not recommended for new projects - but still widely used), PHP's mysqli (MySQL
Improved) Extension ("preferred" for new projects), and PHP Data Objects (PDO)
(recommended, but not "preferred" for new projects). The last two of these support prepared
statements. Both require PHP 5+. We'll use mysqli.

The separation of code and data achieved with the mysqli PHP extension, the underlying
MySQL APIs that it uses, and the (relatively) new MySQL protocol revision can't be perfect -
everything is sent over the same socket connection anyway - but it does appear to be way
better (simpler, and hence less error-prone) than what could be achieved by escaping.
Specifically, in the MySQL binary protocol, the input values are preceded by binary
representations of their lengths in bytes and then are sent verbatim.

Employ the principle of least privilege

Besides avoiding SQL injections, it makes sense to mitigate any that would potentially occur
anyway, as well as possibly some other attacks carried out against or via the database. To this
end, it is a good idea to have your PHP application use an SQL server account with the
minimum privileges required - not an administrative account and not an account that can also
access another database.

This also helps in case your PHP application is somehow fully compromised, such that the
attacker gains direct access to the database with the application's access privileges, yet you
care not to let this compromise directly "propagate" onto other databases that your application
does not use.

Schema

For our sample program, we'll start with just one table in a brand new MySQL database.
Connect to the MySQL server (such as with the command-line mysql client program) and issue
the following:

create database myapp;

use myapp;
create table users (user varchar(60), pass varchar(60));

(We will need to revise this a little bit to deal with an issue that we'll "discover" further down this
article.)
The user column will hold usernames, and the pass column will hold password hashes.
Currently, phpass produces hash encoding strings that are at most 60 characters long.

The sample program is born

The code snippets included in this article generally assume that you're familiar with creating
HTML web pages and PHP scripts. Thus, any opening and closing tags (such as <html> and
<?php), etc. are omitted from here, to keep the article from growing too long. However, the
sample files in the archive provided with the article do include all of those essential bits.

How to create new users

First, we need to put the phpass code in place. (We will use it to hash the new password.) We
place the PasswordHash.php file from the phpass distribution tarball somewhere within our web
"virtual host" "document root" directory and we set proper permissions for the file to be loaded
by the web server's PHP setup (typically, the Unix permission bits will need to be 600 or 644
depending on web server setup).

Then we create a subdirectory for our sample program (this is how multiple revisions of the
program are included in the archive accompanying this article - in separate subdirectories). Let's
call the directory demo (and set its Unix permissions to 711). We'll place two files into this
directory: user-man.html (with permissions set to 644) containing the HTML form below, and
user-man.php (with permissions set the same as we did for PasswordHash.php).

Let's place the following HTML form into user-man.html:

<form action="user-man.php" method="POST">

Username:<br>
<input type="text" name="user" size="60"><br>
Password:<br>
<input type="password" name="pass" size="60"><br>
<input type="submit" value="Create user">
</form>
This form asks for and submits a username and a password to the user-man.php script. Let's
start writing it. First, let's include the phpass code:

require '../PasswordHash.php';

To actually use phpass, we need to decide on and specify the extent of password stretching and
whether we want to force the use of "portable" hashes or not (both of these matters were briefly
discussed above). Let's place those constants into PHP variables:

// Base-2 logarithm of the iteration count used for password stretching

$hash_cost_log2 = 8;
// Do we require the hashes to be portable to older systems (less secure)?
$hash_portable = FALSE;

(In a real application, these should be in a configuration file included from the actual program
code files instead. Alternatively, they may be configurable via the application itself, by an
administrative user.)

To obtain the submitted username and password, let's initially use:

$user = $_POST['user'];
// Should validate the username length and syntax here
$pass = $_POST['pass'];

(This is a bit problematic. We will revise it soon.)

Now we can hash the password with:

$hasher = new PasswordHash($hash_cost_log2, $hash_portable);

$hash = $hasher->HashPassword($pass);
if (strlen($hash) < 20)
fail('Failed to hash new password');
unset($hasher);
This uses the fact that the shortest valid password hash encoding string that phpass can
currently return is 20 characters long (this is the case for CRYPT_EXT_DES, whereas other
hash types use even longer encoding strings). fail() is a custom function that we'll use in our
sample program. Let's define it (earlier in the code) as follows:

function fail($pub, $pvt = '')

{
$msg = $pub;
if ($pvt !== '')
$msg .= ": $pvt";
exit("An error occurred ($msg).\n");
}

(This function as defined above is a bit problematic. We will revise it soon.)

Note that we don't bother producing proper HTML output in fail(). For our sample program, it is
simpler to produce plain text output. Let's set the HTTP header accordingly such that the web
browser does not attempt to parse our script's output as HTML:

header('Content-Type: text/plain');

Indeed, we need to do this before our script possibly produces any output. (In a real PHP
application, you would likely be producing HTML output instead, which requires more code and
extra safety measures.)

Let's also place our database access credentials into PHP variables. For example:

// In a real application, these should be in a config file instead

$db_host = '127.0.0.1';
$db_port = 3306;
$db_user = 'mydbuser';
$db_pass = 'voulDyu0gue$s?';
$db_name = 'myapp';
Let's connect to the database using mysqli, and let's not forget to check for a possible failure:

$db = new mysqli($db_host, $db_user, $db_pass, $db_name, $db_port);

if (mysqli_connect_errno())
fail('MySQL connect', mysqli_connect_error());

Finally, let's try to create the user by inserting the username and the password hash encoding
string (which includes the salt, etc.) into the database table using the prepared statements API:

($stmt = $db->prepare('insert into users (user, pass) values (?, ?)'))

|| fail('MySQL prepare', $db->error);
$stmt->bind_param('ss', $user, $hash)
|| fail('MySQL bind_param', $db->error);
$stmt->execute()
|| fail('MySQL execute', $db->error);

If we got this far, we must have successfully created the user. Let's close the database
connection:

$stmt->close();
$db->close();

In fact, it would be nice to do this on failure as well, but that would make the code more
complicated (the cleanups to perform would vary depending on where the failure occurs).
Instead, we rely on the web server setup to perform any cleanups for terminating PHP scripts,
which it needs to do anyway because scripts may sometimes terminate abnormally.

So that's it. Please find the HTML file and the demo program we've just created, complete with
all details and with the snippets in the proper order (unlike in this article), in the demo1
subdirectory in the accompanying archive (tar.gz, ZIP).

Let's test the program. Go to the URL for the user-man.html HTML page in a web browser, enter
myuser for the username and mypass for the password. If the script completes without error, we
should be able to see the new user account in the users table:
Q- What are the various channel allocation techniques used in cellular
communication?

Ans:-

In radio resource management for wireless and cellular network, channel

allocation schemes are required to allocate bandwidth and communication channels to
base stations, access points and terminal equipment. The objective is to achieve
maximum system spectral efficiency in bit/s/Hz/site by means of frequency reuse, but
still assure a certain grade of service by avoiding co-channel interference and adjacent
channel interference among nearby cells or networks that share the bandwidth. There
are two types of strategies that are followed:-

1.Fixed

FCA, fixed channel allocation: Manually assigned by the network operator

2.Dynamic:

1.DCA, dynamic channel allocation,

2. DFS, dynamic frequency selection

3.Spread spectrum

FCA

In Fixed Channel Allocation or Fixed Channel Assignment (FCA) each cell is

given a predetermined set of frequency channels. FCA requires manual frequency
planning, which is an arduous task in TDMA and FDMA based systems, since such
systems are highly sensitive to co-channel interference from nearby cells that are
reusing the same channel. Another drawback with TDMA and FDMA systems with
FCA is that the number of channels in the cell remains constant irrespective of the
number of customers in that cell. This result in traffic congestion and some calls being
lost when traffic gets heavy in some cells, and idle capacity in other cells.

If FCA is combined with conventional FDMA and perhaps or TDMA, a fixed

number of voice channels can be transferred over the cell. A new call can only be
connected by an unused channel. If all the channel are occupied than the new call is
blocked in this system. There are however several dynamic radio-resource
management schemes that can be combined with FCA. A simple form is traffic-
adaptive handover threshold, implying that that calls from cell phones situated in the
overlap between two adjacent cells can be forced to make handover to the cell with
lowest load for the moment. If FCA is combined with spread spectrum, the maximum
number of channels is not fixed in theory, but in practice a maximum limit is applied,
since too many calls would cause too high co-channel interference level, causing the
quality to be problematic. Spread spectrum allows cell breathing to be applied, by
allowing an overloaded cell to borrow capacity (maximum number of simultaneous
calls in the cell) from a nearby cell that is sharing the same frequency.

FCA can be extended into a DCA system by using a borrowing strategy in

which a cell can borrow channels from neighboring cell which is supervised by Mobile
Switching Center (MSC).

DCA and DFS

Dynamic Frequency Selection (DFS) may be applied in wireless networks with

several adjacent non-centrally controlled access-points. The access-points automatically
selects a frequency channel with low interference level. DFS is supported by the novel
IEEE 802.11h wireless local area network standard. DFS is also mandated in the 5470-
5725 MHz U-NII band for radar avoidance..

A more efficient way of channel allocation would be Dynamic Channel

Allocation or Dynamic Channel Assignment (DCA) in which voice channel are not
allocated to cell permanently, instead for every call request base station request
channel from MSC. The channel is allocated following an algorithm which accounts
likelihood of future blocking within the cell. It requires the MSC to collect real time
data on channel occupancy, traffic distribution and Radio Signal Strength Indications
(RSSI). DCA schemes are suggested for TDMA/FDMA based cellular systems such as
GSM, but are currently not used in any products. OFDMA systems, such as the
downlink of 4G cellular systems, can be considered as carrying out DCA for each
individual sub-carrier as well as each timeslot.

DCA and DFS eliminate the tedious manual frequency planning work. DCA also
handles bursty cell traffic and utilizes the cellular radio resources more efficiently.
DCA allows the number of channels in a cell to vary with the traffic load, hence
increasing channel capacity with little costs.

Spread spectrum

Spread spectrum can be considered as an alternative to complex DCA

algorithms. Spread spectrum avoids cochannel interference between adjacent cells,
since the probability that users in nearby cells use the same spreading code is
insignificant. Thus the frequency channel allocation problem is relaxed in cellular
networks based on a combination of Spread spectrum and FDMA, for example IS95
and 3G systems. Spread spectrum also facilitate that centrally controlled base stations
dynamically borrow resources from each other depending on the traffic load, simply
by increasing the maximum allowed number of simultaneous users in one cell (the
maximum allowed interference level from the users in the cell), and decreasing it in an
adjacent cell. Users in the overlap between the base station coverage area can be
transferred between the cells (called cell-breathing), or the traffic can be regulated by
admission control and traffic-shaping.

However, spread spectrum gives lower spectral efficiency than non-spread

spectrum techniques, if the channel allocation in the latter case is optimized by a good
DCA scheme. Especially OFDM modulation is an interesting alternative to spread
spectrum because of its ability to combat multipath propagation for wideband
channels without complex equalization. OFDM can be extended with OFDMA for
uplink multiple access among users in the same cell. For avoidance of inter-cell
interference, FDMA with DCA or DFS is once again of interest. One example of this
concept is the above mentioned IEEE 802.11h standard. OFDM and OFDMA with
DCA is often studied as an alternative for 4G wireless systems.

DCA on a packet-by-packet basis

In packet based data communication services, the communication is bursty and

the traffic load rapidly changing. For high system spectrum efficiency, DCA should be
performed on a packet-by-packet basis. Examples of algorithms for packet-by-packet
DCA are Dynamic Packet Assignment (DPA), Dynamic Single Frequency Networks
(DSFN) and Packet and resource plan scheduling (PARPS).

Q- What are the main subsystems of GSM architecture?

GSM comes with a hierarchical, complex system architecture comprising many

entities, interfaces, andacronyms. A GSM system consists of three subsystems, the
radio subsystem (RSS), the network and switching subsystem (NSS), and the operation
subsystem (OSS. Generally, a GSM customer only notices a very small fraction of the
whole network – the mobile stations (MS) and some antenna masts of the base
transceiver stations (BTS).

1) Radio subsystem
As the name implies, the radio subsystem (RSS) comprises all radio specific
entities, i.e., the mobile stations (MS) and the base station subsystem (BSS).
Figure shows the connection between the RSS and the NSS via the A interface
(solid lines) and the connection to the OSS via the O interface (dashed
lines). The A interface is typically based on circuit-switched PCM-30 systems
(2.048 Mbit/s), carrying up to 30 64 kbit/s connections, whereas the O interface
uses the Signalling System No. 7 (SS7) based on X.25 carrying management data
to/from the RSS
● Base station subsystem (BSS): A GSM network comprises many BSSs, each
controlled by a base station controller (BSC). The BSS performs all functions
necessary to maintain radio connections to an MS, coding/decoding of
voice, and rate adaptation to/from the wireless network part. Besides a BSC,
the BSS contains several BTSs.
.
Functional architecture of a GSM system
● Base transceiver station (BTS): A BTS comprises all radio equipment, i.e.,
antennas, signal processing, amplifiers necessary for radio transmission. A BTS can
form a radio cell or, using sectorized antennas, several cells , and is connected to MS
via the Um interface (ISDN U interface for mobile use), and to the BSC via the Abis
interface. The Um interface contains all the mechanisms necessary for wireless
transmission (TDMA, FDMA etc.) and will be discussed in more detail below. The Abis
interface consists of 16 or 64 kbit/s connections. A GSM cell can measure between
some 100 m and 35 km depending on the environment (buildings, open space,
mountains etc.) but also expected traffic.
● Base station controller (BSC): The BSC basically manages the BTSs. It
reserves radio frequencies, handles the handover from one BTS to another
within the BSS, and performs paging of the MS. The BSC also multiplexes
the radio channels onto the fixed network connections at the A interface

● Mobile station (MS): The MS comprises all user equipment and software
needed for communication with a GSM network. An MS consists of user independent
hard- and software and of the subscriber identity module (SIM), which stores all user-
specific data that is relevant to GSM.3 While an MS can be identified via the
international mobile equipment identity (IMEI), a user can personalize any MS using
his or her SIM, i.e., user-specific mechanisms like charging and authentication are
based on the SIM, not on the device itself. Device-specific mechanisms, e.g., theft
protection, use the device specific IMEI. Without the SIM, only emergency calls are
possible. The SIM card contains many identifiers and tables, such as card-type, serial
number, a list of subscribed services, a personal identity number (PIN), a PIN
unblocking key (PUK), an authentication key (Ki), and the international mobile
subscriber identity (IMSI) (ETSI, 1991c). The PIN is used to unlock the MS. Using the
wrong PIN three times will lock the SIM. In such cases, the PUK is needed to unlock
the SIM. The MS stores dynamic information while logged onto the GSM system, such
as, e.g., the cipher key Kc and the location information consisting of a temporary
mobile subscriber identity (TMSI) and the location area identification (LAI). Typical
MSs for GSM 900 have a transmit power of up to 2 W, whereas for GSM 1800 1 W is
enough due to the smaller cell size. Apart from the telephone interface, an MS can
also offer other types of interfaces to users with display, loudspeaker, microphone, and
programmable soft keys. Further interfaces comprise computer modems, IrDA, or
Bluetooth. Typical MSs, e.g., mobile phones,comprise many more vendor-specific
functions and components, such as
cameras, fingerprint sensors, calendars, address books, games, and Internet browsers.
Personal digital assistants (PDA) with mobile phone functions are also available. The
reader should be aware that an MS could also be integrated into a car or be used for
location tracking of a container.
2) Network and switching subsystem
The “heart” of the GSM system is formed by the network and switching subsystem
(NSS). The NSS connects the wireless network with standard public
networks, performs handovers between different BSSs, comprises functions for
worldwide localization of users and supports charging, accounting, and roaming
of users between different providers in different countries. The NSS consists of
the following switches and databases:
● Mobile services switching center (MSC): MSCs are high-performance digital
ISDN switches. They set up connections to other MSCs and to the BSCs
via the A interface, and form the fixed backbone network of a GSM system.
Typically, an MSC manages several BSCs in a geographical region. A gateway
MSC (GMSC) has additional connections to other fixed networks, such as
PSTN and ISDN. Using additional interworking functions (IWF), an MSCcan also
connect to public data networks (PDN) such as X.25. An MSC
handles all signaling needed for connection setup, connection release and
handover of connections to other MSCs. The standard signaling system
No. 7 (SS7) is used for this purpose. SS7 covers all aspects of control signaling
for digital networks (reliable routing and delivery of control messages,
establishing and monitoring of calls). Features of SS7 are number portability,
free phone/toll/collect/credit calls, call forwarding, three-way calling etc. An
MSC also performs all functions needed for supplementary services such as
call forwarding, multi-party calls, reverse charging etc.
● Home location register (HLR): The HLR is the most important database in a
GSM system as it stores all user-relevant information. This comprises static
information, such as the mobile subscriber ISDN number (MSISDN), subscribed
services (e.g., call forwarding, roaming restrictions, GPRS), and the
international mobile subscriber identity (IMSI). Dynamic information is
also needed, e.g., the current location area (LA) of the MS, the mobile subscriber
roaming number (MSRN), the current VLR and MSC. As soon as an
MS leaves its current LA, the information in the HLR is updated. This information
is necessary to localize a user in the worldwide GSM network. All these user-specific
information elements only exist once for each user in a single HLR, which also
supports charging and accounting. The parameters will be explained in more detail in
section 4.1.5. HLRs can manage data for several million customers and contain highly
specialized data bases which must fulfill certain real-time requirements to answer
requests within certain time-bounds.
● Visitor location register (VLR): The VLR associated to each MSC is a dynamic
database which stores all important information needed for the MS users currently in
the LA that is associated to the MSC (e.g., IMSI, MSISDN, HLR address). If a new MS
comes into an LA the VLR is responsible for, it copies all relevant information for this
user from the HLR. This hierarchy of VLR and HLR avoids frequent HLR updates and
long-distance signaling of user information. The typical use of HLR and VLR for user
localization will be described . Some VLRs in existence, are capable of managing up to
one million customers.
3) Operation subsystem
The third part of a GSM system, the operation subsystem (OSS), contains the
necessary functions for network operation and maintenance. The OSS possesses
network entities of its own and accesses other entities via SS7 signaling (see Figure).
The following entities have been defined:
● Operation and maintenance center (OMC): The OMC monitors and controls
all other network entities via the O interface (SS7 with X.25). Typical OMC
management functions are traffic monitoring, status reports of network entities,
subscriber and security management, or accounting and billing. OMCs use the concept
of telecommunication management network (TMN) as standardized by the ITU-T.
● Authentication centre (AuC): As the radio interface and mobile stations are
particularly vulnerable, a separate AuC has been defined to protect user identity and
data transmission. The AuC contains the algorithms for authentication as well as the
keys for encryption and generates the values needed for user authentication in the
HLR. The AuC may, in fact, be situated in a special protected part of the HLR.
● Equipment identity register (EIR): The EIR is a database for all IMEIs, i.e.,
it stores all device identifications registered for this network. As MSs are mobile, they
can be easily stolen. With a valid SIM, anyone could use the stolen MS. The EIR has a
blacklist of stolen (or locked) devices. In theory an MS is useless as soon as the owner
has reported a theft. Unfortunately, the blacklists of different providers are not usually
synchronized and the illegal use of a device in another operator’s network is possible
(the reader may speculate as to why this is the case). The EIR also contains a list of
valid IMEIs (white Slist), and a list of malfunctioning devices (gray list).
 Mobile Computing Assignment

Name :-Vedprakash Bisen

Reg No:- 2013MIT007

Q.1 Distinguished Between 3G And 4G Cellular Network?

 Difference between 3G and 4G:

3G 4G

Data Throughput: Up to 3.1mbps Practically speaking, 3 to 5 mbps but

potential estimated at a range of 100 to 300
mbps.

Peak Upload Rate: 50 Mbit/s 500 Mbit/s

Peak Download Rate: 100 Mbit/s 1 Gbit/s

Switching Technique: packet switching packet switching, message switching

Network Architecture: Wide Area Cell Based Integration of wireless LAN and Wide
area.

Services And CDMA 2000, UMTS, EDGE etc Wimax2 and LTE-Advance
Applications:

Forward error 3G uses Turbo codes for error correction. Concatenated codes are used for error
correction (FEC): corrections in 4G.

Frequency Band: 1.8 – 2.5GHz 2 – 8GHz

Bandwidth 5 to 20 MHz 100 MHz or More

 Q2.What are frequencies are used in Forward and Reverse Link Frequency in GSM?

Ans:-

 Forward And Reverse Frequencies in GSM

 Introduction:-

GSM is an acronym that stands for Global System for Mobile Communications

GSM is now an international standard for mobile service. It offers high mobility. Subscribers can easily
roam worldwide and access any GSM network.

GSM offers a number of services including voice communications, Short Message Service (SMS), fax,
voice mail, and other supplemental services such as call forwarding and caller ID

Currently there are several bands in use in GSM. 450 MHz, 850 MHZ, 900 MHz, 1800 MHz, and 1900
MHz are the most common one

 Uplinks/Downlinks & Reverse/Forward:-

GSM allows for use of duplex operation. Each band has a frequency range for the uplink (cell phone to
tower) and a separate range for the downlink (tower to the cell phone). The uplink is also known as
the Reverse and the downlink is also known as the Forward

 Forward link: the link from the base station to the handset.
 Reverse link: the link from the handset to the base station.

Fig: reverse and forward links

 Uplink Frequency(Reverse):-

 In a GSM network, the term uplink frequency is used for a band (or group) of frequencies
dedicated for transmitting data from mobile units (or cell phones) to the BTS towers. The uplink
frequency in a GSM network generally lies between a range of 890 and 915 megahertz (MHz), making it
a 25-MHz band. This band contains multiple frequencies from 890.1 MHz to 914.9 MHz, allotted rapidly
to different users to facilitate maximum number of mobile units under a single BTS.
  Downlink Frequency(Forward):-
 The downlink frequency in a GSM network refers to the transmitting frequency from BTS
antenna to a mobile unit on ground. This frequency lies within the range of 935 to 960 MHz, which
makes it a 25-MHz frequency band. In similarity to uplink frequency scenario, the downlink frequency
band also contains multiple frequencies (ranging from 935.1 MHz to 959.9 MHz) to facilitate multiple
mobile units on ground.

 Uplink/Downlink Band Gap:-

 A significant band gap of 20 MHz (such as 915 to 935 MHz) between uplink and downlink
frequencies is made standard in GSM operations to avoid any interference between uplink and downlink
data transmission sessions. Furthermore, this also makes it possible for BTS to allot unique frequencies to
different roaming users in a standardized manner.
 Aniket B. mandalkar (2013MIT009)

Mobile Computing and M-Commerce

SGGS IE&T, Nanded

-----------------------------------------------------------------------------------------------------------------------------

Q.1 Define location management and handoff management.

Ans: location management-

In a cellular network, a service coverage area is divided into smaller areas of hexagonal shape, referred
to as cells. Each cell is served by a base station. The base station is fixed. It is able to communicate with
mobile stations such as cellular phones using its radio transceiver. The base station is connected to the
mobile switching center (MSC), which is, in turn, connected to the public switched telephone network
(PSTN).

Fig. illustrates a typical cellular network. (A base station is marked with a triangle.)

Location management deals with how to keep track of an active mobile station within the cellular
network. A mobile station is active if it is powered on. Since the exact location of a mobile station must
be known to the network during a call, location management usually means how to track an active
mobile station between two consecutive phone calls.

The continued growth of wireless communication systems, and expansion of network subscription rates,
signals increased demand for the efficient location management. A cellular communication system must
track the location of its users in order to forward calls to the relevant cell within a network. Cells within
a network are grouped into Location Areas (LAs). Users are free to move with a given location area
without updating their location, informing the network only when transitioning to a new LA. If a call is to
be forwarded to a user, the network must now page every cell within the location area to determine
their precise location. Network cost is incurred on location updates and paging, the balance of these
defining the field of Location Management (LM). So there are two basic operations involved with
location management:
• Location Updating: Informing the network of a devices location.
• Paging: Polling a group of cells to determine the precise location of a device

There are two basic operations involved with location management: location update and paging.

The paging operation is performed by the cellular network. When an incoming call arrives for a mobile
station, the cellular network will page the mobile station in all possible cells to find out the cell in which
the mobile station is located so the incoming call can be routed to the corresponding base station. This
process is called paging. The number of all possible cells to be paged is dependent on how the location
update operation is performed.

The location update operation is performed by an active mobile station. A location update scheme can
be classified as either global or local. A location update scheme is global if all subscribers update their
locations at the same set of cells, and a scheme is local if an individual subscriber is allowed to decide
when and where to perform location update. A local scheme is also called individualized or per-user
based.

From another point of view, a location update scheme can be classified as either static or dynamic. A
location update scheme is static if there is a predetermined set of cells at which location updates must
be generated by a mobile station regardless of it mobility. A scheme is dynamic if a location update can
be generated by a mobile station in any cell depending on its mobility. A global scheme is based on
aggregate statistics and traffic patterns, and it is usually static too. Location areas in and reporting
centers are two examples of global static schemes. A global scheme can be dynamic. For example, the
time varying location areas scheme is both global and dynamic. A per-user based scheme is based on the
statistics and/or mobility patterns of an individual subscriber, and it is usually dynamic. The time-based,
movement based and distance based schemes are three excellent examples of individualized dynamic
schemes. An individualized scheme is not necessary dynamic. For example, the individualized location
areas scheme is both individualized and static.

Location management involves signaling in both the wireline portion and the wireless portion of the
cellular network. However, most researchers only consider signaling in the wireless portion due to the
fact that the radio frequency bandwidth is limited while the bandwidth of the wireline network is always
expandable. This chapter will only discuss signaling in the wireless portion of the network. Location
update involves reverse control channels while paging involves forward control channels. The total
location management cost is the sum of the location update cost and the paging cost. There is a trade-
off between the location update cost and the paging cost. If a mobile station updates its location more
frequently (incurring higher location update cost), the network knows the location of the mobile station
better. Then the paging cost will be lower when an incoming call arrives for the mobile station.
Therefore both location update and paging costs can not be minimized at the same time. However, the
total cost can be minimized or one cost can be minimized by putting a bound on the other cost. For
example, many researchers try to minimize the location update cost subject to a constraint on the
paging cost. The cost of paging a mobile station over a set of cells or location areas has been
studied against the paging delay . There is a trade-off between the paging cost and the paging delay. If
there is no delay constraint, the cells can paged sequentially in order of decreasing probability, which
will result in the minimal paging cost. If all cells are paged simultaneously, the paging cost reaches the
maximum while the paging delay is the minimum. many researchers try to minimize the paging cost
under delay constraints.
Handoff Management –
Mobility is the most important feature of a wireless cellular communication system. Usually, continuous
service is achieved by supporting handoff (or handover) from one cell to another. Handoff is the process
of changing the channel (frequency, time slot, spreading code, or combination of them) associated with
the current connection while a call is in progress. It is often initiated either by crossing a cell boundary or
by a deterioration in quality of the signal in the current channel. Handoff is divided into two broad
categories— hard and soft handoffs. They are also characterized by “break before make” and “make
before break.”

In hard handoffs, current resources are released before new resources are used; in soft handoffs, both
existing and new resources are used during the handoff process. Poorly designed handoff schemes tend
to generate very heavy signaling traffic and, thereby, a dramatic decrease in quality of service (QoS). (In
this chapter, a handoff is assumed to occur only at the cell boundary.) The reason why handoffs are
critical in cellular communication systems is that neighboring cells are always using a disjoint subset of
frequency band , so negotiations must take place between the mobile station (MS), the current serving
base station (BS), and the next potential BS. Other related issues, such as decision making and priority
strategies during overloading, might influence the overall performance.

TYPES OF HANDOFFS
Handoffs are broadly classified into two categories—hard and soft handoffs. Usually, the hard handoff
can be further divided into two different types—intra- and intercellular handoffs. The soft handoff can
also be divided into two different types—multiway soft handoffs and softer handoffs.

Fig. Handoff between MS and BSs

A hard handoff is essentially a “break before make” connection. Under the control of the MSC, the BS
hands off the MS’s call to another cell and then drop the call. In a hard handoff, the link to the prior BS is
terminated before or as the user is transferred to the new cell’s BS; the MS is linked to no more than
one BS at any given time. Hard handoff is primarily used in FDMA (frequency division multiple access)
and TDMA (time division multiple access), where different frequency ranges are used in adjacent
channels in order to minimize channel interference. So when the MS moves from one BS to another BS,
it becomes impossible for it to communicate with both BSs (since different frequencies are used).

HANDOFF DECISION
There are numerous methods for performing handoff, at least as many as the kinds of state information
that have been defined for MSs, as well as the kinds of network entities that maintain the state
information. The decision-making process of handoff may be centralized or decentralized (i.e., the
handoff decision may be made at the MS or network). From the decision process point of view, one can
find at least three different kinds of handoff decisions.

Network-Controlled Handoff
In a network-controlled handoff protocol, the network makes a handoff decision based on the
measurements of the MSs at a number of BSs. In general, the handoff process (including data
transmission, channel switching, and network switching) takes 100–200 ms. Information about the
signal quality for all users is available at a single point in the network that facilitates appropriate
resource allocation. Network-controlled handoff is used in first-generation analog systems such as AMPS
(advanced mobile phone system), TACS (total access communication system), and NMT (advanced
mobile phone system).

Mobile-Assisted Handoff
In a mobile-assisted handoff process, the MS makes measurements and the network makes the
decision. In the circuit-switched GSM (global system mobile), the BS controller (BSC) is in charge of the
radio interface management. This mainly means allocation and release of radio channels and handoff
management. The handoff time between handoff decision and execution in such a circuit-switched GSM
is approximately 1 second.

Mobile-Controlled Handoff
In mobile-controlled handoff, each MS is completely in control of the handoff process. This type of
handoff has a short reaction time (on the order of 0.1 second). MS measures the signal strengths from
surrounding BSs and interference levels on all channels. A handoff can be initiated if the signal strength
of the serving BS is lower than that of another BS by a certain threshold.

Q.2 Explain the principle of frequency division multiple access.

Ans:
 FDMA is one of the earliest multiple-access techniques for cellular systems when continuous
transmission is required for analog services. In this technique the bandwidth is divided into a number of
channels and distributed among users with a finite portion of bandwidth for permanent use as
illustrated in fig. The vertical axis that represents the code is shown here just to make a clear
comparison with CDMA (discussed later in this chapter). The channels are assigned only when
demanded by the users. Therefore when a channel is not in use it becomes a wasted resource. FDMA
channels have narrow bandwidth (30Khz) and therefore they are usually implemented in narrowband
systems. Since the user has his portion of the bandwidth all the time, FDMA does not require
synchronization or timing control, which makes it algorithmically simple. Even though no two users use
the same frequency band at the same time, guard bands are introduced between frequency bands to
minimize adjacent channel interference. Guard bands are unused frequency slots that separate
neighboring channels. This leads to a waste of bandwidth. When continuous transmission is not
required, bandwidth goes wasted since it is not being utilized for a portion of the time. In wireless
communications, FDMA achieves simultaneous transmission and reception by using Frequency division
duplexing (FDD). In order for both the transmitter and the receiver to operate at the same time, FDD
requires duplexers. The requirement of duplexers in the FDMA system makes it expensive.
Q. What is cell splitting? What is cell sectoring?
Answer:-
1. Cell Splitting :-

As the density of subscribers to a cellular system grows, there comes a time where the system
capacity is reached. At that point you cannot continue to add new customers without degrading
system performance for all your customers. You could simply refuse to add new users, but
that’s not a very good business plan. Another idea would be to add new channels. Assuming
you can’t get additional spectrum from the FCC, this would require new technology to squeeze
more voice channels into the same bandwidth
- Cell splitting is the process of subdividing a congested cellinto smaller cells.
- Each with its own base station and a correspondingreduction in antenna height and
transmitter power.
- Cell splitting increases the capacity of a cellular systemsince it increases the number of
times that channels arereused.
- By defining new cells which have a smaller radius than the original cells and by installing
these smaller cells (calledmicrocells) between the existing cells,
- Capacity increases due to the additional number of channelsper unit area.

Consider following example to understand cell splitting,

There are 100 people in a specific area. All of them owns a mobile phone (MS) and are quite
comfortable to communicate with each other. So, a provision for all of them to mutually
communicate must be made. As there are only 100 users, a single base station (BS) is built in
the middle of the area and all these users’ MS are connected to it. All these 100 users now
come under the coverage area of a single base station. This coverage area is called a cell. This s
shown in Fig 1

Fig 1. A single BS for 100 MS users

But now, as time passed by, the number of mobile users in the same area increased from 100
to 700. Now if the same BS has to connect to these 700 users’ MS, obviously the BS will be
overloaded. A single BS, which served for 100 users is forced to serve for 700 users, which is
impractical. To reduce the load of this BS, we can use cell splitting. That is, we will divide the
above single cell into 7 separate adjacent cells, each having its own BS. This is shown as

Now, let us look into the big picture. Until now, we have discussed about cell splitting in a
small area. Now, we use this same concept to deal with large networks. In a large network, it is
not necessary to split up all the cells in all the clusters. Certain BSs can handle the traffic well if
their cells (coverage areas) are split up. Only those cells must be ideal for cell splitting. Fig 3
shows network architecture with a few number of cells split up into smaller cells, without
affecting the other cells in the network.

Fig 3. Cell Splitting.

The concept of cell splitting can further be applied to the split cells as well. That is, the split up
cells can further be split into a number of smaller cells to improve the efficiency of the BS even
more. Fig 4 shows a hierarchy of cell splitting.

Here, the master cells which have been split up into smaller cells are known as macro cells.
The split up cells are known as micro cells. The innermost cells, split up by splitting the micro
cells are known as pico cells
2. Cell Sectoring :-

Sectoring is another way to increase capacity. In sectoring, a cell has the same coverage
space but instead of using a single omni-directional antenna that transmits in all directions,
either three or six directional antennas are used and each with beam-width of about 120° or
60° as shown

- Sectoring is another way to increase capacity is to keep the cell radius unchanged.
- It is the technique for decreasing co-channel interference and thus increasing system
capacity by replacing a single omnidirectional antenna at the base station by several
directional antennas, each radiating within a specified sector and transmit with only a
fraction of the available co-channel cells.
- Each sector uses a directional antenna at the BS and isassignedasetofchannels
- The factor by which the co-channel interference is reduced depends on the amount of
sectoring used.
- Sectoring uses directional antennas further
controltheinterference andfrequencyreuseofchannels.
- The number of channels in each sector is the number
of channelsinacelldividedbythenumberofsectors..
- TheamountofCCIisalsoreducedbythenumber ofsectors.
 When sectoring is employed, the channels allocated to a particular cell are divided among the
different sectors. It is done in such a way that channels assigned to a particular sector are always
at the same direction in the different cells.

For example, group A of channels assigned to the sector S2, group B of channels are assigned
to the sector S1 at the top of all cells, and so on. Each sector causes interference to the cells that
are in its transmission angle only. Unlike the case of no sectoring where six interfering co-
channel cells from the first-tier co-channels cells cause interference, with 120° sectoring, two or
three co-channel cells cause interference and with 60° sectoring, one or two co-channel cells
cause interference. Fig.2 shows sectoring for a four-cell pattern.
PROS AND CONS:-
1. Much less costly than cell splitting, only require more antennas at base station.
2. Primary disadvantage is that the available channels in a cell are subdivided into sectored
groups.
3. There are more channels per cell, because of smaller cluster sizes, but those channels are
broken into sectors.
Name: Mr. Swapnil Khobragade

Reg. No. : 2013MIT011

Que. What is trunking Effect??

Trunking
Allow a large number of users to share the relatively small number of channels in a cell by
providing access to each user, on demand, from a pool of available channels. Exploit the
statistical behavior of users. Each user is allocated a channel on a per call basis, and upon
termination of the call, the previously occupied channel is immediately returned to the pool of
available channels.
To design trunked radio systems that can handle a specific capacity at a specific \grade of
service," it is essential to understand trunking theory and queuing theory. The fundamentals of
trunking theory were developed by Erlang, a Danish mathematician who, in the late 19th century
Today, the measure of traffic intensity bears his name.

Definitions of Common Terms Used in Trunking Theory

Blocked Call: Call which cannot be completed at time of request, due to congestion, Also
referred to as a lost call.

Holding Time: Average duration of a typical call, Denoted by H = 1=_.

Traffic Intensity: Measure of channel time utilization, which is the average channel occupancy
measured in Erlangs.
This is a dimensionless quantity and may be used to measure the time utilization of single or
multiple channels.
Denoted by A.

Load: Traffic intensity across the entire trunked radio system, measured in Erlangs.
Grade of Service (GOS): A measure of congestion which is specified as the probability of a call
being blocked (for Erlang B).
The AMPS cellular system is designed for a GOS of 2% blocking. This implies that the channel
allocations for cell sites are designed so that 2 out of 100 calls will be blocked due to channel
occupancy during the busiest hour.
Request Rate: The average number of call requests per unit time. Denoted by _.

There are two types of trunked systems which are commonly used.
(a) Blocked calls cleared.
(b) Blocked calls delayed

M/M/m/m Queue Assumption: We will assume blocked calls cleared trunking with several
further assumptions.
Blocked calls cleared
Offers no queuing for call requests,
For every user who requests service, it is assumed there is no setup time and the user is given
immediate access to a channel if one is available.
If no channels are available, the requesting user is blocked without access and is free to try again
later.

Call arrives as determined by a Poisson distribution.

There is memory less arrivals of requests, implying that all users, including blocked users, may
request a channel at any time.

There are an infinite number of users (with infinite overall request rate).
Remark: While it is possible to model trunked systems with infinite users, the resulting
expressions are much more complicated than the Erlang B result below.
Furthermore, the Erlang B formula provides a conservative estimate of the GOS, as the infinite
user results always predict a smaller likelihood of blocking.

The duration of the time that a user occupies a channel is exponentially distributed, so that longer
calls are less likely to occur.
There are m channels available in the trunking pool.

5.6. Erlang B formula:

C = the number of trunked channels offered by a trunked radio system

A = the total offered traffic.
The probability that a call is blocked is
One Erlang represents the amount of traffic intensity carried by a channel that is completely
occupied
Que. Explain in details Fadding effect in wireless channel

Before frequency modulation, syllabic compounding, pre-emphasis, clipping, and band pass
filtering are used to enhance the voice signal quality

syllabic compandor After frequency discrimination, band pass filtering is first used to eliminate
the random phase effect of the narrowband fading channel. Then de-emphasis and syllabic
expanding are used to recover the original voice signal FM is adopted in the first generation
cellular system because itis simple and it is effective in combating the narrowband fading effect.

Slow versus fast fading

The terms slow and fast fading refer to the rate at which the magnitude and phase change
imposed by the channel on the signal changes. The coherence time is a measure of the minimum
time required for the magnitude change of the channel to become uncorrelated from its previous
value.

 Slow fading arises when the coherence time of the channel is large relative to the delay
constraint of the channel. In this regime, the amplitude and phase change imposed by the
channel can be considered roughly constant over the period of use. Slow fading can be
caused by events such as shadowing, where a large obstruction such as a hill or large
building obscures the main signal path between the transmitter and the receiver. The
received power change caused by shadowing is often modeled using a log-normal
distribution with a standard deviation according to the log-distance path loss model

 Fast fading occurs when the coherence time of the channel is small relative to the delay
constraint of the channel. In this regime, the amplitude and phase change imposed by the
channel varies considerably over the period of use.

In a fast-fading channel, the transmitter may take advantage of the variations in the channel
conditions using time diversity to help increase robustness of the communication to a temporary
deep fade. Although a deep fade may temporarily erase some of the information transmitted, use
of an error-correcting code coupled with successfully transmitted bits during other time instances
(interleaving) can allow for the erased bits to be recovered. In a slow-fading channel, it is not
possible to use time diversity because the transmitter sees only a single realization of the channel
within its delay constraint. A deep fade therefore lasts the entire duration of transmission and
cannot be mitigated using coding.

The coherence time of the channel is related to a quantity known as the Doppler spread of the
channel. When a user (or reflectors in its environment) is moving, the user's velocity causes a
shift in the frequency of the signal transmitted along each signal path. This phenomenon is
known as the Doppler shift. Signals traveling along different paths can have different Doppler
shifts, corresponding to different rates of change in phase. The difference in Doppler shifts
between different signal components contributing to a single fading channel tap is known as the
Doppler spread. Channels with a large Doppler spread have signal components that are each
changing independently in phase over time. Since fading depends on whether signal components
add constructively or destructively, such channels have a very short coherence time.

In general, coherence time is inversely related to Doppler spread, typically expressed as

where is the coherence time, is the Doppler spread. This equation is just an
[1]
approximation, to be exact, see Coherence time.

Selective fading

Selective fading or frequency selective fading is a radio propagation anomaly caused by partial
cancellation of a radio signal by itself — the signal arrives at the receiver by two different paths,
and at least one of the paths is changing (lengthening or shortening). This typically happens in
the early evening or early morning as the various layers in the ionosphere move, separate, and
combine. The two paths can both be skywave or one be groundwave.

Selective fading manifests as a slow, cyclic disturbance; the cancellation effect, or "null", is
deepest at one particular frequency, which changes constantly, sweeping through the received
audio.

As the carrier frequency of a signal is varied, the magnitude of the change in amplitude will vary.
The coherence bandwidth measures the separation in frequency after which two signals will
experience uncorrelated fading.

 In flat fading, the coherence bandwidth of the channel is larger than the bandwidth of the
signal. Therefore, all frequency components of the signal will experience the same
magnitude of fading.
 In frequency-selective fading, the coherence bandwidth of the channel is smaller than
the bandwidth of the signal. Different frequency components of the signal therefore
experience uncorrelated fading.

Since different frequency components of the signal are affected independently, it is highly
unlikely that all parts of the signal will be simultaneously affected by a deep fade. Certain
modulation schemes such as orthogonal frequency-division multiplexing (OFDM) and code
division multiple access (CDMA) are well-suited to employing frequency diversity to provide
robustness to fading. OFDM divides the wideband signal into many slowly modulated
narrowband subcarriers, each exposed to flat fading rather than frequency selective fading. This
can be combated by means of error coding, simple equalization or adaptive bit loading. Inter-
symbol interference is avoided by introducing a guard interval between the symbols. CDMA
uses the rake receiver to deal with each echo separately.
Frequency-selective fading channels are also dispersive, in that the signal energy associated with
each symbol is spread out in time. This causes transmitted symbols that are adjacent in time to
interfere with each other. Equalizers are often deployed in such channels to compensate for the
effects of the intersymbol interference.

The echoes may also be exposed to Doppler shift, resulting in a time varying channel model.

The effect can be counteracted by applying some diversity scheme, for example OFDM (with
subcarrier interleaving and forward error correction), or by using two receivers with separate
antennas spaced a quarter-wavelength apart, or a specially-designed diversity receiver with two
antennas. Such a receiver continuously compares the signals arriving at the two antennas and
presents the better signal.

Fading models

Examples of fading models for the distribution of the attenuation are:

 Dispersive fading models, with several echoes, each exposed to different delay, gain and
phase shift, often constant. This results in frequency selective fading and inter-symbol
interference. The gains may be Rayleigh or Rician distributed. The echoes may also be
exposed to Doppler shift, resulting in a time varying channel model.
 Nakagami fading
 Log-normal shadow fading
 Rayleigh fading
 Rician fading
 Weibull fading
Q- What are different types of handover

In cellular telecommunications, the term handover or handoff refers to the process of transferring an
ongoing call or data session from one channel connected to the core network to another. In satellite
communications it is the process of transferring satellite control responsibility from one earth station to
another without loss or interruption of service.

Main function of handover

In telecommunications there may be different reasons why a handover might be conducted:

 when the phone is moving away from the area covered by one cell and entering the area covered
by another cell the call is transferred to the second cell in order to avoid call termination when the
phone gets outside the range of the first cell;
 when the capacity for connecting new calls of a given cell is used up and an existing or new call
from a phone, which is located in an area overlapped by another cell, is transferred to that cell in
order to free-up some capacity in the first cell for other users, who can only be connected to that cell;
 in non-CDMA networks when the channel used by the phone becomes interfered by another
phone using the same channel in a different cell, the call is transferred to a different channel in the
same cell or to a different channel in another cell in order to avoid the interference;
 again in non-CDMA networks when the user behaviour changes, e.g. when a fast-travelling user,
connected to a large, umbrella-type of cell, stops then the call may be transferred to a smaller macro
cell or even to a micro cell in order to free capacity on the umbrella cell for other fast-traveling users
and to reduce the potential interference to other cells or users (this works in reverse too, when a user
is detected to be moving faster than a certain threshold, the call can be transferred to a larger
umbrella-type of cell in order to minimize the frequency of the handovers due to this movement);
 in CDMA networks a handover (see further down) may be induced in order to reduce the
interference to a smaller neighboring cell due to the "near-far" effect even when the phone still has an
excellent connection to its current cell;
The most basic form of handover is when a phone call in progress is redirected from its
current cell (called source) to a new cell (called target). In terrestrial networks the source and the target
cells may be served from two different cell sites or from one and the same cell site (in the latter case the
two cells are usually referred to as two sectors on that cell site). Such a handover, in which the source
and the target are different cells (even if they are on the same cell site) is called inter-cell handover. The
purpose of inter-cell handover is to maintain the call as the subscriber is moving out of the area covered
by the source cell and entering the area of the target cell.
A special case is possible, in which the source and the target are one and the same cell and only the
used channel is changed during the handover. Such a handover, in which the cell is not changed, is
called intra-cell handover. The purpose of intra-cell handover is to change one channel, which may be
interfered or fading with a new clearer or less fading channel.

Types of handover

 A hard handover is one in which the channel in the source cell is released and only then the
channel in the target cell is engaged. Thus the connection to the source is broken before or 'as' the
connection to the target is made—for this reason such handovers are also known as break-before-
make. Hard handovers are intended to be instantaneous in order to minimize the disruption to the
call. A hard handover is perceived by network engineers as an event during the call. It requires the
 least processing by the network providing service. When the mobile is between base stations, then
the mobile can switch with any of the base stations, so the base stations bounce the link with the
mobile back and forth. This is called ping-ponging.

 A soft handover is one in which the channel in the source cell is retained and used for a while in
parallel with the channel in the target cell. In this case the connection to the target is established
before the connection to the source is broken, hence this handover is called make-before-break. The
interval, during which the two connections are used in parallel, may be brief or substantial. For this
reason the soft handover is perceived by network engineers as a state of the call, rather than a brief
event. Soft handovers may involve using connections to more than two cells: connections to three,
four or more cells can be maintained by one phone at the same time. When a call is in a state of soft
handover, the signal of the best of all used channels can be used for the call at a given moment or all
the signals can be combined to produce a clearer copy of the signal.
Q. Explain the channels in GSM.

. GSM uses a mix of Frequency Division Multiple Access (FDMA) and Time Division Multiple
Access (TDMA). FDMA parts involves the division by frequency of the 25 MHz bandwidth in to
124 carrier frequencies (Also called ARFCN) spaced 200 KHz for GSM-900. For GSM-1800
frequency spectrum of 75 MHz bandwidth is divided in to 374 carrier frequencies spaced 200
KHz. TDMA further divides each carrier frequencies in to 8 time slots such that each carrier
frequency is shared by 8 users. So in GSM, the basic radio resource is a time slot with duration
of 577 µs. 8 Time slots of 577 µs constitutes a 4.615 ms TDMA Frame. GSM uses Gaussian
Minimum Shift Keying (GMSK) modulation scheme to transmit information (data and signaling)
over Air Interface.

GSM uses number of channels to carry data over Air Interface; these channels are broadly
divided in to following two categories:

1. Physical Channels
2. Logical Channels

PHYSICAL CHANNELS

A physical channel is determined by the carrier frequency or a number of carrier frequencies

with defined hopping sequence and the Time Slot number.

8 Time Slots (1 Time Slot = 1 Physical Channel) of 577 µs constitutes a 4.615 ms TDMA Frame.
In GSM standard data on a time slot transmitted in bursts, so time slot is often expressed in BP
(Burst Period). 1 BP represents 1 TS. TDMA frame (4.615 ms of 8 TS) further structured in to
multiframes. There are two types of multiframes in the system:

 26 TDMA Multiframe: Consists 26 TDMA frames with duration of 120 ms and used to
carry the Logical Channels TCH, SACCH, FACCH etc.
  51 TDMA Multiframe: Consists 51 TDMA frames with duration of 234.5 ms and used to
carry the Logical Channels FCCH, SCH, BCCH, CCCH, SDCCH, SACCH etc.

These multiframes further structured in to Superframe and Hyperframe.

SUPERFRAME: Superframe consists of 51*26 TDMA frames with duration of 6.12 sec. This
is corresponding to the smallest cycle for which the organization of all channels is repeated.

HYPERFRAME: Hyperframe consists 2048 superframes (2048*51*26 TDMA frames) with

duration of 3 hrs, 28 min, 53 sec and 760 ms. It is in particular smallest cycle for frequency
hopping, cyphering.

The frame hierarchy is used for synchronization between BTS and MS.

Multiframes in GSM

LOGICAL CHANNELS

Logical Channels are determined by the information carried within the physical channel. Logical
channels used to carry data and signaling information. Different logical channels are mapped in
either direction on physical channels.

Logical channels divided in to following two categories:

 Traffic Channels
 Signaling Channels
TRAFFIC CHANNELS(TCH): GSM uses a TCH to transmit user data (e.g., voice,fax). Two
basic categories of TCHs have been defined, i.e., full-rate TCH(TCH/F) and half-rate TCH
(TCH/H).

 Full Rate Traffic Channels (TCH\F): This channel carries information at rate of 22.8
Kbps.
 Half Rate Traffic Channels (TCH\H): This channel carries information at rate of 11.4
Kbps.

With the voice codecs available at the beginning of the GSM standardization, 13 Kbit/s were
required, whereas the remaining capacity of the TCH/F (22.8 Kbit/s) was used for error
correction (TCH/FS). Improved codes allow for better voice coding and can use aTCH/H. Using
these TCH/HSs double the capacity of the GSM system for voicetransmission. However, speech
quality decreases with the use of TCH/HS and manyproviders try to avoid using them. The
standard codecs for voice are called full rate (FR,13 Kbit/s) and half rate (HR, 5.6 Kbit/s). A
newer codec, enhanced full rate (EFR), provides better voice quality than FR as long as the
transmission error rate is low. The generated data rate is only 12.2 Kbit/s. New codecs, which
automatically choose the best mode of operation depending on the error rate (AMR, adaptive
multi-rate), will be used together with 3G systems. An additional increase in voice quality is
provided by the so-called tandem free operation (TFO). This mode can be used if two MSs
exchange voice data. In this case, coding to and from PCM encoded voice (standard in ISDN)
can be skipped and the GSM encoded voice data is directly exchanged. Data transmission in
GSM is possible at many different data rates, e.g., TCH/F4.8 for 4.8 Kbit/s, TCH/F9.6 for 9.6
Kbit/s, and, as a newer specification, TCH/F14.4 for 14.4 Kbit/s. These logical channels differ in
terms of their coding schemes and error correction capabilities.

SIGNALLING CHANNELS

Signaling channel carries control information to enable the system to operate correctly. There are
three main categories of signaling channels in GSM which are further divided in several
categories:

1. BROADCAST CHANNELS (BCH)

 Broadcast Control Channel (BCCH)

 Frequency Correction Channel (FCCH)
  Synchronization Channel (SCH)
 Cell Broadcast Channel (CBCH)

BROADCAST CONTROL CHANNEL (BCCH) – DOWNLINK

 Broadcasts Network and Cell specific information required to identify the network and
gain access.
 Broadcast parameters include Location Area Code (LAC), Mobile Network Code
(MNC), Control Channel Structures, BCCH frequencies of neighboring Cells and other
access parameters.

FREQUENCY CORRECTION CHANNEL (FCCH) – DOWNLINK

 This channel contains frequency correction bursts, used by the mobiles for frequency
correction.
 Bears information for frequency Synchronization.

SYNCHRONIZATION CHANNEL (SCH) – DOWNLINK

 This channel is used by the MS to learn the Base Station Information Code (BSIC) as
well as the TDMA frame number (FN).
 6 bits of BSIC having two parts. 3 bits NCC and 3 bits BCC. NCC stands for Network
Colour Code and used to identify the BTS for which measurement is made. BCC stands
for Base-Station Colour Code and used for a better transmission in case of interference.
 BICS avoids ambiguity or interference which can arise when a Mobile Station can
receive SCH from two cells using the same BCCH frequency.

CELL BROADCAST CONTROL CHANNEL (CBCH) – DOWNLINK

 This channel is used to broadcast specific information to network subscribers; such as

weather, traffic, sports, stocks and other public services and announcement.
 This channel is assigned with SDCCH and usually occupies the second subslot of the
SDCCH.

2. COMMON CONTROL CHANNELS (CCCH)

 Paging Channel (PCH)

 Random Access Channel (RACH)
 Access Grant Channel (AGCH)

PAGING CHANNEL (PCH) – DOWNLINK

 This channel is used for alerting to Mobile Subscribers for incoming calls, SMS and other
mobility services.
 Every MS in a cell periodically listen to this channel.
RANDOM ACCESS CHANNEL (RACH) – UPLINK

 This channel is used by a MS seeking attention of the BTS. When MS wants to initiate
dialogue with network, this channel is used to send request to the network for a dedicated
resource.
 The real dialogue between MS and Network will take place on the dedicated channel.
 If the request is not granted within a specific time period by the network, the MS repeats
the request on the RACCH.

ACCESS GRANT CHANNEL (AGCH) – DOWNLINK

 This channel is used by a BTS to notify the MS of the assignment of an initial SDCCH
for initial signaling.
 In response to request from MS on RACH, the network allocates a specific dedicated
signaling channel (SDCCH) for further communication. This response is sent on AGCH.

3. DEDICATED CONTROL CHANNELS (DCCH)

 Standalone Dedicated Control Channel (SDCCH)

 Fast Associated Control Channel (FACCH)
 Slow Associated Control Channel (SACCH)

STAND-ALONE DEDICATED CONTROL CHANNEL (SDCCH) –

UPLINK/DOWNLINK

 In response of RACCH, network allocates SDCCH over AGCH for further

communication between MS and BTS.
 This channel is used for the Location Update, Voice Call Set up and SMS.

FAST ASSOCIATED CONTROL CHANNEL (FACCH) – UPLINK/DOWNLINK

 This channel is used to convey Handover information.

 There is no TS and frame allocation dedicated to this channel. This channel can be
associated with SDCCH or TCH and works on the principle of stealing. The burst of
TCH is replaced by FACCH signaling when required.

SLOW ASSOCIATED CONTROL CHANNEL (SACCH) – UPLINK/DOWNLINK

 This channel is always associated with TCH or SDDCH used for control and supervision
of signals associated with the traffic channels.
 Used to convey the periodic carrier-signal strength measurements to the network transmit
power control and timing advance.
 Name: Sapna Buddhapal Kamble
Reg No.: 2013MIT013
1) What is the necessity of Standards?

ans:-
Rather than asking why we need standards, we might usefully ask ourselves what the world
would be like without standards. Products might not work as expected. They may be of inferior
quality and incompatible with other equipment, in fact they may not even connect with them, and
in extreme cases; non-standardized products may even be dangerous. Standardized products and
services are valuable User 'confidence builders', being perceived as:

 safe
 healthy
 secure
 high quality
 flexible
As a result, standardized goods and services are widely accepted, commonly trusted and
highly valued. Standards provide the foundation for many of the innovative communication
features and options we have come to take for granted, and they contribute to the enhancement of
our daily lives - often invisibly. We need look no further for evidence than the GSM™ standard
which facilitates mobile communication the world over between (for example): friends and
relations ,hospitals ,business ,schools ,industry, emergency services, airports ,governments.
Standardization brings important benefits to business including a solid foundation upon which to
develop new technologies and an opportunity to share and enhance existing practices.
Standardization also plays a pivotal role in assisting Governments, Administrations, Regulators
and the legal profession as legislation, regulation and policy initiatives are all supported by
standardization.

The necessity of various standards is described below

1G systems These are the analog systems such as AMPS that grew rapidly in the 1980s
and are still available today. Many metropolitan areas have a mix of 1G and 2G systems,
as well as emerging 3G systems. The systems use frequency division multiplexing to
divide the bandwidth into specific frequencies that are assigned to individual calls.

AMPS(Advance mobile phone service)

 AMPS is a first-generation cellular technology that uses separate frequencies, or

"channels", for each conversation .It therefore required considerable bandwidth for
a large number of users.
 AMPS was very similar to the older "0G" Improved Mobile Telephone Service, but
used considerably more computing power in order to select frequencies, hand off
conversations to PSTN lines, and handle billing and call setup.
  What really separated AMPS from older systems is the "back end" call setup
functionality. In AMPS, the cell centers could flexibly assign channels to handsets
based on signal strength, allowing the same frequency to be re-used in various
locations without interference. This allowed a larger number of phones to be
supported over a geographical area.
 AMPS use frequency modulation for radio transmission. AMPS allocate frequency
ranges within the 800 and 900 Megahertz spectrum to cellular telephone.

ETACS (The European Total Access Communication System)

 It is identical to AMPS,except it is scaled to fit 25khz channels used throughout Europe
 Another difference between ETACS and AMPS is how telephone no. of each subscriber
is formatted,due to need to accommodate different country codes throughout Europe as
opposed to area codes in the US.

N-AMPS (Narrowband AMPS)

 To increase capacity in large AMPS market ,Motorola developed this.

 NAMPS provided three users in a 30khz AMPS channel by using FDMA and 10khz
channels with three AMPS channels at one time,service provider were able to provide
more trunked radio channels at base station in heavily populated area.
2G systems These second-generation systems are digital, and use either TDMA
(Time Division Multiple Access) or CDMA (Code Division Multiple Access) access
methods. The European GSM (Global System for Mobile communications) is a 2G
digital system with its own TDMA access methods. The 2G digital services began
appearing in the late 1980s, providing expanded capacity and unique services such as
caller ID, call forwarding, and short messaging. A critical feature was seamless roaming,
which lets subscribers move across provider boundaries.

USDC (United States Digital Cellular-IS-54 &IS-136)

 AMPS had many disadvantages too. Primarily, it did not have the potential to support the
increasing demand for mobile communication usage. Each cell site did not have much
capacity for carrying higher numbers of calls. It also had a poor security system which
allowed people to steal a phone's serial code to use for making illegal calls. All of these
triggered the search for a more capable system.The quest resulted in IS54,first American
2G standard
 It supports more users in fixed spectrum allocation.Its supports users or six half rate users
on each APS channel,thus USDC offers as much assix times the capacity of AMPS.
 IS-54 employs the same 30 kHz channel spacing and frequency bands (824-849 and 869-
894 MHz) as AMPS. Capacity was increased over the preceding analog design by
dividing each 30 kHz channel pair into three time slots and digitally compressing the
 voice data, yielding three times the call capacity in a single cell. A digital system also
made calls more secure because analog scanners could not access digital signals.
 A pragmatic effort was launched to improve IS-54 that eventually added an extra channel
to the IS-54 hybrid design. IS-136 systems needed to support millions of AMPS phones.
 IS-136 added a number of features to the original IS-54 specification, including text
messaging, circuit switched data (CSD), and an improved compression protocol.

GSM (Global System for Mobile)

 It iwas developed to solve fragmentation problems of the 1G systems inEurope.It
was the 1st cellular system to specify digital modulation and network level
architectures and services
 GSM provides enhanced features over older analog-based systems, which are
summarized below:
 Total Mobility: The subscriber has the advantage of a Pan-European system
allowing him to communicate from everywhere and to be called in any area
served by a GSM cellular network using the same assigned telephone number,
even outside his home location. The calling party does not need to be informed
about the called person's location because the GSM networks are responsible for
the location tasks. With his personal chipcard he can use a telephone in a rental
car, for example, even outside his home location. This mobility feature is
preferred by many business people who constantly need to be in touch with their
headquarters.
 High Capacity and Optimal Spectrum Allocation: The former analog-based
cellular networks had to combat capacity problems, particularly in metropolitan
areas. Through a more efficient utilization of the assigned frequency bandwidth
and smaller cell sizes, the GSM System is capable of serving a greater number of
subscribers. The optimal use of the available spectrum is achieved through the
application Frequency Division Multiple Access (FDMA), Time Division
Multiple Access (TDMA), efficient half-rate and full-rate speech coding, and the
Gaussian Minimum Shift Keying (GMSK) modulation scheme.
 Security: The security methods standardized for the GSM System make it the
most secure cellular telecommunications standard currently available. Although
the confidentiality of a call and anonymity of the GSM subscriber is only
guaranteed on the radio channel, this is a major step in achieving end-to- end
security. The subscriber’s anonymity is ensured through the use of temporary
identification numbers. The confidentiality of the communication itself on the
radio link is performed by the application of encryption algorithms and frequency
hopping which could only be realized using digital systems and signaling.
 Services: The list of services available to GSM subscribers typically includes the
following: voice communication, facsimile, voice mail, short message
transmission, data transmission and supplemental services such as call
forwarding.
CDMA Digital Cellular Standard (IS-95)

 Interim Standard 95 (IS-95) is the first CDMA-based digital cellular standard by

Qualcomm.
It is a 2G mobile telecommunications standard that uses CDMA, a multiple access
scheme for digital radio, to send voice, data and signaling data (such as a dialed
telephone number) between mobile telephones and cell sites.
 It was designed to be compatible with existing US analog cellular system (AMPS)
frequency band, mobiles and base station can be economically produced for dual mode
operation.
 It allows each user within a cell to use the same radio channel,users in adjacent cells also
use the same radio channel .
 Unlike other cellular standards, the user data rate changes in realtime,depending on the
voice activity and requirements in the network

CT2 Standard for Cordless Telephone

 It is designed for use in both domestic and office environments. It is used to provide
telepoint services which allow a subscriber to use CT2 handsets at a public telepoint.
 It’s a digital version of 1G analog,cordless telephone.
 It offers good speech quality, is more resistant to interference, noise and fading and like
personal telephones,uses a handset with built-in antenna.The digital transmission
provides better security
 Some more features:
Standardized on 864-868 MHz.
500 frames/second (alternately base station and handset).
100 kHz carriers.
32 kbit/s ADPCM voice channel compression.
10 mW maximum power output.
GFSK data encoding.
Up to 100 meter (300 ft) range.

Digital European Cordless Telephone(DECT)

 DECT provides a cordless communication framework for high traffic density ,short range
telecommunication ,and covers a board range of application and environments.

 It offers excellent quality and services for voice and data applications.The main function
of DECT is to provide local mobility to protable usres in an building private branch
exchange.

 It provides low power radio acess between portable parts and fixed base station at range
of up to a few 100ms.
  DECT is based on Time Division Duplex (TDD) and Time Division Multiple Access
(TDMA). It has a TDD/TDMA frame structure.

 Operates between 1.88 GHz and 1.9 GHz.

 Modulation: GMSK with BT = 0.5
 10 carriers in the 1880 - 1900MHz band.
 Offers both speech and relatively high data rate transmission capability (<300 kb/s).
 Designed for office type scenarios including short range indoor environments

 3G systems 3G has become an umbrella term to describe cellular data communications

with a target data rate of 2 Mbits/sec. The ITU originally attempted to define 3G in its
IMT-2000 (International Mobile Communications-2000) specification, which specified
global wireless frequency ranges, data rates, and availability dates. However, a global
standard was difficult to implement due to different frequency allocations around the
world and conflicting input. So, three operating modes were specified. According to
Nokia, a 3G device will be a personal, mobile, multimedia communications device that
supports speech, color pictures, and video, and various kinds of information content.

PACS (Personal Access Communication Systems)

 It is able to provide voice ,data,video images for indoor and microcell use.

 PACS is designed to provide coverage within a 500ms range.The main objective is to

integrate all forms of wireless local loop communication into one system with full
telephone features ,in order to provide wireless connectivity for local exchange.

 The usage of a personal access communications system has become increasingly popular
as a way to either augment or replace some other form of communication with a work or
home environment. Because the system requires relatively little power to function and is
capable of interacting with several different devices in order to send and receive
transmissions, the cost savings involved with using this type of communication process
often pays for itself in a short period of time.

PHS (Personal Handyphone System)

 Formerly PHP Developed in Japan. For radio systems Debuted in 1995. Offered two-way
communications, data services and Internet access.
 PHS is essentially a cordless telephone like DECT, with the capability to handover from
one cell to another. This makes PHS suitable for dense urban areas, but impractical for
rural areas, and the small cell size also makes it difficult if not impossible to make calls
from rapidly moving vehicles
  Modern PHS phone support many value-added services such as high speed wireless data/
Internet connection (64 kbit/s and higher), WWW access, e-mailing, text messaging and
even color image transfer.
 In spite of its low-cost base station, micro-cellular system and "Dynamic Cell
Assignment" system, PHS offers higher number-of-digits frequency use efficiency with
lower cost (throughput per area basis), compared with typical 3G cellular telephone
systems
 Features:
o PHS cells are small
o With transmission power of base station a maximum of 500 mW
o Range typically measures in tens or at most hundreds of meters (some can range up to
about 2 kilometers in line-of-sight)
o Uses TDMA/TDD for its radio channel access method, and 32 kbit/s ADPCM for its
voice codec.
4G Systems On the horizon are 4G systems that may become available even
before 3G matures (3G is a confusing mix of standards). While 3G is important in
boosting the number of wireless calls, 4G will offer true high-speed data services. 4G
data rates will be in the 2-Mbit/sec to 156-Mbit/sec range, and possibly higher. 4G will
also fully support IP. High data rates are due to advances in signal processors, new
modulation techniques, and smart antennas that can focus signals directly at users.
OFDM (orthogonal frequency division multiplexing) is one scheme that can provide very
high wireless data rates.

2) What are the applications of a satellite system?

ans:- Reconnaissance Satellites

All of the early And recommendations had been for a ―direct readout‖ satellite—one that
transmitted pictures to the ground electronically. many of the studies assumed a standard
television camera.the Air Force (aided by its rAnd think tank) had begun development of a
reconnaissance satellite,Weapon System 117l (WS-117l), on 16 march 1955. the program,
initially called Advanced reconnaissance Satellite (ArS), then Sentry, and finally Satellite and
missile observation System (SAmoS), was slow to mature. By 1957, members of the presidential
Science Advisory committee (pSAc) were dissatisfied with the Air Force program; they wanted a
―film return‖ satellite and they wanted the program managed by the central intelligence Agency
(ciA).the success of the u-2 seemed to indicate that the ciA was better at bringing new
technology into operation in a short period of time. on 7 February 1958, president eisenhower
authorized the ciA to proceed with coronA.
 Navigation Satellites
In the days immediately following the launch of Sputnik in october 1957, scientists and
engineers worked to analyze the spacecraft’s signal and its orbit. Bill Guier and George
Weiffenbach of the Johns hopkins university Applied physics laboratory (Apl) listened to the
satellite’s signal and monitored the change in its frequency due to the doppler effect.they used
this doppler shift to compute an orbit for the russian satellite. Another Apl engineer, Frank
mclure (1916–1973), realized that if the orbit were known, the doppler information could be used
to determine the position of the radio receiver on the ground. in early 1958, mclure described the
potential for developing a space-based navigation system.Within a few weeks,Apl proposed a
navigation system to the navy.

Transit Satellites
The earliest transits were launched on the thor-Able and thor Able-Star rockets from cape
canaveral.T he very first of these occurred on 17 September 1959. the last two experimental
transit satellites demonstrated that precise navigation was possible using two frequency beacons
broadcasting the satellite ephemerides (orbits).this system was so robust that it was capable of
determining the harmonics of earth’s gravitational field and the effects of propagation through
the ionosphere. the last satellites were also able to demonstrate the availability of the satellites
when in a near-circular orbit at about 1,000 km and inclined about 66 degrees. After the poor
reliability of the naval Avionics Facility indianapolis (nAFi)builttransit satellites,rcA built the
rest.it was always clear thattransit had significant limitations. the accuracy was good enough for
nuclear weapons (<1 km) but not good enough for conventional weapons.the transit position
fixes took some time to obtain, making transit almost useless for moving objects. the navy
continued research—especially at naval research laboratory (nrl)—on improvements. in 1964,
the Air Force started a new navigation satellite program, project 621B.

Societal impact of navigation Satellites

The original purpose of navigation satellites was to maintain the so-called balance of
terror.even if the Soviet union had launched a first strike,the submarine-launched icBms (SlBms)
would have enough navigational accuracy to level most of the cities of the Soviet union—whose
positions were now well-known thanks to reconnaissance satellites. nAvStAr/GpS gave aircraft
the same navigational assurance—and accuracy to within meters, not kilometers.this improved
accuracy led to GpS-guided munitions used in the Gulf wars. in cars and trucks,and by hikers.of
the three applications pioneered by the military, this is by far the greatest success story.
commercial sales of GpS receivers are now a $9 billion industry.

Weather

By the 1950s, the idea of weather satellites was beginning to surface. in 1951, rAnd
published ―inquiry into the Feasibility of Weather reconnaissance from a Satellite vehicle‖ and
Arthur c. clarke depicted polar and geosynchronous ―metsats‖ in the endpapers of The
19
Exploration of Space. in 1954, a tropical storm was discovered accidentally when pictures
taken from an Aerobee sounding rocket were analyzed.Also in 1954,dr.harry Wexler,the
Weather Bureau’s chief scientist, presented a paper on ―observing the Weather from a Satellite
 20
vehicle‖at the third Symposium on Space travel. in 1955, when the decision was made to
launch a satellite during the upcoming international Geophysical year (iGy), weather observation
and radiation balance payloads were considered and eventually were flown on vanguard and
explorer satellites.

Polar Satellites/tiroS

In spite of the influence of scientists such as Wexler and verner Suomi, the first weather
satellite was a product of the military. tiroS (television infra-red observation Satellite) was rcA’s
losing entry in theAir ForceWS-117l competition won by lockheed in 1956. the Army was
persuaded to support development of tiroS as a polar-orbiting weather satellite. the project was
transferred to ArpA and eventually to nASA in 1958.the first launch was on a delta on 1 April
1960. the satellite had two television cameras: one wide-angle and one narrow-angle (high-
resolution) on tiroS-1 and -2, and both wide-angle on succeeding tiroS satellites. tiroS-8
pioneered the Automatic picture transmission (Apt) camera system.tiroS satellites had the
cameras mounted on the base of the satellite, aligned with the spin axis.this meant that the
cameras were earth-pointing for only a small fraction of their orbits.tiroS-9 pioneered the
―cartwheel‖ configuration wherein the cameras were mounted on the sides of the spacecraft; the
spacecraft spin axis was aligned with orbit normal and pictures were taken continuously. All
launches were from cape canaveral into high-inclination (481)° orbits untiltiroS-9 and -10 were
launched into Sun-synchronous (SS) polar orbits. Sun-synchronous orbits allowed pictures to be
taken at the same local time every day (usually early morning).

Weather Forecasting:

Several satellites deliver pictures of the earth using e.g. infra red or visible light. without
help of satellites, the forecasting of hurricanes would be impossible.

Radio and T.V. Broadcasts Satellites

Hundreds of radio and T.V programs are available via satellites. This technology
competes with cable in many places. as it is cheaper to install and in most case no extra fees have
to be paid for this services.

Military Services :
One of the earliest application of satellite was their use for espionage. Many communication
links are managed via satellites because they much safer from attack of enemy.

--------------------------------------------------------------------------------------------------
 Dept. of Information Technology

Shaikh Mussavir Ahemad

2013MIT0015
Mussavir.shaikh33@gmail.com

Q. What is the necessity of standards?

Ans-The necessity of various standards is described below

1G systems These are the analog systems such as AMPS that grew rapidly in
the 1980s and are still available today. Many metropolitan areas have a mix of 1G
and 2G systems, as well as emerging 3G systems. The systems use frequency
division multiplexing to divide the bandwidth into specific frequencies that are
assigned to individual calls.

AMPS(Advance mobile phone service)

 AMPS is a first-generation cellular technology that uses separate frequencies, or

"channels", for each conversation .It therefore required considerable bandwidth for a
large number of users.
 AMPS was very similar to the older "0G" Improved Mobile Telephone Service, but used
considerably more computing power in order to select frequencies, hand off
conversations to PSTN lines, and handle billing and call setup.
 What really separated AMPS from older systems is the "back end" call setup
functionality. In AMPS, the cell centers could flexibly assign channels to handsets
based on signal strength, allowing the same frequency to be re-used in various
locations without interference. This allowed a larger number of phones to be supported
over a geographical area.
 AMPS use frequency modulation for radio transmission. AMPS allocate frequency
ranges within the 800 and 900 Megahertz spectrum to cellular telephone.

ETACS (The European Total Access Communication System)

 It is identical to AMPS,except it is scaled to fit 25khz channels used throughout Europe
 Another difference between ETACS and AMPS is how telephone no. of each subscriber is
formatted,due to need to accommodate different country codes throughout Europe as
opposed to area codes in the US.

N-AMPS (Narrowband AMPS)

 To increase capacity in large AMPS market ,Motorola developed this.

  NAMPS provided three users in a 30khz AMPS channel by using FDMA and 10khz channels
with three AMPS channels at one time,service provider were able to provide more trunked
radio channels at base station in heavily populated area.

2G systems These second-generation systems are digital, and use either

TDMA (Time Division Multiple Access) or CDMA (Code Division Multiple Access)
access methods. The European GSM (Global System for Mobile communications)
is a 2G digital system with its own TDMA access methods. The 2G digital services
began appearing in the late 1980s, providing expanded capacity and unique
services such as caller ID, call forwarding, and short messaging. A critical feature
was seamless roaming, which lets subscribers move across provider boundaries.

USDC (United States Digital Cellular-IS-54 &IS-136)

 AMPS had many disadvantages too. Primarily, it did not have the potential to support the
increasing demand for mobile communication usage. Each cell site did not have much
capacity for carrying higher numbers of calls. It also had a poor security system which
allowed people to steal a phone's serial code to use for making illegal calls. All of these
triggered the search for a more capable system.The quest resulted in IS54,first American 2G
standard
 It supports more users in fixed spectrum allocation.Its supports users or six half rate users on
each APS channel,thus USDC offers as much assix times the capacity of AMPS.
 IS-54 employs the same 30 kHz channel spacing and frequency bands (824-849 and 869-
894 MHz) as AMPS. Capacity was increased over the preceding analog design by dividing
each 30 kHz channel pair into three time slots and digitally compressing the voice data,
yielding three times the call capacity in a single cell. A digital system also made calls more
secure because analog scanners could not access digital signals.
 A pragmatic effort was launched to improve IS-54 that eventually added an extra channel to
the IS-54 hybrid design. IS-136 systems needed to support millions of AMPS phones.
 IS-136 added a number of features to the original IS-54 specification, including text
messaging, circuit switched data (CSD), and an improved compression protocol.

GSM (Global System for Mobile)

 It iwas developed to solve fragmentation problems of the 1G systems inEurope.It
was the 1st cellular system to specify digital modulation and network level
architectures and services
 GSM provides enhanced features over older analog-based systems, which are
summarized below:
 Total Mobility: The subscriber has the advantage of a Pan-European system
allowing him to communicate from everywhere and to be called in any area served by
a GSM cellular network using the same assigned telephone number, even outside his
home location. The calling party does not need to be informed about the called
person's location because the GSM networks are responsible for the location tasks.
With his personal chipcard he can use a telephone in a rental car, for example, even
outside his home location. This mobility feature is preferred by many business people
who constantly need to be in touch with their headquarters.
  High Capacity and Optimal Spectrum Allocation: The former analog-based
cellular networks had to combat capacity problems, particularly in metropolitan areas.
Through a more efficient utilization of the assigned frequency bandwidth and smaller
cell sizes, the GSM System is capable of serving a greater number of subscribers. The
optimal use of the available spectrum is achieved through the application Frequency
Division Multiple Access (FDMA), Time Division Multiple Access (TDMA),
efficient half-rate and full-rate speech coding, and the Gaussian Minimum Shift
Keying (GMSK) modulation scheme.
 Security: The security methods standardized for the GSM System make it the most
secure cellular telecommunications standard currently available. Although the
confidentiality of a call and anonymity of the GSM subscriber is only guaranteed on
the radio channel, this is a major step in achieving end-to- end security. The
subscriber’s anonymity is ensured through the use of temporary identification
numbers. The confidentiality of the communication itself on the radio link is
performed by the application of encryption algorithms and frequency hopping which
could only be realized using digital systems and signaling.
 Services: The list of services available to GSM subscribers typically includes the
following: voice communication, facsimile, voice mail, short message transmission,
data transmission and supplemental services such as call forwarding.

CDMA Digital Cellular Standard (IS-95)

 Interim Standard 95 (IS-95) is the first CDMA-based digital cellular standard by Qualcomm.
It is a 2G mobile telecommunications standard that uses CDMA, a multiple access scheme
for digital radio, to send voice, data and signaling data (such as a dialed telephone number)
between mobile telephones and cell sites.
 It was designed to be compatible with existing US analog cellular system (AMPS) frequency
band, mobiles and base station can be economically produced for dual mode operation.
 It allows each user within a cell to use the same radio channel,users in adjacent cells also use
the same radio channel .
 Unlike other cellular standards, the user data rate changes in realtime,depending on the
voice activity and requirements in the network

CT2 Standard for Cordless Telephone

 It is designed for use in both domestic and office environments. It is used to provide
telepoint services which allow a subscriber to use CT2 handsets at a public telepoint.
 It’s a digital version of 1G analog,cordless telephone.
 It offers good speech quality, is more resistant to interference, noise and fading and like
personal telephones,uses a handset with built-in antenna.The digital transmission provides
better security
 Some more features:
Standardized on 864-868 MHz.
500 frames/second (alternately base station and handset).
100 kHz carriers.
32 kbit/s ADPCM voice channel compression.
10 mW maximum power output.
GFSK data encoding.
Up to 100 meter (300 ft) range.

Digital European Cordless Telephone(DECT)

  DECT provides a cordless communication framework for high traffic density ,short
range telecommunication ,and covers a board range of application and environments.
 It offers excellent quality and services for voice and data applications.The main
function of DECT is to provide local mobility to protable usres in an building private
branch exchange.
 It provides low power radio acess between portable parts and fixed base station at
range of up to a few 100ms.
 DECT is based on Time Division Duplex (TDD) and Time Division Multiple Access
(TDMA). It has a TDD/TDMA frame structure.
 Operates between 1.88 GHz and 1.9 GHz.
 Modulation: GMSK with BT = 0.5
 10 carriers in the 1880 - 1900MHz band.
 Offers both speech and relatively high data rate transmission capability (<300 kb/s).
 Designed for office type scenarios including short range indoor environments

 3G systems 3G has become an umbrella term to describe cellular data

communications with a target data rate of 2 Mbits/sec. The ITU originally
attempted to define 3G in its IMT-2000 (International Mobile Communications-
2000) specification, which specified global wireless frequency ranges, data rates,
and availability dates. However, a global standard was difficult to implement due
to different frequency allocations around the world and conflicting input. So, three
operating modes were specified. According to Nokia, a 3G device will be a
personal, mobile, multimedia communications device that supports speech, color
pictures, and video, and various kinds of information content.

PACS (Personal Access Communication Systems)

 It is able to provide voice ,data,video images for indoor and microcell use.
 PACS is designed to provide coverage within a 500ms range.The main
objective is to integrate all forms of wireless local loop communication into
one system with full telephone features ,in order to provide wireless
connectivity for local exchange
 The usage of a personal access communications system has become increasingly
popular as a way to either augment or replace some other form of communication
with a work or home environment. Because the system requires relatively little
power to function and is capable of interacting with several different devices in
order to send and receive transmissions, the cost savings involved with using this
type of communication process often pays for itself in a short period of time.

PHS (Personal Handyphone System)

 Formerly PHP Developed in Japan. For radio systems Debuted in 1995. Offered
two-way communications, data services and Internet access.
 PHS is essentially a cordless telephone like DECT, with the capability to handover
from one cell to another. This makes PHS suitable for dense urban areas, but
impractical for rural areas, and the small cell size also makes it difficult if not
impossible to make calls from rapidly moving vehicles
 Modern PHS phone support many value-added services such as high speed
wireless data/ Internet connection (64 kbit/s and higher), WWW access, e-
mailing, text messaging and even color image transfer.
 In spite of its low-cost base station, micro-cellular system and "Dynamic Cell
Assignment" system, PHS offers higher number-of-digits frequency use efficiency
with lower cost (throughput per area basis), compared with typical 3G cellular
telephone systems
 Features:
o PHS cells are small
o With transmission power of base station a maximum of 500 mW
o Range typically measures in tens or at most hundreds of meters (some can range
up to about 2 kilometers in line-of-sight)
o Uses TDMA/TDD for its radio channel access method, and 32 kbit/s ADPCM for its
voice codec.

4G Systems On the horizon are 4G systems that may become available even
before 3G matures (3G is a confusing mix of standards). While 3G is important in
boosting the number of wireless calls, 4G will offer true high-speed data services.
4G data rates will be in the 2-Mbit/sec to 156-Mbit/sec range, and possibly
higher. 4G will also fully support IP. High data rates are due to advances in signal
processors, new modulation techniques, and smart antennas that can focus
signals directly at users. OFDM (orthogonal frequency division multiplexing) is
one scheme that can provide very high wireless data rates.
Q. What are the basic unit of cellular system?

There are various cellular systems in the world, such as the GSM and CDMA. The design of these cellular systems

are complicated but the architecture of most cellular systems can be broken down into six basic components.

There are six basic components that can be found in most cellular systems.

The architecture of most cellular systems can be broken down into the following six components:

a)Mobile Station (MS)

A mobile station is basically a mobile/wireless device that contains a control unit, a transceiver and an antenna

system for data and voice transmission. For example, in GSM networks, the mobile station will consist of the mobile

equipment (ME) and the SIM card.

b) Air Interface Standard

There are three main air interface protocols or standards: frequency division multiple access (FDMA), time division

multiple access (TDMA) and code division multiple access (CDMA). These standards are basically the medium

access control (MAC) protocols that define the rules for entities to access the communication medium.

These air interface standards allow many mobile user to share simultaneously the finite amount of radio channels.

c) Base Station (BS)

A base station is a fixed station in a mobile cellular system used for radio communications with mobile units. They

consist of radio channels and transmitter and receiver antenna mounted on a tower.

d) Databases

Another integral component of a cellular system is the databases. Databases are used to keep track of information

like billing, caller location, subscriber data, etc. There are two main databases called the Home Location Register

(HLR) and Visitor Location Register (VLR). The HLR contains the information of each subscriber who resides in the

same city as the mobile switching center (MSC). The VLR temporarily stores the information for each visiting

subscriber in the coverage area of a MSC. Thus, the VLR is the database that supports roaming capability.

e) Security Mechanism

The security mechanism is to confirm that a particular subscriber is allowed to access the network and also to
authenticate the billing.
There are two databases used for security mechanism: Equipment Identify Register (EIR) and Authentication Center

(AuC). The EIR identifies stolen or fraudulently altered phones that transmit identity data that does not match with

information contained in either the HLR or VLR. The AuC, on the other hand, manages the actual encryption and

verification of each subscriber.

f) Gateway

The final basic component of a cellular system is the Gateway. The gateway is the communication links between two

wireless systems or between wireless and wired systems. There are two logical components inside the Gateway:

mobile switching center (MSC) and interworking function (IWF).

The MSC connects the cellular base stations and the mobile stations to the public switched telephone network

(PSTN) or other MSC. It contains the EIR database.

The IWF connects the cellular base stations and the mobile stations to Internet and perform protocol translation if

needed.

(Miss. SNEHAL D. ROKADE)

(2013MIT0016)
Chetan Kadu(2013MIT017)

Q.What are the classifications of Wireless technologies and system?

Computer networking is a very vast and advanced field which has been implemented on many
aspects of technology. In computer networking, there are so many devices which are used for
networking purposes. Wireless technologies really provide the convenient and easy approach to
communications between different areas that are far behind from the different types of the
modern and the latest technologies. Types of wireless technologies were designed by the
scientists because people who are away from their homes, they can avail such opportunity of
using the internet at very high speed and be in touch with others for the sake of their business
enhancement in the best possible ways of communication between two places.
Following are types of wireless technologies

Different types of Wireless Technologies:

Due to the easy approach to the networks and also in many appliances of the daily life wireless
technologies are of many types and almost all the technologies that are used in the present era for
the sake of better working are designed wirelessly. Some important types of wireless
technologies are as follows

Wifi Technology :
WiFi is commonly called as wireless LAN, it is one of those networks in which high frequency
radio waves are required for transmission of data from one place to another. WiFi operates on
several hundreds feet between two places of data transmission. This technology only works on
high frequency radio signals. Otherwise, it will not work properly. Nowadays this technology is
used as office or home network and in many electronic devices. Wireless LAN or WiFi is
divided into three main parts on which its whole working depends and all of its applications also
depend on these parts i.e. infrastructure mode, ad hoc network and mixed network.

Zigbee technology :

A type of low cost, low power and wireless technology which is used for the different purposes
at ultra low power is known as Zig bee technology. Low power radios on the basis of standard
personal wireless networking are used by a protocol to enhance the technology. It offers
excellent wireless control path network. It was not proposed or designed for the excellent or high
speed data transfer rate applications. But it was designed for working on excellent long battery
timings at low cost and also at ultra low power consumption. It is an ideal technology which
operates at low power and low cost and used for wireless monitoring and control.

Wimax technology :
One of the important wireless technologies another technology is present that is called as Wimax
technology. It is defined as a type of the wireless networking technology that is required to
transmit the information in the form of microwaves through different type of methods of wireless
networking from point to point or multi point access top the devices which are portable in nature.
Wimax technology is categorized into two types that are Fixed Wimax and the mobile Wimax.

Voice communication (Voip ):

Voice communication is also the types of wireless technologies. In this types all the types of
technologies related to the communication through the voice is included such as communication
through different types of cell phones, through different types wireless internet technologies etc

Bluetooth technology :
Other important type of wireless technology is the Bluetooth technology that is used to transmit
the data from one device to another device with the help of mobile phone technology.
 2012MIT018
Prashant Mandale

Question 1:State the two different types of fading. Define Rayleigh fading.

Answer:
1. In a wireless communication system the signals may travel through multiple
paths between a transmitter and a receiver. This effect is called multipath
propagation.
2. Due to the multiple paths, the receiver of the signal will observe variations
of amplitude, phase and angle of arrival of the transmitted signal.
3. These variations originate the phenomenon referred as multipath fading. The
variations are characterized by two main manifestations (types), large-scale
and small-scale fading.
4. Furthermore these manifestations give rise to specific types of degradations
of the signal. Based on figure 1 presents the fading manifestations and its
associated degradations.
 Large-scale fading

Large-scale fading, refers to path loss caused by the effects of thesignal

traveling over large areas. Large-scale fading characterizes the losses due to
considerably bigphysical objects in the signal’s path like hills or forests. The
path loss is characterized ,by a meanloss (due to the distance between the
transmitter and the receiver and the propagation environmentcharacteristics)
and a variation around the mean loss.

Small-scale fading

Fluctuation of the signal envelope is Rayleigh distributed when there is no

predominant line of sight
between the transmitter and receiver.
When there is a predominant line of sight between the transmitter and receiver
the fluctuations are statistically described by a Rician.
Figure 1 shows two manifestations of small scale fading.
The first one, signal dispersion, refers to the time spreading of the signal.
Dispersion causes the underlying digital pulses transmitted in thesignal to
spread in time.
The second manifestation reflects the time variant behavior of the channel that
isdue to relative mobility between a transmitter and a receiver or the objects in
the path of the signal. Both
of these manifestations can be characterized in the time and frequency domain
by fading degradation
types.
As shown in figure 1, the degradation types of the dispersion manifestation are
frequency
selective fading and flat fading. From the time domain point of view, frequency
selective fading occurswhen the maximum spread in time of a symbol is greater
than the duration of the symbol. Consequently,another name for this fading
degradation is channel induced intersymbol interference. From thefrequency
domain point of view, frequency selective fading occurs when the spectral
components of asignal are affected in different ways by the channel. In
particular, frequency selective fading occurs whenthe channel’s coherence
bandwidth (the channel’s bandwidth in which all components
experienceapproximately the same fading characteristics) is smaller than the
signal’s bandwidth.
When theconditions described above, for frequency selective fading are not
met, the degradation is referred as flatfading. In this case the channel
characteristics are approximately flat for all frequencies.
Figure 1 also shows the degradation types of the channel’s time variance
manifestation. These
are fast and slow fading. From the time domain point of view, fast fading refers
to the condition in whichthe channel’s coherence time (an expected time
duration during which the channel’s response is invariant)is smaller than the
symbol duration.
Before describing fast fading in the frequency domain it is necessary to
introduce the Doppler
frequency concept.
The Doppler frequency (fm) characterizes the maximum Doppler frequency
shift of
the signals in a mobile environment. This is computed as f v /m . Where
‘ v’ is the relative velocitybetween the transmitter and receiver and ‘λ’ is the
wavelength of the transmitted signal. With this basisone can indicate that from
the frequency domain point of view, fast fading occurs when the
signalbandwidth is less than the maximum frequency Doppler shift.
Both types of small scale fading can be present in a wireless system. In this
tutorial we will look
at flat fading with the associated time variance manifestation. Large scale
fading is reflected only on thestrength of the received signal and will not be
considered here. Having presented some relevant
preliminary information we can proceed to the central topic of this document,
modeling of flat Rayleighfading channels.

LARGE-SCALE FADING: PATH-LOSS MEAN

AND STANDARD DEVIATION

For the mobile radio application, Okumura madesome of the earlier

comprehensive path-loss measurementsfor a wide range of antenna heights
and coverage distances. Hata transformed Okumura’s data into
parametricformulas. For the mobile radio application, the mean pathloss, —
Lp(d), as a function of distance, d, between the transmitterand receiver is
proportional to an nth power of d relativeto a reference distance d0
 isis often stated in decibels, as shown below.
Ls(d0) (dB) + 10 n log (d/d0)

The reference distance d0 corresponds to a point located inthe far field

of the antenna. Typically, the value of d0 is takento be 1 km for large cells,
100 m for microcells, and 1 m forindoor channels. —Lp(d) is the average
path loss (over a multitudeof different sites) for a given value of d.
Linear regressionfor a minimum mean-squared estimate (MMSE) fit
of —Lp(d) versus d on a log-log scale (for distances greater thand0) yields a
straight line with a slope equal to 10n dB/decade

SMALL-SCALE FADING:
STATISTICS AND MECHANISMS

When the received signal is made up of multiple reflectiverays plus a significant

line-of-sight (nonfaded) component,the envelope amplitude due to small-scale
fading has aRician pdf, and is referred to as Rician fading.
The nonfadedcomponent is called the specular component. As the amplitude
of the specular component approaches zero, the Ricianpdf approaches a Rayleigh
pdf, expressed as
Definition of Rayleigh fading
Rayleigh fading is a statistical model for the effect of a propagation environment
on a radio signal, such as that used by wireless devices.
Rayleigh fading models assume that the magnitude of a signal that has passed
through such a transmission medium (also called communications) will vary
randomly, or fade, according to a Rayleigh distribution — the radial component of
the sum of two uncorrelated Gaussian random variables.
Rayleigh fading is viewed as a reasonable model
for troposphere and ionospheres’ signal propagation as well as the effect of heavily
built-up urban environments on radio signals.Rayleigh fading is most applicable
when there is no dominant propagation along a line of sight between the
transmitter and receiver. If there is a dominant line of sight, Rician fading may be
more applicable.
2012MIT018
 Prashant Mandale

Q. Why 800 MHz frequency is selected for mobiles?

Answer:

Fig.1: The Complete Frequency Allocation

A. The frequency Spectrum Allocation

I. Very Low Frequencies:- The frequencies in between 3khz &
30khz fall in this category. These signals are mostly used for Short
Distance Communication like Communication between two nearby
Ships or Submarines.
II. Low Frequencies:- The frequencies in between 30khz to 300 khz
fall in this category. These signals are used for navigational
purposes in guiding the Ship & Airplanes.
III. Medium frequencies:- This band contains the signals transmitted
with frequencies in between 300khz to 3Mhz. These frequencies
 are mainly used Long Range Navigation, Medium Wave
Broadcasting etc. This band is also used for amateur wireless.
IV. High Frequencies:- 30Mhz to 300Mhz communication
frequencies are considered as High frequencies. These are mainly
used for Shortwave broadcasting, International broadcasting,
radiotelephones etc.
V. Very High Frequencies:-This is one of the widely used band.
Various day today applications use this band. FM radio,
Television, cordless phones, wireless systems, aircraft radars etc.
makes use of this band. The frequencies ranging from 30 MHz to
300 MHz fall in this category.
VI. Ultra High Frequencies:- This is the band where the
communication systems like GPS, Mobiles etc. work. The
frequencies in between 300 MHz to 3GHz fall in this category.
VII. Super High Frequencies;- The frequencies ranging from 3GHz to
30Ghz are considered to be Super High Frequencies. The band is
still underdeveloped. Mostly this band is used for Space & Satellite
communication.
VIII. Extremely High Frequencies;- This band contains very high
frequencies. The band has frequencies ranging from 30Ghz to
300Ghz. This band is also underdeveloped. This is mostly used for
astronomical research & radar landing systems.
IX. Infrared, Ultra Violet Frequencies:-This is a band which is used
for optical communications. This band has very high frequencies.
IR transmission is useful in communicating two devices placed at
very short distances. While the visible light is being used as a great
medium to communicate messages directly with the groups since
ancient times.
B. The Radio Frequencies Used
I. Radio frequencies are scarce resources. Many national (economic)
interests make it hard to find common, worldwide regulations. The
International Telecommunications Union Radio communication sector
(ITU-R) handles standardization in the wireless sector, so it also
handles frequency planning.
II. The ITU-R has split the world into three regions: Region 1covers
Europe, the Middle East, countries of the former Soviet Union, and
Africa.Region 2 includes Greenland, North and South America, and
region 3 comprisesthe Far East, Australia, and New Zealand.To
achieve at least some harmonization, the ITU-R holds, the World
Radio Conference (WRC), to periodically discuss and decide
frequency allocations forall three regions.
 Table 1: Frequencies used for Analog & Digital Communiation

III. Table 1 gives some examples for frequencies used for analog and
digital communication like mobile phones, cordless telephones, wireless
LANs, and other radio frequency(RF) systems for countries in the three
regions representing the major economic power. As you can see, the
frequency bands are mostly used for the wireless communication like radio,
WLANs, some old mobile communication system like NMT. Under these
situations, for the mobile communication the most widely available band is
800 MHz band (850MHz to be precise) . So in US this band was allocated as
per the market requirement for mobile communication. When the same GSM
system was to be implemented in Europe, the European countries preferred
to allocate a standard band for mobile communication. They allocated the
900 MHz & 1800 MHz band for GSM. Our country & most of the countries
followed the European Standards later on. So we have the 900MHz &
1800MHz band allocated to GSM communication in INDIA.