GSM architecture

Mobile Station (MS)

MS contains the following two components.

Mobile Equipment (ME)

 Mobile equipment is a piece of hardware which is used to generate the signal, and process the data receiver
and to be transmitted.
 It also contains a number known as the International Mobile Equipment Identity (IMEI). This is installed in the
phone at manufacture and "cannot" be changed.
 It is accessed by the network during registration to check whether the equipment has been reported as stolen.

 Type approval code (TAC). The TAC is issued by an authorized agency on successful testing for type approval.
 Final assembly code (FAC). This uniquely identifies the manufacturer of the mobile equipment.
 Serial number (SNR). Each mobile equipment is identified with a unique serial number within a TAC and FAC.
 The remaining 1 digit/4 bits are not currently used and are a "spare."

Subscriber Identity Module (SIM)

 The SIM or Subscriber Identity Module contains the information that provides the identity of the user to the
network.

Base station subsystem (BSS)

All radio related functions are performed by BSS. It has (BSCs) and BTS

Base Transceiver Station (BTS)

The BTS contains the radio transceivers and antennas, responsible for the radio transmissions with the MS. This
includes the following functions:
 Modulation and demodulation
 Channel coding and decoding
 Encryption process
 RF transmit and receive
Base station controller (BSC)

The BSC handles the management of the radio resource and monitor the BTS. It controls the following functions:

 Allocation and release of radio channels,

 Frequency hopping,
 Power control algorithms,
 Handover management,
 Choice of the encryption algorithm
 Monitoring of the radio link.

Network Subsystem

Network switching subsystem (NSS) (or GSM core network) is the component of a GSM system that carries out call out
and mobility management functions for mobile phones roaming on the network of base stations.
Main Switching Center (MSC)

 It is responsible for the switching of calls between the mobile users and between mobile and fixed network
users.
 It manages outgoing and incoming calls from various types of networks, such as PSTN, ISDN, and PDN.
 It also handles the functionality required for the registration and authentication of a user

Home Location Register (HLR)

 The HLR manages the mobile subscriber database

 This includes the IMSI, the MS ISDN number (MSISDN), and the list of services subscribed by the user
 The HLR also stores the current location of the MS

Visitor Location Register (VLR)

 The VLR temporarily keeps the administrative data of the subscribers that are currently located in a given
geographical area under its control.
 When roaming mobile enter MSC area MSC associated with VLR

Authentication Center (AUC)

 The authentication center (AuC) is a protected database that contains a copy of the secret key stored in each
subscriber's SIM card, for authentication and encryption over the radio channel.
 It provides the codes for both authentication and encryption to avoid undesired violations of the system by
third parties.

Equipment Identity Register (EIR)

 The equipment identity register (EIR) is a database that contains a list of all valid ME on the network, where
each MS is identified by its IMEI. An IMEI is marked as invalid if it has been reported stolen.

Operation and support system (OSS).

Here are some of the OSS functions:

 Administration and commercial operation
 Security Management.
 Network configuration, Operation and Performance Management.
 Maintenance Tasks.
Frequency response of GSM

GSM 900 GSM 1800 GSM 1900

UPLINK FREQ.(MHz) 890-915 1710-1785 1850-1910
DOWNLINK FREQ.(MHz) 935-960 1805-1880 1930-1990
BANDWIDTH.(MHz) 25 95 60
FREQ. INTERVELS.(KHz) 200 200 200

Advantages of GSM over Analog system

 It has reduced RF transmission power
 It provides International roaming capability.
 Security against fraud
 It has encryption capability for information security and privacy and compatibility with ISDN
 Digital transmitted signals can be transported longer distance than analog signals.
 The transmission errors can be detected and corrected more easily and accurately than is possible with analog
signals
Cell
 Cell is a small graphical area
 It represented as hexagonal in diagrams
 Limited in transmitter powers

Frequency Reuse

Frequency reuse is the ability to reuse same radio channel frequency at another adjacent cell sites within the cellular
system

Multiple Access

A major requirement of cellular networks is to provide an efficient technique for multiple devices to access the
wireless system. These techniques include:

FDMA-frequency division multiple access in which every user device uses its own frequency channel.

TDMA-time division multiple access, in which a radio channel is divided in time slots, and use devices use their
allocated time slots.

CDMA-code division multiple access, in which orthogonal codes are used to differentiate user devices. CDMA is very
spectrum efficient, and was used by 3G standards.
 Approach TDMA FDMA CDMA

Transmission Discontinuous Continuous Continuous

scheme

Cell capacity Limited Limited No absolute limit on

channel capacity but it is
an interference limited
system

Advantages Established Simple established Flexible

fully digital robust less frequency planning
flexible needed
soft handover

Disadvantages Guard space needed Inflexible, Complex receivers

(multipath propagation), frequencies are needs more complicated
synchronization difficult scarce resource power control for senders

Handoff

Handoff (or handover) is a control process initiated when a mobile moves from its current cell to its neighboring cell.

Method of increasing capacity in GSM

Cell splitting

 Cell splitting is the process of subdividing a congested cell into smaller cells such that each smaller cell has its
own base station with reduced antenna height and reduced transmitter power.
Cell Sectoring

 In this method, a cell has the same coverage space but instead of using a single omni‐directional antenna
that transmits in all directions, either 3 or 6 directional antennas are used such that each of these antennas
provides coverage to a sector of the hexagon.

Effects of Interference

 Interference is a major limiting factor in the performance of cellular systems.

 It causes degradation of signal quality.
 It introduces bit errors in the received signal.

Sources of Interference
 Another mobile in the same cell.
 A call in progress in the neighboring cell.
 Other base stations operating on the same frequency.
 Any non-cellular system which leaks energy into the cellular frequency band.

Types of interference in GSM

Co-channel interference

 This type of interference is the due to frequency reuse, i.e. several cells use the same set of frequency.
 These cells are called co-channel cells.
 To reduce co-channel interference, co-channel cells must be physically separated by a minimum distance

Adjacent channel interference

 Interference resulting from signals which are adjacent in frequency to the desired signal is called adjacent
channel interference.
 Adjacent channel interference results from imperfect receiver filters which allow nearby frequencies to leak
into the pass band.
 Adjacent channel interference can be minimized through careful filtering and channel assignments.
Types of Switching Techniques

1) Circuit Switching
2) Packet Switching
3) Message Switching

Circuit Switching

 Circuit switching is a technique that directly connects the sender and the receiver in an unbroken path.
 Telephone switching equipment, for example, establishes a path that connects the caller's telephone to the
receiver's telephone by making a physical connection.
 Once a connection is established, a dedicated path exists between both ends until the connection is
terminated.
 Routing decisions must be made when the circuit is first established

Packet Switching
 There are two methods of packet switching: Datagram and virtual circuit.
 In both packet switching methods, a message is broken into small parts, called packets.
 Each packet is tagged with destination addresses.
 Since packets have a strictly defined maximum length, they can be stored in main memory instead of
disk. Therefore access delay and cost are minimized.
 Also the transmission speeds, between nodes, are optimized.
 If the network becomes overloaded, packets are delayed or discarded (``dropped'').
 Packet do not necessarily travel together. They don't even all travel via the same route.
 But eventually they arrive at the right point and at their destination are reassembled into the correct
order, then converted to analog form

Datagram packet switching

 Each packet is a self-contained unit with complete addressing information attached.

 This fact allows packets to take a variety of possible paths through the network.
 So the packets, each with the same destination address, do not follow the same route, and they may
arrive out of sequence at the exit point node (or the destination).
 Reordering is done at the destination point based on the sequence number of the packets.
 It is possible for a packet to be destroyed if one of the nodes on its way is crashed momentarily. Thus all
its queued packets may be lost.
 Virtual circuit packet switching

 In the virtual circuit approach, a preplanned route is established before any data packets are sent.
 A logical connection is established when a sender send a "call request packet" to the receiver and the
receiver send back an acknowledge packet "call accepted packet" to the sender if the receiver agrees on
conversational parameters.
 The conversational parameters can be maximum packet sizes, path to be taken, and other variables
necessary to establish and maintain the conversation.
 In virtual circuit, the route between stations does not mean that this is a dedicated path, as in circuit
switching.

Message Switching

 In case of Message Switching it is not necessary to establish a dedicated path in between any two
communication devices.
 Here each message is treated as an independent unit and includes its own destination source address by its
own.
 Each complete message is then transmitted from one device to another through internetwork
 Each intermediate device receive the message and store it until the nest device is ready to receive it and
then this message is forwarded to the next device.
 The storing and Forwarding introduces the concept of delay.
 Switching Advantages disadvantages
technique
Circuit  The communication is dedicated  Possible long wait to establish a connection,
Switching (10 seconds, more on long- distance or
international calls.) during which no data
can be transmitted.
 More expensive than any other switching
techniques.
 Inefficient use of the communication
channel, because the channel is not used
when the connected systems are not using
it.

Packet  Packet switching is cost effective,  Protocols for packet switching are typically
Switching because it do not need massive more complex.
amount of secondary storage.  If packet is lost, sender needs to retransmit
 Packet switching offers improved the data.
delay characteristics, because  Packet-switched systems still can’t deliver
there are no long messages in the same quality as dedicated circuits in
the queue (maximum packet size applications requiring very little delay - like
is fixed). voice conversations or moving images.
 Packet can be rerouted if there is
any problem, such as, busy or
disabled links.
 Many network users can share
the same channel at the same
time.
Message  Channel efficiency can be greater  Not compatible for interactive applications
Switching compared to circuit-switched such as voice and video. This is due to
systems, because more devices longer message delivery time.
are sharing the channel.  The method is costly as store and forward
 Traffic congestion can be devices are expensive.
reduced, because messages may  It can lead to security issues if hacked by
be temporarily stored in route. intruders.
 As the system is complex, often people are
not aware whether the messages are
transferred successfully or not..

Signaling

In-Band Signaling

 In-band signaling is the exchange of call control information within the same physical channel, or within the
same frequency band, that the telephone call itself is using.
 An example is dual-tone multi-frequency signaling (DTMF), which is used on most telephone lines to
customer premises.
Dual-tone multi-frequency
 DTMF is an in-band telecommunication signaling system using the voice-frequency band over telephone
lines between telephone equipment and other communications devices and switching centers.
 The DTMF system uses a set of eight audio frequencies transmitted in pairs to represent 16 signals,
 They can be transmitted through electrical repeaters and amplifiers, and over radio and microwave links,
thus eliminating the need for intermediate operators on long-distance circuits.
Out-of-band signaling
 Out-of-band signaling is telecommunication signaling on a dedicated channel separate from that used for the
telephone call. Out-of-band signaling has been used in Signaling System No. 7 (SS7)
Channel-associated signaling (CAS)
 Dedicated signaling channel is associated with each data channel
 Signaling flows the same path as data
 Can be controlled by non-digital or digital system
 This information can be transmitted in the same band (in-band signaling) or a separate band (out-of-band
signaling)
Common-channel signaling (CCS)
 Common-channel signaling (CCS) the transmission of signaling information (control information) on a
separate channel
 The most common CCS signaling methods in use today are (ISDN) and (SS7).

Signaling point of ss7

Service switching points (SSP)

Signaling transfer points (STP)
Service control points (SCP)
 The SSP gathers the analog signaling information from the local line in the network (end point) and converts
the information into an SS7 message.
 These messages are transferred into the SS7 network to STPs that transfer the packet closer to its
destination.
 When special processing of the message is required (such as rerouting a call to a call forwarding number),
the STP routes the message to a SCP.

Common Channel Signaling Modes

There are three types of CCS signaling modes:

 Associated
 Quasi-associated
 Non-associated

Associated Signaling

 In associated mode, both the signaling and the corresponding user traffic take the same route through the
network.
 Networks that employ only associated mode are easier to design and maintain; however, they are less
economic, except in small-sized networks.
 Associated mode requires every network switch to have signaling links to every other interconnected switch
(this is known as a fully meshed network design).
Quasi-Associated Signaling
 In quasi-associated mode, signaling follows a different route than the switched traffic to which it refers,
requiring the signaling to traverse at least one intermediate node.
 Quasi-associated networks tend to make better use of the signaling links; however, it also tends to create a
more complex network in which failures have more potential to be catastrophic.
 Quasi-associated signaling can be the most economical way of signaling for lightly loaded routes because it
avoids the need for direct links.
 The signaling is routed through one or more intermediate nodes. Signaling packets arrive in sequence using
quasi-associated signaling because the path is fixed for a given call (or database transaction) at the start of a
call (or transaction)

Non-Associated Signaling

 Because the path is not fixed at a given point in time in non-associated mode, the signaling has many
possible routes through the network for a given call or transaction.
 Therefore, the packets might arrive out of sequence because different routes might have been traversed.
 SS7 does not run in non-associated mode because no procedures exist for reordering out-of-sequence
messages.
 Associated and quasi-associated signaling modes assure sequential delivery, while non-associated signaling
does not.

Four phases of call control

 Pre selection
 Call completion
 Conversation
 Release
Telecommunication Network management
 Telecommunications Network Management originated under the International Telecommunication Union –
Telecommunications (ITU-T) for managing open systems in a communications network
 It allows elements from different manufacturers to be incorporated into a network under a single
management control.
 Modern telecom networks offer automated management functions and are run by operations support
system (OSS) software.
 These manage modern telecom networks and provide the data that is needed in the day-to-day running of a
telecom network.
 OSS software is also responsible for issuing commands to the network infrastructure to activate new service
offerings, commence services for new customers, and detect and correct network faults.

Telecommunication Network management architecture

Business management -Includes the functions related to business aspects, analyzes trends and quality issues, for
example, or to provide a basis for billing and other financial reports.
Service management -Handles services in the network: definition, administration and charging of services.
Network management -Distributes network resources, performs tasks of: configuration, control and supervision
of the network.
Element management -Handles individual network elements including alarm management, handling of
information, backup, logging, and maintenance of hardware and software.
Telecommunications requirements

 Terminals for accessing the network

 Computers that process information and are interconnected by the network
 Telecommunications links that form a channel through which information is transmitted from a sending
device to a receiving device.
 Telecommunications equipment that facilitates the transmission of information.
 Telecommunications software that controls message transmission over the network.

Telecommunication Network management is need because of,

 Size and complexity of the telecommunications network is growing fast

 To improve efficiency
 When a fault is detected it is quite a difficult task because it was done manually and many systems may detect
a fault even when the actual fault may be in only one of them or even somewhere else.
 Time spending to repair the fault is very high because fault is detection it is quite a difficult task
Network Management Applications

Fault Management Responsible for:

 Detection of a problem
 Fault Isolation
 Correction to normal operation
 Uses Polling of managed objects to search for error conditions and/or report alarms/alerts,
 Can also use event reporting
 Illustrates the problem detected either as a graphic or in textual format

Configuration Management Responsible for:

 Changes, additions and deletions on the managed object parameter(s)

 Needs to be co-ordinated with the network management systems personnel (frequently involve some
manual work scheduling)
 Underlies most of the other network management functional areas

Accounting Management Responsible for:

 Usually divided into three stages: metering, tariffing and billing.

 Metering logs a particular usage of the managed object
 Tariffing is the means by which a charge can be calculated
 Billing is the selection & application of a tariffing mechanism on the metered usage and the composition of
the customer bill

Performance Management Responsible for:

 Optimization of managed
 Identification of bottlenecks in network and implementation of corrective action
 Divides into four main functions: Performance data collection, Data analysis, Problem Reporting, Display &
formatting

Security management Responsible for:

 administration of access controls on managed objects

 Issuing of security alarm reports for violations.
 Maintenance and security audit trail

Advantages and disadvantages of networking

Advantages of Computer Networking Disadvantages of Computer Networking

Easy Communication and Speed Breakdowns and Possible Loss of Resources
Ability to Share Files, Data and Information Expensive to Build
Sharing Hardware Security Threats
Sharing Software Bandwidth Issues
Comparison between extranet, internet and intranet

Extranet Internet Intranet

Definition Extranet is for individuals A global network which is An intranet is unique to an
or group of personnel who available to anyone who organization or group of
want to send private wants to access people who work together
information information at a place
Privacy Extremely Private Public Private intermediately
Dependency Dependent on internet Independent Independent yet
and intranet dependent on internet
Firewall Yes No Can have

Network Topologies

 Hierarchical Network
 Star Network
 Ring Network
 Bus Network

Network Devices
Repeater
 A repeater operates at the physical layer
 Its job is to regenerate the signal over the same network before the signal becomes too weak or corrupted so
as to extend the length to which the signal can be transmitted over the same network.
 An important point to be noted about repeaters is that they do not amplify the signal.
Hub
 A hub is basically a multiport repeater.
 Hubs cannot filter data, so data packets are sent to all connected devices. .
 Also, they do not have intelligence to find out best path for data packets which leads to inefficiencies and
wastage.

Bridge
 A bridge operates at data link layer.
 A bridge is a repeater, with add on functionality of filtering content by reading the MAC addresses of source
and destination.
 It is also used for interconnecting two LANs working on the same protocol.

Switch –

 A switch is a multi-port bridge with a buffer and a design that can boost its efficiency and performance.
Switch is data link layer device.
 Switch can perform error checking before forwarding data that makes it very efficient as it does not forward
packets that have errors and forward good packets selectively to correct port only.

Routers

 A router is a device like a switch that routes data packets based on their IP addresses.
 Router is mainly a Network Layer device.
 Routers normally connect LANs and WANs together and have a dynamically updating routing table based on
which they make decisions on routing the data packets.
Plmn service areas