Model Questions

MCA: 5th Semester

CS:- Wireless network ( 501)
Q1) Explain in detail the IEEE 802.11 WEAN.
Ans :- IEEE 802.11 standard, popularly known as WiFi, lays down the architecture and
specifications of wireless LANs (WLANs). WiFi or WLAN uses high-frequency radio waves
instead of cables for connecting the devices in LAN. Users connected by WLANs can move
around within the area of network coverage.
IEEE 802.11 Architecture
The components of an IEEE 802.11 architecture are as follows −
Stations (STA) − Stations comprises of all devices and equipment that are connected to the
wireless LAN. A station can be of two types−

Wireless Access Point (WAP) − WAPs or simply access points (AP) are generally wireless
routers that form the base stations or access.

Client. Clients are workstations, computers, laptops, printers, smartphones, etc.

Each station has a wireless network interface controller.

Basic Service Set (BSS) − A basic service set is a group of stations communicating at the
physical layer level. BSS can be of two categories depending upon the mode of operation−

Infrastructure BSS − Here, the devices communicate with other devices through access points.

Independent BSS − Here, the devices communicate in a peer-to-peer basis in an ad hoc

manner.

Extended Service Set (ESS) − It is a set of all connected BSS.

Distribution System (DS) − It connects access points in ESS.

Frame Format of IEEE 802.11

The main fields of a frame of wireless LANs as laid down by IEEE 802.11 are −

Frame Control − It is a 2 bytes starting field composed of 11 subfields. It contains control

information of the frame.

Duration − It is a 2-byte field that specifies the time period for which the frame and its
acknowledgment occupy the channel.
Address fields − There are three 6-byte address fields containing addresses of source,
immediate destination, and final endpoint respectively.

Sequence − It a 2 bytes field that stores the frame numbers.

Data − This is a variable-sized field that carries the data from the upper layers. The maximum
size of the data field is 2312 bytes.

Check Sequence − It is a 4-byte field containing error detection information.

Q2) Explain the Physical Layer and services offered by IEEE 802.11.
Ans :- IEEE 802.11 Services
The 802.11 standard defines services that provide the functions that the LLC layer requires for
sending MAC Service Data Units (MSDUs) between two entities on the network. These
services, which the MAC layer implements, fall into two categories:

Station Services These include Authentication, Deauthentication, Privacy, and MSDU delivery.

Distribution System Services These include Association, Disassociation, Distribution,

Integration, and Reassociation.

The following sections define the station and distribution system services.

Station Services
The 802.11 standard defines services for providing functions among stations. A station may be
within any wireless element on the network, such as a handheld PC or handheld scanner. In
addition, all access points implement station services. To provide necessary functionality, these
stations need to send and receive MSDUs and implement adequate levels of security.

Authentication
Because wireless LANs have limited physical security to prevent unauthorized access, 802.11
defines authentication services to control LAN access to a level equal to a wired link. Every
802.11 station, whether part of an independent BSS or an ESS network, must use the
authentication service prior to establishing a connection (referred to as an association in 802.11
terms) with another station with which it will communicate. Stations performing authentication
send a unicast management authentication frame to the corresponding station.

The IEEE 802.11 standard defines the following two authentication services:
Open system authentication This is the 802.11 default authentication method. It is a very simple
two-step process. First the station wanting to authenticate with another station sends an
authentication management frame containing the sending station's identity. The receiving station
then sends back a frame indicating whether it recognizes the identity of the authenticating
station.

Shared key authentication This type of authentication assumes that each station has received a
secret shared key through a secure channel independent from the 802.11 network. Stations
authenticate through shared knowledge of the secret key. Use of shared key authentication
requires implementation of the Wired Equivalent Privacy algorithm (WEP).

Deauthentication
When a station wants to disassociate from another station, it invokes the deauthentication
service. Deauthentication is a notification and cannot be refused. A station performs
deauthentication by sending an authentication management frame (or group of frames to
multiple stations) to advise of the termination of authentication.

Privacy
With a wireless network, all stations and other devices can hear data traffic taking place within
range on the network, seriously affecting the security level of a wireless link. IEEE 802.11
counters this problem by offering a privacy service option that raises the security level of the
802.11 network to that of a wired network.

The privacy service, applying to all data frames and some authentication management frames,
is based on the 802.11 Wired Equivalent Privacy (WEP) algorithm that significantly reduces
risks if someone eavesdrops on the network.

Q3) (a)What are the benefits of adhoc networ Wireless networks.

(b)Explain the applications of adhoc Wireless network.
Ans :- The Advantages of Ad Hoc Networks
Ad hoc networks are wireless connections between two or more computers and/or wireless
devices (such as a Wi-Fi enabled smart phone or tablet computer). A typical wireless network is
based on a wireless router or access point that connects to the wired network and/or Internet.
An ad hoc network bypasses the need for a router by connecting the computers directly to each
other using their wireless network adapters.

Router Free
Connecting to files on other computers and/or the Internet without the need for a wireless router
is the main advantage of using an ad hoc network. Because of this, running an ad hoc network
can be more affordable than a traditional network---you don't have the added cost of a router.
However, if you only have one computer an ad hoc network won't be possible.

Mobility
Ad hoc networks can be created on the fly in nearly any situation where there are multiple
wireless devices. For example: emergency situations in remote locations make a traditional
network nearly impossible, but "The medical team can utilize 802.11 radio NICs in their laptops
and PDAs and enable broadband wireless data communications as soon as they arrive on the
scene."

Speed
Creating an ad hoc network from scratch requires a few settings changes and no additional
hardware or software. If you need to connect multiple computers quickly and easily, then an ad
hoc network is an ideal solution.
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(b)Explain the applications of adhoc Wireless network.
Ans:- In the Windows operating system, ad hoc is a communication mode (setting) that allows
computers to directly communicate with each other without a router. Wireless mobile ad hoc
networks are self-configuring, dynamic networks in which nodes are free to move.

Such wireless networks lack the complexities of infrastructure setup and administration,
enabling devices to create and join networks "on the fly".[3]

Each device in a MANET is free to move independently in any direction, and will therefore
change its links to other devices frequently. Each must forward traffic unrelated to its own use,
and therefore be a router. The primary challenge in building a MANET is equipping each device
to continuously maintain the information required to properly route traffic.
This becomes harder as the scale of the MANET increases due to
1) the desire to route packets to/through every other node,
2) the percentage of overhead traffic needed to maintain real-time routing status,
3) each node has its own goodput to route independent and unaware of others needs, and
4) all must share limited communication bandwidth, such as a slice of radio spectrum.
Such networks may operate by themselves or may be connected to the larger Internet. They
may contain one or multiple and different transceivers between nodes. This results in a highly
dynamic, autonomous topology. MANETs usually have a routable networking environment on
top of a link layer ad hoc network.

Q4) Explain and compare the media access control mechanism of DCF Methods adopted
in IEEE 802.11 WEAN.
Ans :- Distributed coordination function (DCF) is a mandatory technique used to prevent
collisions in IEEE 802.11-based WLAN standard (Wi-Fi). It is a medium access control (MAC)
sublayer technique used in areas where carrier-sense multiple access with collision avoidance
(CSMA/CA) is used.

Technique
The basic technique of DCF is as follows −

When a station has a frame to transmit, it waits for a random backoff time. The random backoff
time is defined by a contention window having a random number of time slots. The backoff time
is given by the following equation −

Timebackoff = random()×Timeslots

Here, the function generates a random number and is the time period for one slot.

If the station senses that the channel is busy during the contention period, it pauses its timer till
the channel is clear.

At the end of the backoff period, if the channel is clear, the station will wait for an amount of time
equal to DIFS (Distributed Inter-Frame Space) and sense the channel again.

If the channel is still clear, the station transmits a RTS (request to send) frame.

The destination station responds using a CTS (clear to send) frame if it is available.

Then the transmitting station sends the data frames.

After the frames are sent, the transmitting station waits for a time equal to SIFS (Short
Inter-Frame Space) for the acknowledgement.

At the end of this transmission process, the station again waits for the backoff time before the
next transmission.

The technique is shown in the following diagram −

Q5) Draw and explain WAP Architecture in detail.

Ans:-
WAP architecture & WAP services
Wireless Application Protocol -
The Wireless Application Protocol (WAP) is a worldwide standard for the delivery and
presentation of wireless information to mobile phones and other wireless devices. The idea
behind WAP is simple: simplify the delivery of Internet content to wireless devices by delivering
a comprehensive, Internet-based, wireless specification. The WAP Forum released the first
version of WAP in 1998. Since then, it has been widely adopted by wireless phone
manufacturers, wireless carriers, and application developers worldwide. Many industry analysts
estimate that 90 percent of mobile phones sold over the next few years will be WAP-enabled.

WAP architecture
WAP services

1. WAP Architecture
i. It provides a scalable and extensible environment for application development of mobile
ii. This is achieved using layered design of protocol stack. The layers resemble the layers of OSI
model.
iii. Each layer is accessible by layers above as well as by other services and applications
through a set of well defined interface.
iv. External applications may access session, transaction, security and transport layers directly.

2. Wireless Application Environment

i. WAE is the uppermost layer in the WAP stack. It is general purpose environment based on
combination of WWW and mobile telephony technologies.
ii. Its primary objective is to achieve interoperable environment that allows operators and service
providers to build applications that can reach wide variety of wireless platforms.
iii. It uses URL and URI for addressing. Language used is WML and WML script. WML script
can be used for validation of user input.

3. Wireless Telephony Application

i. WTA provides a means to create telephony services using WAP. It uses WTA Interface (WTAI)
which can be evoked from WML and for WML script.
ii. The Repository makes it possible to store WTA services in device which can be accessed
without accessing the network. The access can be based on any event like call disconnect, call
answer etc.
iii. Sometimes, there can be notification to user based on which WTA services are accessed by
users. The notification is called WTA service indication.

4. Wireless Session Protocol.

i. WSP provides reliable, organized exchange of content between client and server.
ii. The core of WSP design is binary form of HTTP. All methods defined by HTTP 1.1 are
supported.
iii. Capability negotiation is used to agree on common level of protocol functionality as well as to
agree on a set of extended request methods so that full compatibility to HTTP applications can
be retained.
iv. An idle session can be suspended to free network resources and can be resumed without
overload of full-blown session establishment.
v. WSP also supports asynchronous requests. Hence, multiple requests will improve utilization
of air time.

5. Wireless Transaction Protocol

i. WTP is defined as light-weight transaction-oriented protocol suitable for implementation in thin
clients.
ii. Each transaction has unique identifiers, acknowledgements, duplicates removal and
retransmission.
iii. Class 1 and Class 2 enable user to confirm every received message, however, in class 0,
there is no acknowledgement.
iv. WTP has no security mechanisms and no explicit connection set-up or tear-down phases.

6. Wireless Transport Layer Security

i. WTLS is security protocol based on industry standard transport layer security (TLS). It
provides transport layer security between a WAP client and the WAP Gateway/ Proxy.
ii. The goals of WTLS are data integrity, privacy, authentication, Denial-of-service protection.
iii. It has features like datagram support, optimized handshake and dynamic key refreshing.

7. Wireless Datagram Protocol

i. WDP provides application addressing by port numbers, optional segmentation and
reassembly, optional error detection.
ii. It supports simultaneous communication instances from higher layer over a single underlying
WDP bearer service. The port number identifies higher level entity above WDP.
iii. The adaptation layer of WDP maps WDP functions directly on to a bearer based on its
specific characteristics.
iv. On the GSM SMS, datagram functionality is provided by WDP.

8. Optimal WAP Bearers

i. The WAP is designed to operate over a variety of different service like SMS,’ Circuit Switched
Data (CSD)’, GPRS,’ Unstructured Supplementary Services Data(USSD)’.

2. WAP services
In addition to a new protocol stack, WAP 2.x introduced many other new features and services.
These new features expand the capabilities of wireless devices and allow developers to create
more useful applications and services. The following is a summary of the features of interest:
WAP Push. WAP Push enables enterprises to initiate the sending of information on the server
using a push proxy. This capability was introduced in WAP 1.2, but has been enhanced in WAP
2.x. Applications that require updates based on external information are particularly suited for
using WAP Push. Examples include various forms of messaging applications, stock updates,
airline departure and arrival updates, and traffic information. Before WAP Push was introduced,
the wireless user was required to poll the server for updated information, wasting both time and
bandwidth.
User Agent Profile (UAProf). The UAProf enables a server to obtain information about the client
making the request. In WAP 2.x, it is based on the Composite Capabilities/Preference Profiles
(CC/PP) specification as defined by the W3C. It works by sending information in the request
object, allowing wireless servers to adapt the information being sent according to the client
device making the request.
External Functionality Interface (EFI). This allows the WAP applications within the WAE to
communicate with external applications, enabling other applications to extend the capabilities of
WAP applications, similar to plug-ins for desktop browsers.
Wireless Telephony Application (WTA). The WTA allows WAP applications to control various
telephony applications, such as making calls, answering calls, putting calls on hold, or
forwarding them. It allows WAP WTA-enabled cell phones to have integrated voice and data
services.
Persistent storage interface. WAP 2.x introduces a new storage service with a well-defined
interface to store data locally on the device. The interface defines ways to organize, access,
store, and retrieve data.
Data synchronization. For data synchronization, WAP 2.x has adopted the SyncML solution. As
outlined in Chapter 10, "Enterprise Integration through Synchronization," SyncML provides an
XML-based protocol for synchronizing data over both WSP and HTTP.
Multimedia Messaging Service (MMS). MMS is the framework for rich-content messaging.
Going beyond what is possible for SMS, MMS can be used to transmit multimedia content such
as pictures and videos. In addition, it can work with WAP Push and UAProf to send messages
adapted specifically for the target client device.

Q6) (a) What is direct sequence spread

Spectrum (D555).
(b) Give the features of D555.

Ans :- Direct sequence spread spectrum (DSSS) is a transmission technology used in local
area wireless network transmissions. In this technology, a data signal at the sending station is
combined with a high data rate bit sequence, which divides user data based on a spreading
ratio.

The benefits of using DSSS are resistance to jamming, sharing single channels among multiple
users, less background noise and relative timing between transmitter and receivers.

This term is also known as direct sequence code division multiple access.
TABLE OF CONTENTS
Direct sequence spread spectrum (DSSS) is a transmission technology used in local area
wireless network transmissions. In this technology, a data signal at the sending station is
combined with a high data rate bit sequence, which divides user data based on a spreading
ratio.

The benefits of using DSSS are resistance to jamming, sharing single channels among multiple
users, less background noise and relative timing between transmitter and receivers.

This term is also known as direct sequence code division multiple access.

Advertisement

Techopedia Explains Direct Sequence Spread Spectrum (DSSS)

DSSS is a spread spectrum modulation technique used for digital signal transmission over
airwaves. It was originally developed for military use, and employed difficult-to-detect wideband
signals to resist jamming attempts. It is also being developed for commercial purposes in local
and wireless networks.

The stream of information in DSSS is divided into small pieces, each associated with a
frequency channel across spectrums. Data signals at transmission points are combined with a
higher data rate bit sequence, which divides data based on a spreading ratio. The chipping code
in a DSSS is a redundant bit pattern associated with each bit transmitted. This helps to increase
the signal's resistance to interference. If any bits are damaged during transmission, the original
data can be recovered due to the redundancy of transmission.

The entire process is performed by multiplying a radio frequency carrier and a pseudo-noise
(PN) digital signal. The PN code is modulated onto an information signal using several
modulation techniques such as quadrature phase-shift keying (QPSK), binary phase-shift keying
(BPSK), etc. A doubly-balanced mixer then multiplies the PN modulated information signal and
the RF carrier. Thus, the TF signal is replaced with a bandwidth signal that has a spectral
equivalent of the noise signal. The demodulation process mixes or multiplies the PN modulated
carrier wave with the incoming RF signal. The result produced is a signal with a maximum value
when two signals are correlated. Such a signal is then sent to a BPSK demodulator. Although
these signals appear to be noisy in the frequency domain, bandwidth provided by the PN code
permits the signal power to drop below the noise threshold without any loss of information.

Q7)Explain with an example the DSR routine protocol. And discuss how power 802.11
infrastructure architecture.

And :- The Dynamic Source Routing protocol (DSR) is a simple and efficient routing protocol
designed specifically for use in multi-hop wireless ad hoc networks of mobile nodes. DSR allows
the network to be completely self-organizing and self-configuring, without the need for any
existing network infrastructure or administration.
DSR has been implemented by numerous groups, and deployed on several testbeds. Networks
using the DSR protocol have been connected to the Internet. DSR can interoperate with Mobile
IP, and nodes using Mobile IP and DSR have seamlessly migrated between WLANs, cellular
data services, and DSR mobile ad hoc networks.

The protocol is composed of the two main mechanisms of "Route Discovery" and "Route
Maintenance", which work together to allow nodes to discover and maintain routes to arbitrary
destinations in the ad hoc network. All aspects of the protocol operate entirely on-demand,
allowing the routing packet overhead of DSR to scale automatically to only that needed to react
to changes in the routes currently in use.

The protocol allows multiple routes to any destination and allows each sender to select and
control the routes used in routing its packets, for example for use in load balancing or for
increased robustness. Other advantages of the DSR protocol include easily guaranteed
loop-free routing, support for use in networks containing unidirectional links, use of only "soft
state" in routing, and very rapid recovery when routes in the network change. The DSR protocol
is designed mainly for mobile ad hoc networks of up to about two hundred nodes, and is
designed to work well with even very high rates of mobility.
DSR is officially defined by an Internet-Draft. As of August 2003, the most current is "The
Dynamic Source Routing Protocol for Mobile Ad Hoc Networks (DSR)"

The most current explanation of the DSR protocol can be found in David B. Johnson, David A.
Maltz, and Josh Broch.

To achieve a long run-time for battery-operated portable electronic devices that incorporate
wireless transceivers, efficient power management of the radio is a critical requirement. The
power management function of IEEE 802.11 wireless local area networks (WLANs) allows
stations (STAs) to operate in the doze mode so that their power consumption is significantly
reduced. Hence, efficient algorithms to manage when and how often a STA enters and exits
doze mode are crucial to battery-operated STAs. We address this problem by developing a
novel model for stochastic analysis of timer-based power management in infrastructure IEEE
802.11 WLANs. Based on this model, the probabilities that a STA is active, idle, or dozing are
derived, and the power consumption of the STA, number of frames buffered, and average delay
per frame are obtained. These results enable an efficient power management algorithm that
optimizes the idle timer and doze duration at the STA and the frame buffer at the access point.
Moreover, similar statistics for the basic power management method in the IEEE 802.11
standard are derived as a special case of the proposed timer-based power management
scheme. Numerical results are presented to demonstrate the effectiveness of the proposed
algorithms.

Q8) Discussin detail about spread spectrum techniques.

Ans:- A collective class of signaling techniques are employed before transmitting a signal to
provide a secure communication, known as the Spread Spectrum Modulation. The main
advantage of spread spectrum communication technique is to prevent “interference” whether it
is intentional or unintentional.

The signals modulated with these techniques are hard to interfere and cannot be jammed. An
intruder with no official access is never allowed to crack them. Hence, these techniques are
used for military purposes. These spread spectrum signals transmit at low power density and
has a wide spread of signals.

Pseudo-Noise Sequence
A coded sequence of 1s and 0s with certain auto-correlation properties, called as Pseudo-Noise
coding sequence is used in spread spectrum techniques. It is a maximum-length sequence,
which is a type of cyclic code.

Narrow-band and Spread-spectrum Signals

Both the Narrow band and Spread spectrum signals can be understood easily by observing their
frequency spectrum as shown in the following figures.

Narrow-band Signals
The Narrow-band signals have the signal strength concentrated as shown in the following
frequency spectrum figure.

Following are some of its features −

Band of signals occupy a narrow range of frequencies.

Power density is high.
Spread of energy is low and concentrated.
Though the features are good, these signals are prone to interference.

Spread Spectrum Signals

The spread spectrum signals have the signal strength distributed as shown in the following
frequency spectrum figure.

Following are some of its features −

Band of signals occupy a wide range of frequencies.

Power density is very low.
Energy is wide spread.
With these features, the spread spectrum signals are highly resistant to interference or jamming.
Since multiple users can share the same spread spectrum bandwidth without interfering with
one another, these can be called as multiple access techniques.

FHSS and DSSS / CDMA

Spread spectrum multiple access techniques uses signals which have a transmission bandwidth
of a magnitude greater than the minimum required RF bandwidth.

These are of two types.

Frequency Hopped Spread Spectrum FHSS

Direct Sequence Spread Spectrum DSSS

This is frequency hopping technique, where the users are made to change the frequencies of
usage, from one to another in a specified time interval, hence called as frequency hopping. For
example, a frequency was allotted to sender 1 for a particular period of time. Now, after a while,
sender 1 hops to the other frequency and sender 2 uses the first frequency, which was
previously used by sender 1. This is called as frequency reuse.

The frequencies of the data are hopped from one to another in order to provide a secure
transmission. The amount of time spent on each frequency hop is called as Dwell time.

Q9) Briefly explain about the system and protocol architecture of 802.11.
Ans :- Wireless LANs are those Local Area Networks that use high frequency radio waves
instead of cables for connecting the devices in LAN. Users connected by WLANs can move
around within the area of network coverage. Most WLANs are based upon the standard IEEE
802.11 or WiFi.

IEEE 802.11 Architecture

The components of an IEEE 802.11 architecture are as follows

1) Stations (STA) − Stations comprise all devices and equipments that are connected to the
wireless LAN. A station can be of two types:

Wireless Access Pointz (WAP) − WAPs or simply access points (AP) are generally wireless
routers that form the base stations or access.
Client. − Clients are workstations, computers, laptops, printers, smartphones, etc.
Each station has a wireless network interface controller.

2) Basic Service Set (BSS) −A basic service set is a group of stations communicating at
physical layer level. BSS can be of two categories depending upon mode of operation:
Infrastructure BSS − Here, the devices communicate with other devices through access points.
Independent BSS − Here, the devices communicate in peer-to-peer basis in an ad hoc manner.
3) Extended Service Set (ESS) − It is a set of all connected BSS.

4) Distribution System (DS) − It connects access points in ESS.

Advantages of WLANs
They provide clutter free homes, offices and other networked places.
The LANs are scalable in nature, i.e. devices may be added or removed from the network at a
greater ease than wired LANs.
The system is portable within the network coverage and access to the network is not bounded
by the length of the cables.
Installation and setup is much easier than wired counterparts.
The equipment and setup costs are reduced.
Disadvantages of WLANs
Since radio waves are used for communications, the signals are noisier with more interference
from nearby systems.
Greater care is needed for encrypting information. Also, they are more prone to errors. So, they
require greater bandwidth than the wired LANs.
WLANs are slower than wired LANs.

Q10) Describe the architecture and protocol stack of Bluetooth technology.

Ans :-Bluetooth network technology connects mobile devices wirelessly over a short-range to
form a personal area network (PAN). The Bluetooth architecture has its own independent model
with a stack of protocols, instead of following the standard OSI model or TCP/IP model.

The protocols in the Bluetooth standard can be loosely grouped into the physical layer, data link
layer, middleware layer, and application layer as shown in the following diagram −

Protocols in the Bluetooth Protocol Architecture

Physical Layer − This includes Bluetooth radio and Baseband (also in the data link layer.
Radio − This is a physical layer equivalent protocol that lays down the physical structure and
specifications for transmission of radio waves. It defines air interface, frequency bands,
frequency hopping specifications, and modulation techniques.

Baseband − This protocol takes the services of radio protocol. It defines the addressing
scheme, packet frame format, timing, and power control algorithms.

Data Link Layer − This includes Baseband, Link Manager Protocol (LMP), and Logical Link
Control and Adaptation Protocol (L2CAP).

Link Manager Protocol (LMP) − LMP establishes logical links between Bluetooth devices and
maintains the links for enabling communications. The other main functions of LMP are device
authentication, message encryption, and negotiation of packet sizes.

Logical Link Control and Adaptation Protocol (L2CAP) − L2CAP provides adaption between
upper layer frame and baseband layer frame format. L2CAP provides support for both
connection-oriented as well as connectionless services.

Middleware Layer − This includes Radio Frequency Communications (RFComm) protocol,

adopted protocols, SDP, and AT commands.

RFComm − It is short for Radio Frontend Component. It provides a serial interface with WAP.

Adopted Protocols − These are the protocols that are adopted from standard models. The
commonly adopted protocols used in Bluetooth are Point-to-Point Protocol (PPP), Internet
Protocol (IP), User Datagram Protocol (UDP), Transmission Control Protocol (TCP), and
Wireless Application Protocol (WAP).

Service Discovery Protocol (SDP)− SDP takes care of service-related queries like device
information so as to establish a connection between contending Bluetooth devices.

AT Commands − ATtention command set.

Applications Layer − This includes the application profiles that allow the user to interact with the
Bluetooth applications.

Q11) (a) List and explain the inter-frame spacing

(b) State the significance of Radio transmission over infrared.
Ans:-Inter – frame Spaces
Inter − frame spaces (IFS) are waiting periods between transmission of frames operating
in the medium access control (MAC) sublayer where carrier-sense multiple access with
collision avoidance (CSMA/CA) is used. These are techniques used to prevent collisions
as defined in IEEE 802.11-based WLAN standard (Wi-Fi).
IFS is the time period between completion of the transmission of the last frame and
starting transmission of the next frame apart from the variable back-off period.

The diagram below shows the different types of inter − frame spacing starting from the
shortest duration (highest priority) to the longest duration (lowest priority). Among these,
DISF is mandatory and SISF is the next most common one.

Reduced Inter-frame Space (RISF)

Reduced Inter-frame Space is a very short duration inter − frame spacing used in high priority
frames, which is used to send a burst of frames. RISF was introduced in 802.11e QoS
amendment and has a duration of RISF of only 2μs.

When a station needs to send multiple frames, RISF is introduced between the individual
frames. It ensures that no other station finds a chance to occupy the channel within the frame
burst.

Short Inter-frame Space (SISF)

Short Inter-frame Spacing (SIFS), is the time interval required by a wireless device in between
receiving a frame and responding to the frame. It is used in Distributed coordination function
(DCF) scheme, which is a mandatory technique used to prevent collisions.

The duration of SIFS is equal to the sum of delays in radio frequency (RF), Physical Layer
Convergence Procedure (PLCP) and processing delay of MAC (medium access control) layer.

In IEEE 802.11 networks, SIFS is the inter-frame spacing maintained before and after the
transmission of an acknowledgment frame and Clear To Send (CTS) frame. The duration of
SISF is usually 10 μs.

PCF Inter-frame Space (PISF)

Point coordination function (PCF) is an optional technique used to prevent collisions in
centralised controlled WLANs. PCF is used additionally along with the mandatory distributed
coordination function (DCF).

The Access Point (AP) which coordinates the communications centrally, waits for PIFS duration
to grasp the channel. Since PIFS is less than DIFS duration, the AP always has the priority to
access the channel over the other stations. PISF is calculated as the sum of SISF and slot time.
DCF Inter-frame Space (DISF)
Distributed coordination function (DCF) is a mandatory technique used to prevent collisions in
IEEE 802.11-based WLAN standard (Wi-Fi). It is a medium access control (MAC) sublayer
technique used in areas where carrier-sense multiple access with collision avoidance
(CSMA/CA) is used.

Using DCF technique, a station needs to sense the status of the wireless channel before it can
place its request to transmit a frame. The time interval that a station should wait before it sends
its request frame is known as DCF Interframe Spacing (DIFS). DISF is calculated as the sum of
SISF and twice the slot time.

Arbitrary Inter-frame Space (AISF)

In arbitrary inter-frame spacing, the stations are prioritized based upon the Access Category, i.e.
the type of data to be transmitted. In this scheme, the waiting period (equal to the AISF) of a
wireless station, is shortened or expanded before the station can transmit its frame. Higher
priority stations are assigned shorter AISF. This implies that a higher priority station has to wait
for a shorter time interval before it can transmit its frame. This is particularly important in delay −
critical transmissions like video streaming or voice streaming.

AISF is calculated as – AISF number * slot time + SISF.

Extended Inter-frame Space (EISF)

Extended inter-frame spacing is an additional waiting period used in addition to the mandatory
DISF in case of corrupted frames.

( B) The Difference between Radio Waves and Infrared Light

According to NASA, radio waves have a longer wavelength than infrared light, in the
electromagnetic spectrum. Radio waves are able to reach the earth’s surface, unlike most
infrared light. Majority of what is contained in the universe, including people, emit infrared light.
Stars, the sun, black holes and neutron stars create magnetic fields, which creates radio waves.
According to Harvard University, radio waves and infrared light are fundamentally different in
wavelengths, frequency and energies of light. Radio waves are used for many things each day,
this includes communication, cell phone use, internet, cable television and security alarm
systems. Infrared light only functions through line of sight, whereas radio waves can effectively
be used from a long distance. A remote control uses infrared light to change the channel on
your TV, while radio waves are used to receive TV shows. If you want to communicate through
the use of infrared technology, you will not be regulated; however, if you want to operate a radio
station, you must obtain a Federal Communications Commission (FCC) license. According to
the National Radio Astronomy Observatory (NRAO), devices used to detect infrared light and
radio waves are designed differently because the wavelengths differ significantly.

Q12) Explain about DCF and PCF in IEEE 802.11 MAC layer.
Ans:- WLAN DCF vs PCF-Difference between DCF and PCF medium access types
WLAN stands for Wireless Local Area Network. It is a small network consisting of Access Points
and Stations. Access points are like Base stations and Stations are like clients or work stations
or PCs or laptops. Both DCF and PCF are 802.11 medium access types. DCF stands for
distributed coordination function and PCF stands for Point coordination function. In this page we
will see difference between DCF and PCF medium access types defined in IEEE 802.11 WLAN
standard. These are the mechanisms for the implementation of CSMA/CA in Wireless LAN.
CSMA/CA refers to Carrier Sense Multiple Access with Collision Avoidance.
Before we see difference between DCF and PCF; let us understand about carrier sense. The
carrier sense refers to sensing of the medium by the station to know whether the medium is
occupied by some other station or it is free to transmit. There are basically two mechanism to
sense the medium.
• Check layer-1 i.e. physical layer to check for carrier presence
• Virtual Carrier Sense function i.e. NAV(Network Allocation Vector)
Pls. note that once NAV is set, stations can update it further provided frame duration exceeds
currently set value of the NAV. If the frame duration is less than the NAV already set, it will not
be changed/altered.

WLAN DCF-Distributed Coordination Function

The DCF is the basis of the standard CSMA/CA access mechanism. Like Ethernet, it first
checks to see that the radio link is clear before transmitting. To avoid collisions, stations use a
random backoff after each frame, with the first transmitter seizing the channel. In some
circumstances, the DCF may use the CTS/RTS clearing technique to further reduce the
possibility of collisions.

Almost most of the wlan traffic uses DCF mac access type, this is because this access type is
same as standard ethernet which provides contention based service. DCF makes it possible for
multiple STAs to communicate without the need of central control. Hence it can be used in IBSS
as well as infrastructure network type. As mentioned earlier, each station first checks for
medium and transmits only when medium is idle. If the medium is not idle they will defer the
transmission and employ simple exponential back-off algorithm in order avoid collisions.
There are two main rules which can be applied for all the transmissions based on DCF concept.
These rules are as mentioned below.
WLAN PCF-Point Coordination Function
coordinators are used to ensure that the medium is provided without contention. Point
coordinators reside in access points, so the PCF is restricted to infrastructure networks. To gain
priority over standard contention based services, the PCF allows stations to transmit frames
after a shorter interval. The PCF is not widely implemented.

However, the IEEE designed the PCF so stations that implement only the distributed
coordination function (DCF) will interoperate with point coordinators.
Contention-free service is not provided full-time. Periods of contention-free service arbitrated by
the point coordinator alternate with the standard DCF-based service.

Q13) Explain the Addressing Mechanism in IEEE 802.11.

Ans :- MAC layer provides functionality for several tasks like control medium access, can also
offer support for roaming, authentication, and power conservation. The basic services provided
by MAC are the mandatory asynchronous data service and optional time-bounded service.
IEEE 802.11 defines two MAC sub-layers:-

Distributed Coordination Function (DCF) –

DCF uses CSMA/CA as access method as wireless LAN can’t implement CSMA/CD. It only
offers asynchronous service.
Point Coordination Function (PCF) –
PCP is implemented on top of DCF and mostly used for time-service transmission. It uses a
centralized, contention-free polling access method. It offers both asynchronous and
time-bounded service.
MAC Frame:
The MAC layer frame consists of 9 fields. The following figure shows the basic structure of an
IEEE 802.11 MAC data frame along with the content of the frame control field.

Frame Control(FC) –
It is 2 bytes long field which defines type of frame and some control information. Various fields
present in FC are:
Type:
It is a 2 bit long field which determines the function of frame i.e management(00), control(01) or
data(10). The value 11 is reserved.
Subtype:
It is a 4 bit long field which indicates sub-type of the frame like 0000 for association request,
1000 for beacon.
To DS:
It is a 1 bit long field which when set indicates that destination frame is for DS(distribution
system).
From DS:
It is a 1 bit long field which when set indicates frame coming from DS.
More frag (More fragments):
It is 1 bit long field which when set to 1 means frame is followed by other fragments.
Retry:
It is 1-bit long field, if the current frame is a retransmission of an earlier frame, this bit is set to 1.
Power Mgmt (Power management):
It is 1-bit long field that indicates the mode of a station after successful transmission of a frame.
Set to 1 the field indicates that the station goes into power-save mode. If the field is set to 0, the
station stays active.
More data:
It is 1-bit long field that is used to indicate receiver that a sender has more data to send than the
current frame. This can be used by an access point to indicate to a station in power-save mode
that more packets are buffered or it can be used by a station to indicate to an access point after
being polled that more polling is necessary as the station has more data ready to transmit.
WEP:
It is 1 bit long field which indicates that the standard security mechanism of 802.11 is applied.
Order:
It is 1 bit long field, if this bit is set to 1 the received frames must be processed in strict order.
Duration/ID –
It is 4 bytes long field which contains the value indicating the period of time in which the medium
is occupied(in µs).

Address 1 to 4 –
These are 6 bytes long fields which contain standard IEEE 802 MAC addresses (48 bit each).
The meaning of each address depends on the DS bits in the frame control field.
SC (Sequence control) –
It is 16 bits long field which consists of 2 sub-fields, i.e., Sequence number (12 bits) and
Fragment number (4 bits). Since acknowledgement mechanism frames may be duplicated
hence, a sequence number is used to filter duplicate frames.
Data –
It is a variable length field which contain information specific to individual frames which is
transferred transparently from a sender to the receiver(s).

CRC (Cyclic redundancy check) –

It is 4 bytes long field which contains a 32 bit CRC error detection sequence to ensure error free
frame.

Q14) Compare and contrast CSMA/CD with CSMA/CA.

Ans:- Difference between CSMA/CA and CSMA/CD
CSMA/CA
CSMA/CA stands for Carrier Sense Multiple Access/ Collision Avoidance. It is a network
protocol for transmission. It operates in the Medium Access Control Layer. This protocol is
effective before the collision.
CSMA/CD
CSMA/CD stands for Carrier Sense Multiple Access/ Collision Detection. It is also a network
protocol for transmission and operates in the Medium Access Control Layer. In this protocol,
each station senses the collision by broadcast sensing. In case of collision, the transmission is
stopped and they send a jam signal and then wait for a random time context before
retransmission.

The following are some of the important differences between CSMA/CA and CSMA/CD.

CSMA/CD CSMA/CA

CSMA / CD is effective after a collision. Whereas CSMA / CA is effective before a collision.

CSMA / CD is used in wired networks. Whereas CSMA / CA is commonly used in wireless

networks.

It only reduces the recovery time. Whereas CSMA/ CA minimizes the possibility of
collision.

CSMA / CD resends the data frame Whereas CSMA / CA will first transmit the intent to
whenever a conflict occurs. send for data transmission.

CSMA / CD is used in 802.3 standard. While CSMA / CA is used in 802.11 standard.

It is more efficient than simple While it is similar to simple CSMA(Carrier Sense

CSMA(Carrier Sense Multiple Access). Multiple Access).

Q15)(a) What is Time-Hopping spread spectrum (-MSS)?

(b) What is chirp spread spectrum (CSS)?
Ans :- To achieve LPI, the transmission time is changed randomly by varying the period and
duty cycle of the pulse (carrier) using a pseudo-random sequence. ... Spreading of the spectrum
is caused by other factors associated with TH, such as using pulses with low duty cycle having
a wide frequency response.
Different spread-spectrum techniques are available, but all have one idea in common: the key
(also called the code or sequence) attached to the communication channel. The manner of
inserting this code defines precisely the spread-spectrum technique. The term "spread
spectrum" refers to the expansion of signal bandwidth, by several orders of magnitude in some
cases, which occurs when a key is attached to the communication channel.

The formal definition of spread spectrum is more precise: an RF communications system in

which the baseband signal bandwidth is intentionally spread over a larger bandwidth by injecting
a higher frequency signal (Figure 1). As a direct consequence, energy used in transmitting the
signal is spread over a wider bandwidth, and appears as noise. The ratio (in dB) between the
spread baseband and the original signal is called processing gain. Typical spread-spectrum
processing gains run from 10dB to 60dB.

To apply a spread-spectrum technique, simply inject the corresponding spread-spectrum code

somewhere in the transmitting chain before the antenna (receiver). (That injection is called the
spreading operation.) The effect is to diffuse the information in a larger bandwidth. Conversely,
you can remove the spread-spectrum code (called a despreading operation) at a point in the
receive chain before data retrieval. A despreading operation reconstitutes the information into its
original bandwidth. Obviously, the same code must be known in advance at both ends of the
transmission channel. (In some circumstances, the code should be known only by those two
parties.)