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Mobile Computing





Mobile Computing








1.0 Introduction

1.1 Application

1.2 History of Wireless Communication

1.3 A simplified reference model

1.4 Cellular Systems


Protocol TCP and IP Suite

2.1 The need for Protocol Arch

2.2 The TCP/IP Protocol Architecture

2.3 Internetworking

2.4 Internet Protocol (IP)

2.5 Transmission Control Protocol (TCP)

2.6 User Datagram Protocol (UDP)


Medium Access Control

3.0 Introduction to MAC

3.1 Motivation for a specialized MAC

.2 Hidden and Exposed Terminals

3.3 Near and Far Terminals


3.5 Classical Aloha

3.6 Slotted Aloha

3.7 Types of Carrier Sense Multiple Access

3.8 MACA

3.8.1 Solution of MACA

3.8.2 Concept of Polling

3.8.3 ISMA

3.9 Comparison of S/T/F and CDMA


Wireless LAN

4.1 Introduction

4.2Goals,Advantages & Disadvantages

4.3Infrared Versus Radio Transmission

4.4. Infrastructure of Adhoc Networks

4.5 IEEE 802

4.5.1Architecture of IEEE 802.11 and Adhoc
Wireless LAN’

4.6 Protocol Architecture

4.7 MAC Management



5.1 Bluetooth Introduction

5.2 Bluetooth Applications

5.3 Architecture

5.4 Concepts of Networking and Bluetooth

5.5 Piconet

5.6 Bluetooth Scartternet

5.7 Protocol Stack

5.8 Core Protocols of B

5.9 L2CAP

5.10 Bluetooth Security

5.11 Power Management

Mobile Computing



Mobile Transport Layer

6.1 Introduction

6.2 Differences of UDP and TCP

6.3 Traditional TCP

6.3.1 Congestion Control

6.3.2 Slow Start

6.3.3 Fast Retransmit and Fast Recover

6.3.4 Implications on Mobility

6.4 Classical TCP Improvement

6.5 Indirect TCP(I

6.5.1 Advantages of I

6.5.2 Disadvantages of I

6.5.3 Snooping TCP


Mobile Network Layer

7.1 Introduction to Mobile IP

7.2 Entities and Term

7.3 IP Packet Delivery

7.4 Agent Discovery

7.5 Agent Solicitation

7.6 Registration

Mobile Computing


Chapter 1


1.0 Introduction

There would not be any accurate prediction for how computers and the networks will be in future.
Definitely it
would be more and more wireless and mobile. In today’s world many devices are wireless
and mobile for instance cameras, mobile phones, CD
players, headsets, keyboards etc.At this juncture
we need to know many jargons of this mobile world.


User Mobility

This refers to user who has access to the same or similar telecommunication services at different
places. i.e., the user may be mobile or moving and the services will follow him or her.


Device Portability

Here the communication device moves (may be with

or without a user).Typical example is our mobile
phone system, where the system itself hands the device from one base station (also called as Radio
Transmitter) to the other if the signal becomes too weak.

The Mobile Communication here uses user mobility

and the device portability at the same time, with
this regard to the device, the term wireless is used. In case of wireless the communication is not by
wire and is replaced by the transmission of electromagnetic waves through the air.

Any communication s
ystem can be broadly classified into the following 4 characteristics:


Fixed and Wired

This is our typical desktop at home, office or college. It is the weight and power consumption of

these desktop which makes them immobile. These devices use fi
xed networks.


Mobile and Wired

Many of today’s laptop fall into this category and users carry the laptop from one hotel to the

other, reconnecting to the company’s network via the telephone network and a modem.


Fixed and Wireless

In the histo
rical buildings and at trade shows they use fixed & wireless category of communication

system to avoid damages to the building due to the installation of wires.


Mobile and Wireless

This is the most interesting case; no cable restricts the user,
who can roam between different

wireless networks.

1.1 Applications

Many applications can benefit from wireless networks and mobile communication. Some of them are
listed below:


Vehicles(Road Traffic)

Today’s cars already have many te
chnologies but future cars will have wireless communication

systems and mobility aware applications. Music, news, road conditions, weather reports and

other broadcast information are received via the digital audio broadcasting (DAB)
with 1.5 Mbs

Mobile Computing


For personal communication a universal mobile telecommunication system (UMTS) phone may

be available for voice and data connectivity with 348kbps.For remote areas, satellite

communication can be used while th
e current position

of the car is determined via the

global positioning system (GPS). Cars driving in

the same area build a local ad
hoc network for

the fast exchange of information in emergency situation or help each other keep a sa
fe distance.

Many cars already have this technology, in future we may have cars which slows down even

before the driver recognizes the accident.



If we have a high quality wireless system in the ambulance which give t
he vital information

about the injured person’s situation and the necessary arrangements can be done in the

hospital before the patient /victim arrives.

Wireless networks are the only means of

communication in case of the natural


In wo
case only decentralized wireless

hoc network services.



A salesman job in today’s world needs lot of traveling .He may need
instant access

to the company’s

database for a presentation, for this he always carriers a

laptop which
would be his mobile office, but

there must be efficient and powerful

synchronization mechanisms are needed to ensure data



Replacement of wired networks

In some cases, wireless networks can also be used to replace wired netwo
rks like remote sensors, for

tradeshows or in historic buildings. For weather forecasts it could use satellite connections

Tradeshows needs a highly dynamic infrastructure for the old historical buildings they use wireless

access points in co
rners of the rooms.


Information and more

When you are traveling you can have a travel guide about the place either in your Laptop or a CD

or a DVD got via the internet at home; but the wireless network can provide you up

n about the appropriate location via the GPS or by contacting a local base station. Another

growing field of wireless network applications lies in entertainment and games.


Location dependent services

Many research efforts in mobile computing an
d wireless networks try to hide the fact that the

network access has been changed (e.g. from mobile phones to WLAN or between different

access points) or that a wireless link is more error prone than a wired one.

Mobile and Wireless Devices


following list gives some examples of mobile and wireless devices graded by increasing


A sensor transmitting state information represents a very simple wireless device.

E.g.: Switch sensing the office door.

Mobile Computing


Embedded Controllers:

ny appliances have a simple or sometimes more complex controller. Keyboards, mice, headsets
coffee machines, hair dryers are some examples. Why not have the hair dryer as a simple mobile and
wireless device that is able to communicate with the mobile phone


As a very simple receiver, a pager can only display short text message, has a tiny display and cannot
send any messages; where as the mobile’s SMS has replaced pagers. Still pages may be useful where it
may be necessary to page a large number if u
sers reliably within short time.

Mobile Phones

Traditional mobile phones have only simple black and white text display which send and receive voice
and short text messages .Today mobile phones has graphic color display ,touch screen and internet

Personal Digital Assistant (PDA)

PDA’s typically accompany a user and offer simple version of office software like calendar, notepad
and software packages like web browser and input device like a pen. It has inbuilt character
recognition translating hand
writing into characters.

Pocket Computer

A pocket computer with tiny keyboards, color displays and the simple programs like text
spreadsheets on the desktop.

Notebook /Laptop

Laptops more or less have the same performance as a standard deskto
p but technically its size &
weight are less or small, it runs on a
battery. If

operated via sensitive display like touch screen then
device is known as notepads or tablet PC’s.

1.2 History of Wireless Communication

The use of light for wireless communic
ation has roots in the ancient times. We will view the different
stages in the history of the wireless communication as several landmarks as described below:

Landmark 1:

Formerly ,”mirrors” were used to create on/off in the light pattern, similarly flags
were used to signal
code words.

Use of light and flags for wireless communication remained important for the navy until radio
transmission was introduced; this system is followed even today by many sailors if all other means of
wireless communication fail.

Landmark 2:

By the end of 18

Century Claude Chappe invented the optical telegraph; as a result the long distance
wireless communication was possible with technical means.

Landmark 3:

Alexander Graham Bell (1876) invented and marketed the wired communi
cation system by the name
“telephone” .The first commercial telegraph line was between Washington and Baltimore in 1843.

Philip Reis (1843
1847) discovered the telephone principle in 1861.

The first regular public voice and video service i.e., our multime
dia was already available in 1986
between Berlin & Leipzig.

Generally all optical transmission system suffers from the obstacle of shadows, rain and fog which
make communication almost impossible. But today by means of the laser all these are overcome.

Mobile Computing


andmark 4:

The wireless communication era started by the efforts of Guglielmo Marconi (1874

He gave the first demonstration of wireless telegraphy in 1895 using long wave transmission with very
high transmission power. The first transatlantic trans
mission followed in 1901.

Landmark 5:

Then the first radio broadcast took place in 1906 when Reginald A. Fessenden (1866

transmitted voice and music for Christmas.

Landmark 6:

The first commercial radio station started in 1920 (KDKA from Pittsbur
gh). Here the sender and
receiver need huge antennas and high transmission power.

Landmark 7:

One of the first “mobile” transmitters was on board by Zeppelin in 1911.

Landmark 8:

On big slip forward in this respect was the invention of frequency modulati
on in 1993 by Edwin .H.
Armstrong. After the Second World War, many national and international projects in this area of
wireless communications were triggered off.

Landmark 9:

The Northern European countries of Denmark, Finland, Norway and Sweden agreed
upon the Nordic
Mobile Telephone (NMT) System.

The analogue NMT uses a 450 MHz carrier and is still the only available system for mobile
communication in some very remote places.

Landmark 10:

The pan
European Mobile phone standard evolved which found a
new development by the new
Groupe Speciale Mobile(GSM) which uses new spectrum of 900Mhz,allows roaming throughout
Europe, is fully digital and offers voice and data service.

In 1983, the US system advanced mobile phone system (AMPS) started working at 85
0 MHz.

Landmark 11:

Telephone at home went wireless with the standard CT (Cordless Telephone) in 1984.

Landmark 12:

After many years of discussion and field trials, GSM was standardized. The first version of GSM now
MUNICATION worked at 900 MHz and uses 124 full
duplex channels, it also offers full international roaming, automatic location services, authentication,
encryption on the wireless link, efficient interoperation with ISDN systems and relatively high audio

Landmark 13:

It was soon discovered that the analog AMPS in US and the digital GSM at 900 MHz in Europe are not
sufficient for the high user densities in cities. While in the US, no new spectrum was allocated for a
new system, in Europe a new freq
uency band at 1800 MHz was chosen. In the Us, different
companies developed different new, more bandwidth technologies to operate side
side with AMPS
in the same frequency band. There was birth of 3 incompatible systems, the analog narrowband AMPS
two digital systems TDMA and CDMA.

Mobile Computing


Landmark 14:

19918 marked the beginning of mobile communications using the satellite with Iridium system. The
Iridium marked the beginning of small truly portable mobile satellite telephones including data service.

Landmark 15:

1990 saw several more powerful WLAN standards. IEEE published 802.11b offering 11Mbit/s at
2.4GHz .The same spectrum is used by Bluetooth, which is a short range technology to set

wireless personal area networks with gross data rate less
than 1Mbit/s.

The wireless application protocol (WAP) started at the same time as i
mode in Japan, while WAP did
not succeed at the beginning; i
mode became a tremendous success.

Landmark 16:

The year 2000, came with higher data rates and packet

transmission for GSM. This is the year
which found lot of hype about the communication business. Most of the hype is over, but the third
generation of mobile communication started in 2001 in Japan with the FDMA service.

IEEE released a new WLAN standard
802.11a, operating at 5GHz and offering gross data rate of

Landmark 17:

In 2002 new WLAN developments followed.

1.3 A Simplified Reference Model

It uses the basic reference model, which is used to structure communication systems.

Fig1. S
hown below is a Personal Digital Assistant (PDA), which

provides an example for
a wireless and portable device.

The PDA communicates with a base station in the middle of the picture. The Base station
consists of a radio transceiver (sender and receiver) a
nd an interworking unit connecting the
wireless link with the fixed

The communication partner of the PDA a conventional computer on the right side of the

It also shows the protocol stack implemented in the according to the reference model.

The PDA and the computers which are the end
systems needs the full protocol stack
comprising of the Application Layer, Transport Layer , Network Layer ,Data Link and the
Physical Layer.

These two communicate with each other using the Ap
plication Layer, which takes the
services provided by the lower layer. The internetworking unit, which are intermediate system
directly in this scenario

Mobile Computing


Fig.1.3 Personal Digital Assi
stant for Wireless and Portable Devices

Functions of each layer

Physical Layer

Lowest layer in a communication system

Converts a stream of bits into signals that can be transmitted on the sender side.

The physical layer of the receiver then transforms th
e signal back into a bit stream.

For wireless communication, the physical layer is responsible for frequency selection,
generation of the carrier frequency, signal detection.

Data Link Layer

The main tasks of this layer include accessing the medium,

multiplexing of different data
streams, correction of transmission errors and synchronization.

This layer is responsible for a reliable point
point connection between two devices or a
multipoint connection between one sender and several receiv


Base Station

Conventional Computer




Data Link





Data Link




Data Link

Data Link




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Network Layer

This is responsible for routing packers through a network or establishing a connection
between two entities over many other intermediate systems.

Important topics are addressing, routing, device location and handover between different

Transport Layer

This layer is used in the reference model to establish an end
end connection.

Topics like quality of service, flow and congestion control are relevant, especially if the
transport protocols known from the internet, TCP and UDP
are to be used over a wireless link.

Application Layer

This layer is situates on top of all transmission
oriented layers.

Topics of interest in this context are service location, support for multimedia applications
adaptive applications that can handle t
he large variations in transmission characteristics and
wireless access to the World Wide Web using a portable device.

1.4 Cellular Systems

Cellular Systems for mobile communications implement SDM. Each transmitter generally called as base
station cove
rs a certain area called as cell. Cell radii could be 10mts to 10’s of kilometers in the

Fig. 1.4 Cellular System





































Mobile Computing


Cells are never perfect circles or hexagons,
but they depend on environment i.e., buildings, mountains,
valleys etc weather condition and sometimes even on system load.

Typical systems using this approach are mobile telecommunication systems, where a mobile station
within the cell around a base stat
ion communicates with this base station and vice versa.

To avoid interference, different transmitters within each other’s interference range use FDM. If
FDM is combined with TDM the hopping pattern has to be coordinated.

The general goal is never to use
the same frequency at the same time within the interference range.
The above Fig. 2a and 2b depicts the two possible models to create cell patterns with minimal
interference .The cells are combined in clusters. The first one has three cell clusters and the

one uses the seven cell clusters.

In real

life the transmission patterns differ and hexagon is used to illustrate the model in the
simplest way.

In this pattern we can observe that the frequency has repeated. The transmission power of a sender
as to be limited to avoid interference with the next cell using the same frequencies.

Further you could
sectorized antennas
to even reduce further interference

On case of traffic variation, assignment of fixed assignment of frequencies to cell cluster is

efficient, like a cell may have heavy load whereas a neighboring cell has light load then we could
think of borrowing the frequencies. That is cells with more traffic is dynamically allotted more
frequencies and this scheme is called as Borrowing Chan
nel Allocation (BCA) Scheme whereas
the fixed frequency is called as Fixed Channel Allocation (FCA) Scheme.

The FCA is used in the GSM system, as it is simpler to use, but needs careful traffic analysis
before installation.

We could also have DCA Dynamic

Channel Allocation Scheme where the frequencies can only be
borrowed, but it is also possible to freely assign frequencies to cell. But here the danger of
interference with cells using the same frequency exists.

Cellular systems uses CDM instead of FDM a
s it does not need elaborate channel allocation
schemes and complex frequency planning.

Here users are separated through the code they use and not through the frequency.

CDM cells are commonly not congested and are said to “
”. While a cell can cov
er a larger
area under a light load, it shrinks if the load increases. The reason for this is the growing noise
level if more users are in a cell.

The higher the noise, the higher is the path loss and transmission errors. Finally, mobile stations
away from the base station drop out of the cell.

Advantages of Cellular System with small cells

1. Higher Capacity

Implementing the SDM allows frequency reuse. If one transmitter is far away from another
(outside the interference range) it can reuse t
he same frequency. As most mobile phone systems assign
frequencies to certain users and this frequency is blocked for other users. But frequencies are a scare
resource and the number if concurrent users per cell are very limited.

Mobile Computing


Huge cells do not allow mo
re users, they limit to less possible user/km2. That is why in cities where
there are many mobile phone users, there are no huge cells but rather many small cells.



The cellular systems are decentralized are thus more robust against the fail
ure of single
components. If one antenna fails, then it affects only small area.

3. Less

Transmission Power

Power is a big for mobile station than the base stations. A receiver far away from a base station
needs much more transmit power, but energy is s
erious problem for the mobile handheld devices.

4. Local

Interference Only

The long distances between sender and the receiver results in more interference problems. With small
cells the mobile stations and the base stations have to deal with ‘loc
al’ interference.

Disadvantages of small cells:


Infrastructure needed

Cellular systems need a complex infrastructure to connect all base stations. It needs many
antennas; switches for call forwarding, location register to find a mobile station etc, wh
ich makes the
whole concept expensive.


Handover needed

The mobile station has to perform a handover when changing from one cell to another. Depending
on the cell size and the speed of movements, this can happen quite often.


Frequency planning

To avoi
d interference between transmitters using the same frequencies need to be carefully
distributed. It must be taken care that interference need to be avoided as well as only a limited number
of frequencies is available.

Mobile Computing


Chapter 2


2.1 The need for Protocol Architecture

When two computers, terminals and /or any other data processing device exchange data the
procedure is quite complex. We know that there must be a high degree of co
operation between the two

computer systems. Instead of applying the logic for a single module, every task is broken into subtasks,
each of which is implemented separately.

In the protocol architecture, the modules are arranged in a vertical stack. Each layer in the stack
s a related subset of the functions required to communicate with another system. It relies on the
next lower layer to perform more primitive functions and to conceal the details of those functions. Ideally,
layers should be defined so that changes in one l
ayer do not require changes in other layer.

Communication is achieved by having the corresponding or peer layers in two systems
communicate. The peer layers communicated by means of formatted block dos data that obey a set of rules
or conventions known a
s a protocol.

The key features of the protocol are as follows:


Concerns the format of the data blocks


Includes control information for co
ordination and error handling


Include speed matching and sequencing

2.2 TCP/IP Protocol


About TCP/IP Protocol Architecture

This is the outcome of the research and development conducted on the ARPANET (an experimental

switched network) funded by the Defense Advanced Research Project Agency (DARPA). It is
generally called

as TCP/IP Protocol Suite. It has large collection of protocols that have been issued as
Internet Standards by the Internet Architecture Board (IAB).

TCP/IP Layers

Communication involves three agent applications, computer and networks. Examples of
ation include file transfer and electronic mail. Applications are concerned with exchange of data
between two computer systems.

Computers are connected to networks and the data to be exchanged are transferred by the network from one
computer to another. T
hus, the transfer of data from one applications to another involves first getting the
data to the computer in which the application resides and then getting the data to the intended application
within the computer.

The communication task is organized into

5 relatively independent layers.


Physical Layer


Network Access Layer


Internet Layer


host or transport Layer


Application Layer

Physical Layer

The physical layer covers the physical interface between a data transmission device i.e. computer,
ation and a transmission medium or network.

This layer is concerned with specifying the characteristics of the transmission medium the nature
of the signals, the data rate and related matters.

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Network Access Layer

This layer is concerned with the exchange
of data between an end system(which could be a server,
workstation etc) and the network to which it is attached.

The sending computer must provide the network with the address of the destination computers, so
that the network may route the data to the appr
opriate destination.

The specific software used at this layer depends on the type of network to be used, there are
different standards for circuit switching and packet switching (x.25, Ethernet etc).

The layers above the network access layer need not worry

about the specifics of the network to be
used, the high

layer software should function properly regardless of the particular network to
which the computer is attached.

The network access layer is concerned of routing the data from one system to other in
the same
network, but if the systems are in different network, and then it is handled by the Internet layer.

Internet Layer

In cases where two devices are attached to different networks, procedures are needed to allow data
to traverse multiple interconnect
ed networks. This is the function of internet layer.

It uses Internet Protocol (IP) at this layer to provide the routing function across multiple networks.

The IP protocol is implemented in the end systems and routers.

Transport Layer

The next concept is t
he reliability of the data i.e all the data arrive at the destination application in
the same order as it was sent.

These mechanisms must be independent of the nature of application. Thus we need to collect those
mechanisms in a common layer shared by all
applications and it is called host
host or transport

It uses the Transmission Control Protocol (TCP) to provide this functionality.

Application Layer

This contains the logic needed to support the various user applications.

For each different typ
e of application like a file transfer, a separate module is needed that is
peculiar to that application.

TCP/IP Concepts

The fig.2.2 below indicates how these protocols are configured for communications. To

Make clear that the total communications facility

may consists of multiple networks, the
constituent networks are usually referred to as subnetworks.

Some sort of network access protocol such as the Ethernet logic, is used to connect a computer to
a subnetwork.

The protocol enables the host to send data
across the subnetwork to another host to, in the case of
a host on another subnetwork, to a router.

The IP is implemented in all the end systems and the routers. This helps to move block of data
from one host to another through one or more routers

The IP n
eed not be told the identity of the destination port, all it needs to know is that the data are
intended for host B. The TCP is implemented only in the end systems; it keeps track of the blocks
of data to assure that all are delivered reliably to the appro
priate application.

For any successful communication there must be some unique address. Actually, two levels of
addressing are needed.

1. Global Internet Address which is a unique address each host on a subnetwork must have.

2. Ports or Service Access Poin
t (SAP) this is an address, which each process with a host must

have and it must be unique within the host. This allows the host
host protocol(TCP) to
deliver data to the actual process.

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Fig 2.2 a

Now let us cons
ider that a process in port 1 at the host A wishes to send a message to another
process associated with the port 3 at host B.



Network Access

Protocol #1


App Y

App X



Network Access

Protocol # 2


App Y

App X





Network 1

Network 2

Logical Connection
(TCP )

Subnetwork attachment

point address

Router J

(e.g., virtual

Port or service access point (SAP)




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The process at A hands the message down to TCP with instructions to send it to host B port 3.TCP
hands the message down to IP wit
h instructions to send it to host B.

IP hands the message down to the network access layer with instructions to send it to router J

To control this operation, control information as well as user data must be transmitted as shown in the
figure show
n below

Fig .2.2 b Protocol

Data Units (PDUs) in the TCP/IP Architecture

To control this operation, control information as well as user data must be transmitted as suggested in the

Fig.2.2. b above. Let us say that the se
nding process generates a block of data and passes this to TCP. TCP

may break this block into smaller pieces to make it more manageable.

The TCP to each of these pieces it appends the control information known as the TCP header, forming

a TCP segment. Th
e control information is to be used by the peer TCP protocol entity at host B (like the
destination port, sequence number and the checksum)

The TCP hands each segment over to IP, with instructions to transmit it to B. These segments must be
transmitted ac
ross one or more subnetwork and relayed through one or more intermediate routers. This

also needs the use or the control information. Thus IP appends a header of control information to each
segment to form an IP datagram.

Finally, each IP datagram is pre
sented to the network access layer for transmission across the first

subnetwork in its journey to the destination. The network access layer appends its own header,

creating a packet or frame. The packet is transmitted across the subnetwork to router J. T
he packet

header contains the information that the subnetwork needs to transfer the data across the subnetwork.

At router J, the packet header is stripped off and the IP header examined. On the basis of the destination

address information in the IP heade
r, the IP module in the router directs the datagram out across

subnetwork 2 to B. To do this, the datagram is again augmented with a network access header.

When data is received at B, the reverse process occurs. At each layer , the corresponding header i
removed, and the remainder is passed on to the next higher layer, until the original user data are delivered
to the destination process.







Application byte stream

TCP Segment

IP Datagram



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As an aside, the generic name for a block of data exchanged at any protocol level is referred to as a
protocol data u
nit (PDU). Thus, a TCP segment is a TCP PDU.

Applications of TCP/IP Applications

The most common application are :


The Simple Mail Transfer Protocol (SMTP)

This provides a basic electronic mail facility.

It provides a mechanism for transferring message
s among separate hosts.

It has mailing lists, return receipts and forwarding.

Once message is created SMTP accepts the message and makes use of TCP to send it to an
SMTP module on another host.

2. File Transfer Protocol (FTP)

It is used to send files f
rom one system to another under user command.

It can accommodate both text and binary files

When user wishes to engage in file transfer, FTP sets up a TCP connection to the target
system for the exchange of control messages

The file is transferred over the

data connection, without the overhead of any headers or
control information at the application level.

When the transfer is complete, the control connection is used to signal the completion and to
accept new file transfer commands.


This provides
the remote logon capability, which enables a user at a terminal or PC to logon to a

remoter computer and function as if directly connected to that computer.


Internetworking (Terms in Internetworking)

Communication Network

A facility that pr
ovides a data transfer service among device attached to the network. Local Area Network
(LAN’s), Metropolitan Area Network (MAN’s) and Wide Area Network (WAN’s) are all examples of
communication networks.


Local Area Network

is a communication network

that interconnects a variety of devices and
provides a means of information exchange among those devices. Scope of LAN is small, typically a single
building or a cluster of buildings. Usually a LAN is owned by the same organization that owns the attached


. Shared

Transmission Medium



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All devices are attached t
o a shared transmission medium
.A transmission from any one device can be
received by all other devices attached to the same network
. Today we have high speed LAN’s with a data
rate of 100Mbps to 10Gbps.

Wide Area Network (WAN)

WAN’s cover a large geographical area, may require the crossing of public rights of ways and may rely at
least in part on circuits provided by a common carrier

Typically a WAN consists of a number of interconnected switching nodes. Traditionally, WAN’s provides
only relatively modest capacity to subscribers. For data attachment, either to a data network or to a
telephone network by means of a modem, data rates

of 64,000bps or less have been common.

With the optical fiber facilities high speed WAN’s of a speed around 10’s and 100’s of Mbps have

Metropolitan Area Network (MAN

MAN occupies a middle ground between the LAN and WAN. MAN’s cover greater di
stances at higher
data rates than LAN’s although there is some overlap in geographical coverage.

Interest in MAN’s has come about as a result of recognition that the traditional point
point and switched
network technique used in WAN’s may be inadequate
for the growing needs of organizations.

The primary markets of MAN’s are the customers who have high capacity needs in a metropolitan area.


It is a collection of communication networks interconnected by bridges and/or routers.


An intern
et used by a single organization that provides the key internet applications especially the World
Wide Web. An intranet operates within the organization for internal purposes and can exist as an isolated
contained internet, or may have links to the in

End System (ES)

A device attached to one of the networks of a internet that is used to support end user applications or

Intermediate System (IS)

A device used to connect two networks and permit communication between end systems attached

different networks.


An IS used to connect two LAN’s that use similar LAN protocols. The bridge acts as address filter, picking
up packets from one LAN that are intender for a destination on another LAN and padding those packets on.
Bridge does
not modify the contents of the packets; neither adds anything to the packets. It acts at the layer
2 of the OSI model.


This is an Intermediate system that is used to connect two networks that may or may not be similar. It
employs the IP protocol.
It operates at layer 3 of the OSI model. Essential functions that the router must
perform include the following.


Provide a link between networks.


Provide for the routing and delivery of data between processes on end systems attached to different


Provide these functions in such a way as not to require modifications of the networking architecture of
any of the attached subnetworks.

Examples of Internetworking

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This Fig 2.3 a. depicts a configuration that illustrates the interactions among protocol
s for internetworking.

Here the server is attached to an ATM WAN and workstation is attached to an IEEE 802 LAN and Router
is used to connect two networks.

Fig.2.3a Internet Protocol, Transmission Control Protocol and User

Datagram Protocol

2.4 Internet Protocol

Now let us look at Internet Protocol (IP) and the Transmission Control Protocol (TCP) and the User
Datagram Protocol.

The most important of the TCP/IP suite is the Internet Protocol (IP). It has two versions IP

and IPv


has been standardized but is not yet widely deployed. The diagram below depicts the IPv

header format




























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Version (4 bits)

Represents version number, it allows the evolution of protocol and the value is 4.

ternet Header Length (IHL) (4 bits)

Its length is 32 bit words and minimum value is 5

Type of Service (8 bits)

Provides guidance to end systems IP modules and to routers along the packet’s path in terms of the
priority of the packets.

Total Length (16

Depicts the total length of the IP packet

Identification (16 bits)

This is used to identify the packet uniquely. It has source address, destination address and user
protocol. It is a sequence number; thus it must be unique number.

Flags (3 bits

Only two of the bits are currently defined. More bit and don’t fragment bit. When a packet is fragmented
that more bit indicates whether this is the last fragment in the original packet. The Don’t Fragment bit
prohibits fragmentation if set. This bit ma
y be useful if it is known that the destination does not have the
capability to reassemble fragments.



Type o
f Service

Total Length



Fragment Offset

Time to live


Header Checksum

Source Address

Destination Address

Options + Padding


4 8
16 19 31









Fig.2.4a IPv


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Fragment Offset (13 bits)

Indicates where in the original packet this fragment belongs; this is measured in 64
bit units.(This implies
that fragments
other than the last fragment must contain a data field that is a multiple of 64 bits in length.

Time to Live TTL (8 bits)

This specifies how long a packet is allowed to remain in the internet.

Protocol (8 bits)

Indicates the next higher level protocol w
hich is to receive the data field at the destination; thus this field
identifies the type of the next header in the packet after the IP header.

Header Checksum (16 bits)

An error
detecting code applied to the header only. Because some header fields may c
hange during transit,
this is re
verified and recomputed at each router. This field is 16 bit ones complemented addition of all 16
bit words in the header. Checksum field is itself initialized to a value to zero.

Source Address (32 bits)

Coded to allow a

variable allocation of bits to specify the network and the end system attached to the
specified network.

Destination Address (32 bits)

This has the same characteristics of the source address.


Encodes the options requested by sending users; thes
e may include security label, source routing, record
routing and time stamping


Used to ensure that the packet header is a multiple of 32 bits in length.;

IPV6 Header

The Internet Engineering Task Force (IETF) in 1996, which develops protocol st
andards for the Internet,
issued a specification for a next

generation IP, known then as IP
, this led to the development of standard
known as IPv

The IPv
provides a number of functional enhancements over IPv
, it provides facility for graphics and

uses 32
bit address to specify a source and destination. With the explosive growth of the internet and
of private networks attached to the internet, this address length became insufficient to accommodate all
systems needing addresses. .IPv

udes 128
bit source and destination addresses.

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Fig 2.4b. Ipv


IP Address Formats are

The source and the destination address fields in the IP header each contain a 32
bit global internet address,

consisting of a network identifier and a host identifier. The address is coded to allow a variable
allocation of bits to specify network and host as shown in the diagram above.

This encoding provides flexibility in assigning addresses to hosts and allow a

mix of network sizes on an
internet. The three principal network classes are best suited to the following conditions:

Class A: Few networks, each with many hosts

Class B: Medium number of networks, each with a medium number of hosts

Class C: Many networ
ks, each with a few hosts.

Class A network addresses begin with a binary 0.Network addresses with a first octet of 0 (binary
00000000) and 127 (binary 01111111) are reserved so there are 126 potential class A network numbers.

Class B network addresses be
gin with a binary 10, so that the range of the first decimal numbers in a class
B is 128 to 191.

Class C addresses, the first decimal number ranges from 192 to 223.


Traffic Class

Flow Label

Payload length

Next header

Hop Limit

Source Address

Destination Address















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Transmission Control Protocol

Most of the application
s make use of TCP/IP protocol suite , the application relies on TCP to assure
reliable delivery of data; TCP in turn relies on IP to handle addressing and routing chores.

TCP Flow Control

This is the central mechanism of TCP.

It uses the sliding window p
rotocol /mechanism

The flow control mechanism used by TCP is known as a credit allocation scheme. In this scheme,
each individual octet of data that is transmitted is considered to have a sequence number.

In addition to data each transmitted segment, the h
eader has 3 fields related to flow control, and
they are sequence number (SN), acknowledgement number (AN) and the Window (w).

When a transport entity sends a segment, it includes the sequence number of the first octet in the
segment data field.

A transpor
t entity acknowledges an incoming segment with a return segment that includes

(AN =i, w=j) with the following interpretation


All octets through sequence number SN=i
1 are acknowledge; the next expected octet has
sequence number “i “.


Permission is

granted to send an additional window of W=j octets of data; that is the “j”
octets corresponding to sequence number “i”
through i

represents the mechanism


Network (7 bits)

Host (24 bits)

1 0

Network (14bits)

Host (16 bits)

1 1 0 Network (21bits)

Host (8bits)

1 1 1 0


1 1 1 1 0 Future Use

Class A

Class B

Class C

Class D

Class E

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For simplicity, let us consider that the data flow is only in one direction.
Assuming that 200 octets of data
are sent in each segment. Initially, through the connection establishment process the sending and the
receiving sequence numbers are synchronized and A is granted an initial credit allocation of 1400 octets
beginning with
the octet number 1001.

After sending 600 octets in three segments “A” has shrunk its window to a size of 800 octets. Following
receipt of these segments B acknowledges receipt of all octets through 1601 and issues a credit of 1000
octets. i.e. A can send
octets 1601 through 2600.

However, by the time that B’s message has arrived at A, A has already sent two segments, containing octets
1601 through 2000.Thus A’s remaining credit at this point is only 400 octets.

As the exchange proceeds, A advances the trai
ling edge of its window each time that it transmits and
advances the leading edge only when it is granted credit. The edit allocation mechanism is quite flexible.

For example consider that the last message issued by B was (AN=i , W=j) and that the last oc
tet of data
received by B was octet number i
1, then

To increase credit to am amount k (k >j) when no additional data have arrived B issues (AN = i, W = k).To
acknowledge an incoming segment containing ‘m’ octets of data (m<j> without granting additional
B issues (AN = i+m, W = j
m) Please refer to the Credit Allocation Diagram

TCP uses only a single type of protocol data unit called a TCP segment.

These fields are as follows:

Source Port

Destination Port

Sequence Number

Header Length

Unused Flags



Urgent pointer

Options + padding















Fig.2.5 TCP


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1. Source Port (16 bits): Denotes

the Source TCP user

2. Destination Port (16 bits) : Denotes the Destination TCP user

3. Sequence Number (32 bits) : Sequence number of the first data octet in this segment except when the

SYN flag is set. ( If SYN is set , it is the initial seque
nce number (ISN) and the first data octet is ISN +1).

4. Acknowledge Number (32 bits)

A piggybacked acknowledgement. It contains the sequence number of the next data octet that the TCP


expects to receive.

5. Data Offset (4 bits)

It i
s a number of 23
bit words in the header.

6. Reserved (6 bits): Reserved for the future use.

7. Flags (6 bits):

URG: Urgent pointer field significant

ACK: Acknowledgement field significant

PSH: Push function

RST: Reset the connection

SYN: Synch
ronize the sequence number

FIN: No more data from sender.

8. Window (16 bits):

Flow control credit allocation in octets. Contains the number of data octets, beginning with the one
indicated in the acknowledgement field that the sender is willing to acc

9. Checksum (16 bits):

The ones complement of the sum modulo 2
1 of the entire 16

bit words in the segment plus a

pseudo header.

10. Urgent Pointer (16 bits):

Points to the last octet in a sequence of urgent data. This all
ows the receiver to know how much urgent

data are coming.

. Options


An example is the option that specifies the maximum segment size that will be accepted.

2.6 User Datagram Protocol (UDP)

The User Datagram Protocol provides a

connectionless service for application

level procedures.

UDP is basically an unreliable service, here the delivery and duplicate protection are not guaranteed.
However this does reduce the overhead of the protocol and may be adequate in many cases. Conne
service, however is more appropriate in some context. At lower layers(internet, network ) connectionless
service is more robust.

A connectionless service represents a “least common denominator” of service to be expected at higher
layers. Further,

even at transport and above there is justification for a connectionless service.UDP sits on
top of IP as it is connectionless. Essentially, it adds a port addressing capability to IP

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The UPD header is show below:

Source an
d Destination Port: The header includes a source port and destination port . Length Field contains
the length of the entire UDP segment including header and data. Checksum is the same algorithm used for
TCP and IP.

For UDP, the checksum applies to the ent
ire UDP segment plus a pseudoheader prefixed to the UDP header
at the time of calculation and is the same pseudoheader used for TCP. This is optional

If an error is detected, the segment is discarded an no further action is taken. If not used, then it is
set to

Source Port

ination Port

Segment Length



16 31








Bit :


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Chapter 3

Medium Access Control

3.0 Introduction

The basics…..

The Protocol Architecture defined specifically for LAN and MAN address the issues for transmitting
blocks of data over the network.

Here the higher layer

protocols i.e. 3 or 4 and above are independent of network architecture. Thus we need
to concentrate on the lower layers of the OSI model for discussing the LAN protocols.

The LAN protocols to the OSI architecture as specified by the IEEE 802 committee .I
t is generally referred
to as IEEE 802 reference model.



MAC (Medium Access Control)

The MAC’s are needed in the wireless domain .It comprises of all the mechanisms that regulate user access
a medium using the SDM, TDM, FDM or CDM.

The MAC algorithms are specifically adapted to the wireless domain.

3.1 Motivation for a specialized MAC

Can we use CSMA/CD (Carrier Sense Multiple Access with Collision Detection) which is an elaborate
MAC schem
es from wired networks as specified by IEEE 802.3 networks (Ethernet).






Data Link







Upper layer

pe of
IEEE 802

OSI Reference Model

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What is CSMA/CD and How does it work? [Carrier Sense Multiple Access with Collision Detection.]

This is a technology for the wired network. The medium used is the wire or the coaxia
l cable as the
technology for the wired medium.

A sender senses the medium to see if it is free. If the medium is busy then the sender waits until it is free. It
the medium is free, and then the sender starts transmitting data and continues to listen into

the medium. If
the sender detects a collision while sending, it stops at once and sends a jamming signal.

CSMA/CD fails in wireless networks why?

The CSMA/CD is not interested about the collision at the sender, but ensures no collision takes place at th
receiver side; but sender is the one detecting the collisions.

In case of wired network, if the collision occurs somewhere in the wire it is noticed by everybody, it is not
the situation in case of the wireless networks.

Here the strength of the signal
unlike the wired ones decreases proportionally to the square of the distance
to the sender. The sender may now apply carrier sense and detect an idle medium, the sender starts sending
but a collision happens at the receiver due to second sender.

The same c
an happen to the collision detection .The sender detects no collision and transmits the data at the
receiver; but actually the collision might have destroyed the data at the receiver. Thus collision detection is
very difficult in wireless scenarios as the
transmission power in the area of the transmitting antenna is
several magnitude higher than the receiving power. Thus the common MAC schemes from the wired
network fails in the wireless scenario.


Hidden and Exposed Terminals


Let us consider three mobile phones A,B and C

Transmission range of A reaches B but not C, C reaches B, but not A

B reaches both A and C

A starts sending to B but C does n
ot receive this transmission ,C also wants to send something to
B and senses the medium

For C medium appears to be free and carrier sense fails, then C sends but it collides at B. Now A
cannot detect collision at B and A also transmits, now A is hidden for

C and vice versa. A is hidden
terminal and these hidden terminals may cause collision and unnecessary delay.

Let us consider another scenario where ‘B’ sends something to ‘A’ and ‘C’ wants to transmit data to
some other mobile phones outside the interfer
ence ranges pf A and B.

C senses the carrier and detects that the carrier is busy ; C postpones its transmission until it detects
the medium is free ; but as A is outside the interference range of C, waiting is not necessary. I.e.
collision at B does not

matter because the collision is too weak to propagate to A.




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3.3Near and Far Terminals


Let us consider that A and B both sending with the same transmission power. When the signal
strength decreases proportionall
y to the square of the distance B’s signal drowns out A’s
signal .As a result ‘C’ cannot receive ‘A’s transmission.

Now think of ‘C’ as being an arbiter for sending rights. In this case terminal B would already

drown out terminal A on the physical layer

‘C’ in return would have no chance of applying a fair scheme as it would only hear B.

The near/far effect is a severe problem of the wireless network using the CDM.All signals
should arrive at the receiver with more or less the same strength.

ise a person standing closer to somebody could always loud than a person farther
away. Even if the sender were separated by code , the closest one would simply drown out the


Space division multiple access (SDMA)

This i
s used for allocating a separate space to user in wireless network.

A typical application involves assigning an optimal base station to a mobile phone user.

The mobile phone may receive several base stations with different quality. A MAC algorithm
could no
w decide which base station is best, taking into account which frequencies (FDM), time
slots (TDM) or code (CDM) are still available.

SDMA typically , is not used in isolation but always in combination with one or more schemes.

The basis for the SDMA algor
ithm is formed by cells and sectorized antennas which constitute the
infrastructure implementing space division multiplexing.

Single users are separated in space by individual beams. This can improve the overall capacity of a
cell tremendously.

y division multiple access (FDMA)

This comprises all algorithms allocating frequencies to transmission channels according to the
frequency division multiplexing (FDM).




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Frequency can be fixed or dynamic

Channels can be assigned to the same frequency at all
times which is called pure FEMA or change
frequencies according to a certain pattern i.e. FDMA combined with TDMA.

The second concept of changing frequencies based on some pattern is commonly used in many
wireless systems to circumvent narrowband interfere
nce at certain frequencies known as
frequency hopping.

Sender and receiver have to agree on a hopping pattern, otherwise the receiver could not tune to
the right frequency. The fact that it is not possible to arbitrarily jump in the frequency space is one
of the main differences between FDM schemes and TDM schemes.

Furthermore, FDM is often used for simultaneous access to the medium by the base station and
mobile station in cellular networks. Here the two partners typically establish a duplex channel i.e.,
a channel that allows for simultaneous transmission in both directions. The two directions, mobile
station to base station and vice versa are now separated using different frequencies. This scheme is
then called frequency division duplex (FDD).

Again, bot
h partners have to know the frequencies in advance; they cannot just listen into the
medium. These two frequencies are known as the uplink and downlink frequencies.

Uplink is from mobile station to the base station or from ground to satellite.

Downlink is
from the base station to the mobile station or from the satellite to the ground control.

The following diagram depicts the frequency division multiplexing for multiple access and duplex.

The figure shows the situation in a mobile phone network based on the

GSM standard for
900MHz (The basic frequency for GSM is fixed and regulated by national authorities.

All uplinks use the range of frequency between 890.2 MHz and 915 MHz and all downlinks from
935.2 to 960 MHz.

In the above diagram the base station which
is at the right, allocates a certain frequency for up
as well as downlink to establish a duplex channel with a mobile phone.

Note the up and the downlinks have a fixed relation.

If uplink frequency fu =890MHz + n.0.2 MHz

Then downlink frequency fd = f
u + 45 MHz for a certain channel ‘n’.

The base station selects the channel and each channel has a bandwidth of 200 MHz.

This illustrates the use of the FDM for multiple access and duplex according to a predetermined



960 MHz

935.2 MHz

915 MHz

890.2 MHz


20 MHz

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Time Division Multiple Access (TDMA)

This is more flexible scheme when compared to FDMA, which comprises all technologies
that allocate certain time slots for communication i.e. controlling TDM.

Now tuning into a certain freq
uency is not necessary i.e. the receiver can stay at the same
frequency the whole time. As we already know listening to different frequencies at the same
time is quite difficult, but listening to many channels separated in time at the same frequency
is sim

Many wired network MAC schemes like Ethernet, ATM, and Token Ring works according to
this principle.

The TDMA has to have synchronization between sender and receiver in the time domain. It is
achieved by allocating a certain time slot for a channel or

by using a dynamic allocation

Dynamic allocation scheme and fixed scheme

The dynamic allocation scheme requires identification for each transmission as this is the case for
typical wired MAC schemes or the transmission has to be announced befor
ehand. The MAC
addresses are often used as identification. This enables a receiver in a broadcast medium to
recognize if it really is the intended receiver of a message.

On contrary, the fixed schemes do not need identification, but are not as flexible con
varying bandwidth requirements.

Fixed TDM

The simplest algorithm for using TDM is allocating time slots for channels in a fixed pattern.
This results in a fixed bandwidth and is the typical solution for wireless phone systems.

MAC is quite simple
, as the only crucial factor is accessing the reserved time slot at the right
moment. If this synchronization is assured, each mobile station knows its turn and no
interference will happen.

This fixed pattern can be assigned by the base stations, where the

competition between different
mobile stations that want to access the medium is solved.

Fixed access patterns fit perfectly well for connections with a fixed bandwidth.

The following fig3.4b depicts the time division multiplexing for multiple access and d














417 µs

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This depicts how these fixed TDM patterns are used to implement multiple access and a
duplex channel between a base station and mobile station. Assigning different slots for
uplink and downlink using the same

frequency is called time division duplex (TDD).

The above figure shows the base station uses one out of 12 slots for downlink, where as
the mobile station uses one out of 12 slots for the uplink.

The uplink and downlink are separated in time.

Using this s
cheme up to 12 different mobile stations can use the same frequency without
interference. Here each connection is allotted its own up and downlink pair.

In case of DECT cordless phone system, the pattern is repeated every 10ms, which means
each slot, has d
uration of 417µs. This repetition guarantees access to the medium every
10ms, independent of any other connections.

This system used by DECT is perfectly apt for connections with a constant data rate but
are inefficient for bursty data or asymmetric connec

In this scheme there is waste of lot of bandwidth. It is very static and very inflexible for
data communication. For such cases the connectionless, demand
oriented TDMA
schemes can be used.

3.5 Classical Aloha

This is a scheme which was invented

at the University if Hawaii and was used in the
ALOHANET for wireless connection if several stations.

Aloha neither coordinates medium access nor does it resolve contention on the MAC layer.
Instead each station can access the medium at any time as shown
in the figure below.

This is a random access scheme, without a central arbiter controlling access and without
coordination among the stations.

If two or more stations access the medium at the same time, a collision occurs and the
transmitted data is destr


Resolving this problem is left to higher layers

A simple Aloha works fine for light load and does not require any complicated mechanisms.

Here the assumption is that the data packet arrival follows a Poisson dist
ribution (where a maximum
throughput is achieved for an 18 percent load).

Sender A

Sender B

Sender C


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3.6 Slotted Aloha


This is a refinement of the classical Aloha scheme.

It introduces time slots, where all senders have to be synchroniz
ed and transmission can only start
at the beginning of a time slot as depicted in the figure above.

Here still the access is not coordinated , here the introduction of slots raise the

throughput from 18 percent to 36 percent (doubling the throughput).

Note that the Aloha systems work perfectly well under a light load, but they cannot give any hard
transmission guarantees, like the m
aximum delay before accessing the medium or minimum
throughput.The UMTS system of the mobile communication relies on the slotted Aloha for medium
access in certain situation.

3.7 Types of Carrier Sense Multiple Access

CSMA is definitely an improvement ove
r the Aloha for sensing the medium.

Sensing the carrier and accessing the medium only when it is free is good means to avoid collision, but
here the problem of the hidden and exposed terminals are not solved.

Several versions or types of CSMA exists, they


persistent CSMA

Here the stations sense the carrier and starts sending immediately if the
medium is idle. If the medium is busy, the station waits for random amount of time before sensing
the medium again and repeats the pattern .



Here the nodes also senses the medium but only transmits with the
probability of p, with the next slot with the probability of 1


persistent CSMA

Here all systems wanting to transmits as soon as the medium is idle or free.
This will cause ma
ny collisions as they block each other. In order to give a fair chance for the
stations waiting for longer time back
off algorithms are used, which are sensitive to the waiting
time, which is generally followed in Ethernet.


Elimination yield

ive multiple access (EY


where several phases of
sensing the medium for contention resolution are interleaved before one “winner “ is got the
access to the medium. It uses the priority schemes to give preferences to the station to access the
. This is generally used in the HIPERLAN 1 specification

Sender A

Sender B

Sender C


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In case of Wireless LAN specified by IEEE 802.11 CSMA/CA is used, where the

Carrier sensing is combined with the back

off scheme in case the medium is busy to achieve
fairness among the compe
ting stations.

3.8 Multiple Access with Collision Avoidance (MACA)

This is a simple scheme, which solves the problem of hidden terminals.

It does not need a base station, but adapts the random access aloha scheme with dynamic reservation.

the problem of the hidden terminals:

A and C are trying to communicated with B, A has already started the transmission, but as C is hidden
it will not know if even C has begin the transmission, thus causing the collision at B.

What MACA does at this



A if it has to transmit to B, it does not start immediately, rather it sends a Request to Send (RTS)

B receives the RTS that contains the name of sender and receiver, as well as the length of the
future tra

C does not hear this transmission, but triggers and acknowledgement from B, called clear to send

The CTS also contains the names of the sender (A) and the receiver (B) of the user data band the
length of the future transmission.

C, thus res
erving the use of the medium for future use by A is now reserved for the duration of the
transmission, hears the CTS.

After receiving CTS; C is not allowed to send anything for the duration indicated in the CTS
toward B.

A collision cannot occur at B durin
g data transmission, and the hidden terminal problem is solved

provided that the transmission conditions remain the same .

Still the problem of the collision can occur when sending an RTS, where both A and C could send
the RTS that collides at B.

Now th
e next question is about
can MACA also help in solving the problem of the exposed terminals?

The exposed terminal problem B wants to communicate to A and C to someone else .But C senses the
medium before transmitting the data and senses a busy medium cause
d by the transmission from B. Thus C
defers, even though it not cause any collision at A.

3.8.1 Solution of MACA

B has to transmit an RTS first containing the name of the receiver A and the sender B.

It sends RTS to C also but C does not react to this b
ecause it is not the receiver, whereas A acknowledges it
by sending an CTS to B.







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At this point C does not receive any CTS and thus finds that A it outside its range, thus it starts transmitting
assuming it will not cause a collision at A.

One problem of M
ACA is clearly the overheads associated with the RTS and CTS transmissions. For short