All Chapter

To Whom It May Concern
This is to certify that Sumon Paul, ID: EEE 02605120; MD. Arifur Rahman,
ID: EEE 02605139; MD. Jakir Hossain, ID: EEE 02605156; MD. Ariful Huq, ID: EEE
02605160, the students of STAMFORD UNIVERSITY, Siddeshwari, Dhaka, have done
“Implementation of WLAN Project” in fulfillment of Hon’s project under my direct
supervision. In my consideration, they have done a good job and successfully
completed the project work.
------------------------
Project supervisor
Tanbir Ibne Anower
Lecture
Dept. of EEE
Stamford University

Acknowledgement
We acknowledge with gratitude the inspiration and encouragement provided by Prof. Dr.
Enamul Basher, Head of the Department, Department of Electrical and Electronic
Engineering, Stamford University, Bangladesh.
We are also thankful for his kind co-operation, extensive guidance and valuable
instructions during the project work by our highly respected project supervisor Tanbir
Ibne Anowar, Lecturer, Department of Electrical and Electronic Engineering, Stamford
University, Bangladesh. Without his support and sincere help it wouldn’t have been
possible for us to successfully finish the project work.
We are also grateful to our parents and friends for their continuous encouragement,
inspiration and support to carry out this project work and we are also indebted to the
almighty Allah for giving us this opportunity and to let us finish the project work
successfully.
Last but not least, we are thankful to Stamford University, Bangladesh and also our
deepest appreciation and thanks to all the respected teachers included guiding us to the
brightness of knowledge from the darkness of illiteracy.

Abstract
The IEEE 802.11 protocol or Wi-Fi is a network access technology for providing
connectivity between wireless stations and wired networking infrastructures using radio
waves.
Computers, laptops, cell phones and palm pilots are examples of mechanisms that can
grant the user internet access. Although computers and laptops are capable of having
normal internet connection (i.e. Ethernet connection), they are also able to have internet
access through wireless technology. Cell phones and palm pilots can only be connected
to the internet by wireless connection. Wireless networking is possible through the
technology of wireless-fidelity. Wireless-fidelity or Wi-Fi allows a ubiquitous internet
connection to be broadcasted through radio waves. Its purpose serves directly to the
users looking for internet access without any cords or wires.
The scope of this project is the WLAN based on its properties, characteristics and
operations. Before understanding any wireless communication, the cellular concept
should be studied. So, our project also covers the cellular concept along with the
principles of transmission of data using communication channels and their properties.
Our project also focuses on the networking technology where the OSI model is
described is general enough to illustrate the basic mechanisms and utilizations along
with explanations of each layer and their usage. Our study also projects the properties
and usage of Direct Sequence Spread Spectrum (DSSS) and Frequency Hopping
Spread Spectrum (FHSS). After explaining the communication principles comes the
communication process, where the main technique of Wi-Fi communication is explained
and described with elaborate examples along with data frame descriptions, making it
easy to understand.
In this project work, we tried to focus on the technology within the wireless networking
and its characteristics.

Chapter
Introduction
Wi-Fi or IEEE 802.11 WLAN standard is being accepted widely and rapidly for many
different environments today. Main characteristics of the 802.11 networks are their
simplicity and robustness against failures due to the distributed approach. Using the
ISM band at 2.4 GHz, the 802.11b version provides data rates of up to 11 Mbit/s at the
wireless medium. Now, the new 802.11a version can achieve data rates of up to 54
Mbit/s at the wireless medium using the OFDM modulation technique in the unlicensed
5 GHz band. Today, 802.11 WLAN can be considered as a wireless version of Ethernet,
which supports best-effort service.
Radio waves are the keys which make Wi-Fi networking possible. These radio signals
are transmitted from antennas and routers and are picked up by Wi-Fi receivers such as
computers and cell phones that are equipped with Wi-Fi cards. Whenever a computer
receives any of the signals within the range of a Wi-Fi network which is usually 300 –
500 feet for antennas and 100 – 150 feet for routers, the Wi-Fi card will read the signals
and thus create an internet connection between the user and the network without the
use of a cord. Usually the connection speed is increases as the computer gets closer to
the main source of the signal and decreases when the computer gets further away. With
that in mind, let’s think of the Wi-Fi card as being an invisible cord that connects the
computer to the antenna for a direct connection to the internet. Once a connection is
established between the user and the network, the user will be prompted with a login
screen and password if it is a fee-based type network. Though there’re also free-based
network connections as well in some areas. Wi-Fi networking around the world is
creating hot spots in cities where anyone with a laptop can wirelessly plug into the
internet. A hotspot is a connection point for a Wi-Fi network. It is a small box that is
hardwired into the internet. There are many Wi-Fi hotspots now available in public
places like restaurants, hotels, libraries and airports.

In this project paper the in-depth analysis of Wi-Fi and the description of our project will
be described in many chapters.
First of all, we will describe the basic data communication concept. This chapter has
three parts: the introduction of data communication, the components of data
communication and their definitions, and the flow of data and their characteristics.
After that, we will focus on the networking. In this chapter the basic structure and the
construction of a network will be described. This chapter has three parts: network and
its definition, basic criteria of a network, the description of network layers with OSI
model. In this chapter The OSI model is explained with examples and with the definition
of each layers.
Now that the basics of a network are briefed, we will describe the cellular concept of
wireless communication. In this chapter methods of wireless communications will be
discussed regarding the cellular concept. This chapter includes the discussions about
cell, its coverage, different models, sectorization, co-channel interference, and
frequency reuse plans.
After describing the cellular concept we will focus on our main concern, the Wi-Fi
communication system. In this chapter we will describe the main technique and
operations of this system. This chapter includes the RF regulations of wireless LAN and
its properties, OFDM technique, Spread spectrum techniques, Chipping and correlating,
and different standards of WLAN in view of this chapter.
After that, we will focus on the heart of the Wi-Fi communication system, the
communication process. In this chapter we will know how Wi-Fi devices communicates,
the transmission and reception, the handshaking process, the bit formats and their
descriptions.
Now that all the theories have been known, we will proceed to the hardware portion of
this system. In this chapter, different parts and networking topologies of a Wi-Fi network
will be shown and described. The main concern of this chapter is to introduce to the
physical architecture of WLAN.

Chapter
Data Communications
2.1 Communication
When we communicate, we are sharing information. This sharing can be local or
remote. Between individuals, local communication usually occurs face to face, while
remote communication takes place over distance. The term telecommunication, which
includes telephony, telegraphy, and television, means communication at a distance.
The word ‘data’ refers to information presented in whatever form is agreed upon by the
parties creating and using the data.
Data communications are the exchange of data between two devices via some form of
transmission medium such as a wire cable. For data communications to occur, the
communicating devices must be part of a communication system made up of a
combination of hardware (physical equipment) and software (programs). The
effectiveness of a data communications system depends on four fundamental
characteristics: delivery, accuracy, timeliness and jitter.
I. Delivery: The system must deliver data to the correct destination. Data must be
received by the intended device or user and only by that device or user.
II. Accuracy: The system must deliver the data accurately. Data that have been altered
in transmission and left uncorrected are unusable.
III. Timeliness: The system must deliver data in a timely manner. Data delivered late
are useless. In the case of video and audio, timely delivery means delivering data as

they are produced, in the same order that they are produced and without significant
delay. This kind of delivery is called real-time transmission.
IV. Jitter: Jitter refers to the variation in the packet arrival time. It is the uneven delay in
the delivery of audio or video packets. For example, let us assume that video packets
are sent every 30 ms. If some of the packets arrive with 30-ms delay and others with
40-ms delay, an uneven quality in the video is the result.
2.2 Communication components
A data communication system has five components
Protocol Protocol
Figure 2-1: Five components of data communication
1. Message: The message is the information (data) to be communicated. Popular forms
of information include text, numbers, pictures, audio and video.
2. Sender: The sender is the device that sends the data message. It can be a
computer, workstation, telephone handset, video camera and so on.
3. Receiver: The receiver is the device that receives the message. It can be a
computer, workstation, telephone handset, television and so on.
4. Transmission medium: The transmission medium is the physical path by which a
message travels from sender to receiver. Some examples of transmission media
include twisted-pair wire, coaxial cable, fiber-optic cable, and radio waves.
5. Protocol: A protocol is a set of rules that governs data communications. It represents
an agreement between the communicating devices. Without a protocol, two devices
may be connected but not communicating, just as a person speaking French cannot be
understood by a person who speaks only Japanese.
Sender Receiver
Message
Rule 1:
Rule 2:
Rule n
Rule 1:
Rule 2:
Rule n
Medium

3.3 Data flow
Communication between two devices can be simplex, half-duplex, or full-duplex.
1. Simplex: In simplex mode (Fig: 2-2), the communication is unidirectional, as on a
one-way street. Only one of the two devices on link can transmit; the other can only
receive. Keyboards and traditional monitors are examples simplex devices. The
keyboard can only introduce input; the monitor can only accept output. The simplex
mode can use the entire capacity of the channel to send data in one direction.
2. Half-duplex: In half-duplex mode (Fig: 2-3), each station can both transmit and
receive but not at the same time. When one device is sending, the other can only
receive and vice versa. The half-duplex is like a one-lane road with traffic allowed in
both directions. When cars are traveling in one direction, cars going the other way must
wait. In a half-duplex transmission, the entire capacity of a channel is taken over by
whichever of the two devices is transmitting at the time. Walkie-talkies and CB (citizens
band) radios are both half-duplex system. The half-duplex mode is used in cases where
there is no need for communication in both directions at the same time; the entire
capacity of the channel can be utilized for each direction.
Direction of data
Monitor
Figure 2-2: Data flow in Simplex
Station Station
Direction of data at time 1
Direction of data at time 2
Figure 2-3: Data flow in Half-duplex
Mainframe

3. Full-duplex: In full-duplex mode (Fig: 2-4) (also called duplex), both stations can
transmit and receive simultaneously. The full-duplex mode is like a two-way street with
traffic following in both directions at the same time. In full duplex mode, signals going in
one direction share the capacity of the link with signals going in the other direction. This
sharing can occur in two ways: Either the link must contain two physically separate
transmission paths, one for sending and the other for receiving; or the capacity of the
channel is divided between signals traveling in both directions. One common example
of full-duplex communication is the telephone network. When two people are
communicating by a telephone line, both can talk and listen at the same time. The full-
duplex mode is used when communication in both directions is required all the time.
The capacity of the channel, however, must be divided between the two directions.
Station Station
Direction of data all the time
Figure 2-4: Data flow in Full-duplex

Chapter
Networks
3.1 What is Network
A network is a set of devices (often referred to as nodes) connected by communication
links. A node can be a computer, printer or any other device capable of sending and/or
receiving data generated by other nodes on the network. A network is a group of
connected communicating devices such as computers and printers. An internet is two or
more networks that can communicate with each other. The most notable internet is
called the Internet, a collaboration of more than hundreds of thousands of
interconnected networks. Private individuals as well as various organizations such as
government agencies, schools, research facilities, corporations, and libraries in more
than 100 countries use the internet. Millions of peoples are users. Most networks use
distributed processing, in which a task is divided among multiple computers instead of
one single large machine being responsible for all aspects of a process. Separate
computers (usually a personal computer or work station) handle a subset. A network is
a combination of hardware and software that sends data from location to another. The
hardware consists of the physical equipment that carries signals from one point of the
network to another. The software consists of instruction sets that make possible
services that we expect from a network.

3.2 Network Criteria
A network must be able to meet a certain number of criteria. The most important of
these are performance, reliability, and security.
1. Performance: Performance can be measured in many ways, including transit time
and response time. Transit time is the amount of time required for message to travel
from one device to another. Response time is the elapsed time between an inquiry and
a response. The performance of a network depends on a number of factors, including
the number of users, the type of transmission medium, the capabilities of the connected
hardware, and the efficiency of the software. Performance is often evaluated by two
networking matrices: throughput and delay. We often need more throughputs and less
delay. However, these two criteria are often contradictory. If we try to send more data to
the network, we may increase throughput but we increase the delay because of traffic
congestion in the network.
2. Reliability: In addition to accuracy of delivery, network reliability is measured by the
frequency of failure, the time it takes a link to recover from a failure, and the network’s
robustness in a catastrophe.
3. Security: Network security issues include protecting data from unauthorized access,
protecting data from damage and development, and implementing policies and
procedure from breaches and data losses.
3.3 Network Layers
3.3.1 Layer Concept
We use the concept of layers in our daily life. For example, the task of sending an e-
mail from one point in the world to another can be broken into several tasks, each
performed by a separate software package. Each software package uses the services
of another software package. At the lowest layer, a signal, or a set of signals, is sent
from the source the source computer to the destination computer. In this chapter, we
give a general idea of the layers of a network and discuss the functions of each. The
OSI model is a layered framework for the design of network system that allows
communication between all types of computer systems. It consists of seven separate
but relative layers, each of which defines a part of the process of moving information
across a network. An understanding of the fundamentals of the OSI model provides a
solid basis for exploring data communications.

3.3.2 Layered Architecture (OSI model)
The Open Systems Interconnect (OSI) model was developed by the International
Standards Organization (ISO) to provide a guideline for the development of standards
for interconnecting computing devices. The OSI model is a framework for developing
these standards rather than a standard itself — the task of networking is too complex to
be handled by a single standard.
The OSI model breaks down device to device connection, or more correctly application
to application connection, into seven so-called “layers” of logically related tasks (see
Table 3-1).
The seven layers of the OSI reference model; are concerned with tasks ranging from
how electrical signals are generated and bits are encoded, to the interface with user
applications.
Layer Description Standards and
Protocols
7 — Application layer Standards to define the provision of services to
applications — such as checking resource
availability, authenticating users, etc.
HTTP, FTP,
SNMP, POP3,
SMTP
6 — Presentation
layer
Standards to control the translation of incoming
and outgoing data from one presentation format
to another.
SSL
5 — Session layer Standards to manage the communication
between the presentation layers of the sending
and receiving computers. This communication is
achieved by establishing, managing and
terminating “sessions”.
ASAP, SMB
4 — Transport layer Standards to ensure reliable completion of data
transfers, covering error recovery, data flow
control, etc. Makes sure all data packets have
arrived.
TCP, UDP
3 — Network layer Standards to define the management of network
connections — routing, relaying and terminating
connections between nodes in the network.
IPv4, IPv6, ARP
2 — Data link layer Standards to specify the way in which devices
access and share the transmission medium
(known as Media Access Control or MAC) and to
ensure reliability of the physical connection
(known as Logical Link Control or LLC).
ARP Ethernet
(IEEE 802.3),
Wi-Fi (IEEE
802.11),
Bluetooth
(802.15.1)
1 — Physical layer Standards to control transmission of the data
stream over a particular medium, at the level of
coding and modulation methods, voltages, signal
durations and frequencies.
Ethernet, Wi-Fi,
Bluetooth,
WiMAX
Table 3-1: The Seven Layers of the OSI Model

An example will show how these layers combine to achieve a task such as sending and
receiving an e-mail between two computers on separate local area networks (LANs)
that are connected via the Internet.
The process starts with the sender typing a message into a PC e-mail application
(Figure 3-1). When the user selects “Send”, the operating system combines the
message with a set of Application layer (Layer 7) instructions that will eventually be read
and actioned by the corresponding operating system and application on the receiving
computer.
The message plus Layer 7 instructions are then passed to the part of sender’s operating
system that deals with Layer 6 presentation tasks.
These include the translation of data between application layer formats as well as some
types of security such as Secure Socket Layer (SSL) encryption. This process
continues down through the successive software layers, with the message gathering
additional instructions or control elements at each level.
By Layer 3 — the Network layer — the message will be broken down into a sequence of
data packets, each carrying a source and destination IP address. At the Data Link layer
the IP address is “resolved” to determine the physical address of the first device that the
sending computer needs to transmit frames to the so-called MAC or media access
control address. In this example, this device may be a network switch that the sending
computer is connected to or the default gateway to the Internet from the sending
computer’s LAN.
At the physical layer, also called the PHY layer, the data packets are encoded and
modulated onto the carrier medium a twisted wire pair in the case of a wired network, or
electromagnetic radiation in the case of a wireless network and transmitted to the
device with the MAC address resolved at Layer 2.
Transmission of the message across the Internet is achieved through a number of
device-to-device hops involving the PHY and Data Link layers of each routing or
relaying device in the chain. At each step, the Data Link layer of the receiving device
determines the MAC address of the next immediate destination, and the PHY layer
transmits the packet to the device with that MAC address.

Figure 3-1: The OSI Model in Practice — an E-mail Example
On arrival at the receiving computer, the PHY layer will demodulate and decode the
voltages and frequencies detected from the transmission medium, and pass the
received data stream up to the Data Link layer. Here the MAC and LLC elements, such
as a message integrity check, will be extracted from the data stream and executed, and
the message plus instructions passed up the protocol stack. At Layer 4, a protocol such
as Transport Control Protocol (TCP), will ensure that all data frames making up the
message have been received and will provide error recovery if any frames have gone
missing. Finally the e-mail application will receive the decoded ASCII characters that
make up the original transmitted message.
Sender writes e-mail message
Message is prepared and
“sent’ from an e-mail application
Message is broken into
presentation and session
elements. Presentation and
session layer control headers
are successively added
Message is broken into packets
and transport layer control
header added
Data frame created from data
packet + network addresses +
Layer 3 header.
Data frame encrypted, frame
control header added, network
addresses translated into MAC
addresses
Access gained to physical
medium, bit stream coded and
modulated onto PHY layer
signals and transmitted
Recipient reads e-mail message
Message is received by the e-mail
application and read by the addressee
Session and Presentation layer control
headers are successively removed.
Messages reassembled into a specific
format for the receiving e-mail application
Bit stream structured into frames,
decrypted, and checked for destination
MAC addresses
Frame headers removed, payloads
reassembled into data packets
Packet reception and sequencing
controlled, data reassembled into
Layer 5 messages.
Received signals are continuously
demodulated, decoded and bits stream
are set to Data Link Layer
Layer 7
Application layer
Layer 6
Presentation layer
Layer 5
Session layer
Layer 4
Transport layer
Layer 3
Network layer
Layer 2
Data Link layer
Layer 1
Physical layer

3.3.2.1 The Lower Layers: Physical, Data Link and Network
The three lower layers of the OSI reference model are responsible for transferring the
data between the end systems hence constitute the communications portion of the
model. These layers run on both end systems and intermediate nodes.
Physical Layer
The physical layer is concerned with the transmission of bits between adjacent systems
(nodes). Its functions include interfacing with the transmission hardware, physical
connector characteristics, and voltage levels for encoding of binary values. Repeaters,
which are responsible for reading and regenerating pulses, operate at this layer. Some
well-known physical layer standards include RS-232 and its successor RS-449.
Data Link Layer
The data link layer provides reliable transmission of data (frames) between adjacent
nodes, built on top of a raw and unreliable bit transmission service provided by the
physical layer. To achieve this, the data link layer performs error detection and control,
usually implemented with a Cyclic Redundancy Check (CRC). Note that the data link
layer provides reliable transmission service over a single link connecting two systems. If
the two end systems that communicate are not directly connected then their
communication will go through multiple data links, each operating independently. In this
case, it is the responsibility of higher layers to provide reliable end-to-end transmission.
Bridges, which connect two similar or dissimilar local area network segments, operate at
this layer. Some well-known protocols for the data link layer include High-level Data Link
Control (HDLC), LAN drivers and access methods such as Ethernet and Token Ring,
and the LAP-D protocol in ISDN networks.
Network Layer
The network layer provides the transparent transfer of data packets from the source to
the destination system, thus relieving the higher layers from having to know about the
underlying network configuration and topology. The end systems can belong to different
sub-networks, with different transmission and switching technologies and procedures. It
is the responsibility of the network layer to hide all the heterogeneous transmission and
switching used to connect end systems and intermediate nodes from its upper layer
(transport layer). Two basic functions performed by the network layer are routing, which
involves determining the path a packet must follow to reach its destination, and packet
forwarding, which involves moving the packet from one sub-network to another. Routing
is performed based on the network layer address, which uniquely identifies each
connection of an end-system with the network. Note that in the simple case where the
two end systems are located on the same sub-network (e.g., they are directly
connected), there may be little or no need for a network layer.

Network protocols can be connection-oriented or connectionless. Connection-oriented
protocols require some initial interaction between the communicating entities before
data transfer begins. This interaction leads to the creation of a logical connection or
virtual circuit between the communicating entities. On the other hand, connectionless
protocols do not require any initial interaction between the communicating entities.
Furthermore, one message is handled independently of any other messages between
the same entities.
The network layer is also responsible for segmenting messages into data units that can
be accepted by the data link layer. Such functionality is required due to the different
technologies used in local and wide area networks. Furthermore, since it would be
insufficient to enforce a single data unit size, segmentation can occur more than once.
Reassembly, which refers to creating the original message prior to segmentation, can
be performed in the intermediate nodes or the end systems. Finally, it is also possible
for the network layer to perform error and flow control.
Routers, which provide the necessary functionality for connecting local area networks
and/or wide area networks, operate at the network layer. Some well-known protocols for
the network layer include the Internet Protocol (IP), the Inter-network Packet Exchange
(IPX) protocol, and the X.25 Layer 3 protocol.
3.3.2.2 The Higher Layers: Transport, Session, Presentation and
Application
The four higher layers of the OSI model provide services to users of end systems,
hence constitute the end system or end-to-end portion of the model. These layers
typically, but not always (e.g., in the case of gateways or Layer 4 switches) run on end
systems.
Transport Layer
The transport layer provides a reliable and transparent transfer of data between end
systems, on top of a possibly unreliable network layer. In order to provide a reliable
transfer service, the transport layer uses mechanisms such as error detection and
recovery, and flow control. Note that such mechanisms can also exist in lower layers,
such as the data link layer. The difference is that the data link layer is responsible for
the reliable transmission of data over a single link, whereas the transport layer is
responsible for the reliable transmission of data from the source to the destination,
which can involve a number of independent links.
The transport layer is also responsible for segmenting long messages into smaller units
or packets that can be accepted by the network layer, and then reassembling the
packets into the original message. Furthermore, similar to network layer protocols,
transport layer protocols can be connection-oriented or connectionless. Finally,

transport layer protocols are capable of multiplexing data from different higher layer
protocols.
The complexity of the transport layer depends both on the service it is expected to
provide to the session layer and on the service it receives from the network layer.
Hence, if the network layer provides an unreliable connectionless (datagram) service
and the transport layer is to provide an error-free, in sequence and zero loss or
duplications transmission of data, then the transport layer will need to implement
extensive error and duplicate detection, retransmission and recovery, and congestion
control mechanisms.
Examples of transport layer protocols include TCP (Transmission Control Protocol),
which is a connection-oriented protocol, and UDP (User Datagram Protocol), which is a
connectionless protocol (Feit, 1998).
Session Layer
The session layer is responsible for controlling the dialogue between the end systems.
This involves establishing and terminating the dialogue, called session, between
applications. The session layer can also include determination of the dialogue type used
and synchronization between the end systems through a checkpointing mechanism.
Presentation Layer
The presentation layer is responsible for the encoding or bit pattern representation of
the transferred data. Its objective is to resolve any differences in the format or encoding
of application data. Two examples of the presentation layer functions are data
compression and data encryption.
Application Layer
Finally, the application layer provides end user services, such as file transfer, electronic
message transfer, virtual terminal emulation, etc. Some well-known examples of
application layer protocols include TELNET (Remote Login), FTP (File Transfer
Protocol), SMTP (Simple Mail Transfer Protocol), SNMP (Simple Network Management
Protocol), X.400 (Message Handling System), and X.500 (Directory Services).

Chapter
Cellular Concept
4.1 What is Cell
The cell is a geographical area covered by RF signals. The RF source is located at the
center of the cell as shown in figure 4.1.
This essentially a radio communication center comprising radios, antennas, and much
supporting equipments enabling communication between mobile to land, land to mobile,
and mobile to mobile units. The entire communication process is controlled and
monitored by the system intelligence resident within the MSC.
The shape and size of the cell depends on several parameters such as ERP, antenna
radiation pattern, and propagation environments. Traditionally, a practical cell is
assumed to be highly irregular having regular RSL at the cell boundary. On the other
hand, the analytical cell, generally used for planning and engineering is assumed to be
a perfect hexagon (as shown in figure 4-1).

Figure 4-1: The Cell
Consequently, a discrepancy arises between the analytical cell and the practical cell. As
mentioned the analytical cell is used for system planning and design, and its initial
deployment is based on computer-aided prediction tools that closely approximate a
practical cell in a given propagation environment. Traffic engineering also plays an
important role in determining the size of the cell.
4.2 Cell Coverage
Cell coverage primarily depends on user-defined parameters such as transmitting
power, antenna height, antenna gain, antenna location, and antenna directivity. Several
other parameters such as propagation environment, hills, tunnels, foliage, and buildings
greatly affect the overall RF coverage. These types of parameters are not user defined,
vary from place to place, and are difficult to predict. As a result, a practical cell is highly
irregular in multi-path environment as depicted in figure 4-2.
CELL
RSL

Figure 4-2: A practical cell having different coverage in different directions due to
multipath, shadowing, hills, vegetation, foliage & building clutter factors.
Consequently, several prediction models have been developed in recent years. The two
most widely used propagation models, accommodating most of this anomalies of
propagation, are the Okumura-Hata and Walfich-Ikegami propagation models. The
foundation of most computer-aided prediction tools available today is also based on
these models. This prediction models are based on extensive experimental data and
statistical analysis that enable us to compute the received signal level in a given
propagation medium.
In these models the path loss characteristics follow the equation of a straight line of the
following form:
L(dB) = L0(dB) + 10 γ log(d/d0)
Where,
d0 = fresnel zone break point (d0 ≈4h1h2/ λ)
d = coverage in a particular direction
h1 = base station antenna height
h2 = mobile antenna height
90°
180°
270°
360°
90°

Where the coverage is d0,
γ = Propagation constant in same direction (function of environment)
L0 = intercept (function of environment, antenna height, location, etc.) dB
L = Path loss in the same direction.
With RSL being the received signal level, we predict that,
d ≈ d0 10 (ERP - L0 – RSL)/10 γ
Which indicates that for a given propagation environment and cell site location, the
coverage depends on parameters such as ERP, RSL, and antenna height, which are
user defined, and on several clutter factors, determining L0, which is the intercept.
Consequently, coverage prediction and cell site deployment, classified as RF
engineering, is a major discipline within the cellular industries. It is also an ongoing
process even in a fully developed cellular system for cell site optimization, performance,
and capacity enhancement.
4.2.1 Okumura Model
Okumura’s model is one of the most widely used for signal prediction in urban areas.
This model is applicable for frequencies in the range 150MHz-1920 MHz (although it is
typically extrapolated up to 3000 MHz) and distances of 1Km- 100Km. It can be used for
base station antenna heights ranging from 30m- 1000m.
Okumura developed a set of curves giving the medium attenuation relative to free space
(Amu), in an urban area over a quasi-smooth terrain with a base station effective
antenna height (hte) of 200m and a mobile antenna height (hre) of 3m. These curves
were developed from extensive measurements using vertical omnidirectional antennas
at both the base and mobile, and are plotted as a function of frequency in the range
100MHz to 1920MHz and as a function of distance from the base station in the range
1km to 100km. To determine path loss using Okumura’s model, the free space path loss
between the points of interest is first determined, and then the value of Amu (f,d) (as
read from the curves) is added to it along with correction factors to account for the type
of terrain. The model can be expressed as:
L50 (dB) = Lf+Amu(f,d)-G(hte)-G(hre)-GArea
Where, L50 is the 50th
percentile (median) value of propagation path loss. Lf is the free
space propagation loss. Amu is the median attenuation relative to free space. G(hte) is
the base station antenna height gain factor. G(hre) is the mobile antenna height gain
factor. And GArea is the gain due to the type of environment. Note that the antenna
height gains are strictly a function of height and have nothing to do with antenna
patterns.
Plots of Amu(f,d) and GArea for a wide range of frequencies are shown in Figure 4.23 and
Figure 4.24. Furthermore, Okumura found that G(hte) varies at a rate of 20dB/deacde
and G(hre) varies at a rate of 10dB/decade for heights less than 3m.
Okumura’s model is considered to be among the simplest and best in terms of accuracy
in path loss prediction for mature cellular and land mobile radio systems in cluttered

environments. It is very practical and has become a standard for system planning in
modern land mobile radio system in Japan. The major disadvantage with the model is
its slow response to rapid changes in terrain. Therefore the model is fairly good in urban
and sub-urban areas, but not as good in rural areas.
4.2.2 Hata Model
The Hata model is an emperical formulation of the graphical path loss data provided by
Okumura and is valid from 150MHz to 1500MHz. Hata presented the urban area
propagation loss as a standard formula and supplied correction equations for
application to other situations. The standard formula for median path loss in urban areas
is given by:
L50(urban)(dB)=69.55+26.16logfc-13.82loghte-a(hre)+(44.9-6.55loghte)log d
Where fc is the frequency of the carrier, hte is the effective transmitter antenna height
ranging from 30m to 200m, hre is the effective receiver antenna height ranging from 1m
to 10m, d is the T-R separation distance (in km) and a(hre) is the correction factor for
effective mobile antenna height which is a function of the size of the coverage area. For
a small to medium sized city, the mobile antenna correction factor is given by:
a(hre)=(1.1logfc- 0.7) hre-(1.56logfc-0.8) dB
And for the large city, it is given by:
a(hre)=8.29(log1.54 hre)2
-1.1 dB for fc ≤ 300MHz
a(hre)=3.2(log11.75 hre)2
-4.97 dB for fc ≥ 300MHz
To obtain the path loss in the suburban area, the standard Hata formula is modified as:
L50 (dB) = L50 (urban)-2[log(fc /28)]2
-5.4
and for path loss in open rural areas, the formula is modified as:
L50 (dB) = L50 (urban)-4.78(log fc)2
+18.33log fc-40.94
Although Hata’s model does not have any of the path specific corrections which are
available in Okumura’s model, the above expression have significant value. The
prediction of the Hata model compare very closely with the original Okumura model, as
long as d exceeds1km. This model is well suited for large cell mobile systems, but not
personal communications systems (PCS) which have cells on the order of 1km radius.
4.3 Co-channel Interference
A co-channel interferer has the same nominal frequency as the deserved frequency. It
occurs as the result of the multiple usage of the same frequency. A cell site, radiating in
all directions (OMNI site), is represented by a carrier-to-interference ratio as follows:
C/I = 10 log [1/j(D/R)γ
]

Sector
1
Sector
2
Sector
3
Where
j = number of co-channel interferers (j = 1,2,3…,6)
γ= Propagation constant
D= frequency reuse distance
R= cell radii
4.4 Sectorization
The 120-deg sectorization is achieved by dividing a cell in to 3 sectors of 120 degree
each (figure 4-3a).
(a) (b)
Figure 4-3: Sectorization: (a) 3-Sector configuration & (b) 6-Sector configuration
Directional antennas are used in each sector for a total of three antennas per cell. The
60 degree sectorization is achieved by dividing a cell into 6 sectors of 60 degree each
(figure 4-4b). Each sector is treated as logical OMNI cell; directional antennas are used
in each sector for a total of 6 antennas per cell. Because of the antenna beam width,
channels can be repeated more often, thus enhancing the capacity. The configuration is
generally used in dense urban environments.
Because of directionalization, the C/I equation presented is now modified as:
C/I = 10 log [(1/j)(φ1/φ2)(D/R)γ
]
Where,
φ1/φ2 is the antenna directivity factor
j = number of co-channel interferers (j = 1,2,3…,6)
γ= Propagation constant
D= frequency reuse distance
R= cell radii
Sector
1
Sector
2
Sector
3
Sector
4
Sector
5
Sector
6

4.5 Cellular Antennas
An antenna is a signal processing device that transmits and receives electromagnetic
signals at the same time. It available in two general categories;
1. Passive antenna
2. Active antenna
The radiation pattern of passive antenna depends on the type and construction of the
device because the radiation pattern is not fixed until after construction of the device.
However, it can be guided to a certain degree by mechanical means. Mechanical down
tilt is a common practice to control the signal within a cell.
The radiation of a active antenna depends on the type, construction, and built-in signal
processing technique of the device. Generally, digital signal processing techniques are
used to generate a desired radiation pattern. The radiation pattern can be steered in a
given direction as well. Also, there are two general classes of radiation pattern:
1. OMNI directional (in all direction)
2. Directional (in certain direction)
OMNI antennas are used in cell sites and directional antennas are used in sectored
sites. Some of the antenna parameters, essential for cell cite engineering, are given
below:
a. Antenna directivity and gain
b. Antenna beam width
c. Antenna front to back ratio
d. Frequency response and bandwidth.
4.6 Cellular Network and Its General Characteristics
4.6.1 Cellular Network
A cellular network is a radio network made up of a number of a radio cells (or just cells)
each served by a fixed transmitter, known as cell site or base station. These cells are
used to cover different areas I order to provide radio coverage over a wider area than
the area of one cell. Cellular networks are inherently asymmetric with a set of fixed main
transceivers each serving a cell and a set of distributed transceivers which provide
services to the network’s users.
Cellular networks offer a number of advantages over alternative solutions:
• Increased capacity
• Reduced power usage
• Better coverage
A good (and simple) example of cellular system is a Policeman’s radio system where
the Police will have several transmitters around a city each operated by an individual
operator.

4.6.2 General Characteristics
The primary requirement for a network to be succeeded as a cellular network is for it to
have developed a standardized method for each distributed station to distinguish the
signal emanating from its own transmitter from the signals received from other
transmitters. Presently, there are two standardized solutions to this issue: frequency
division multiple access (FDMA) and code division multiple access (CDMA).
FDMA works by using varying frequencies for each neighboring cell. By tuning to the
frequency of a chosen cell the distributed stations can avoid the signal from other cells.
The principle of CDMA is more complex, but achieves the same result; the distributed
transceivers can selct one cell and listen to it. Other available methods of multiplexing
such as polarization division multiple access (PDMA) and time division multiple access
(TDMA) cannot be used to separate signals from one cell to the next since the effect of
both vary with position and this would make signal separation partially impossible. Time
division multiple access, however, is used in combination with either FDMA or CDMA in
a number of systems to give multiple channels within the coverage are of single cell.
In the case of aforementioned Police network, each radio has a knob, the knob acts as
a channel selector and allows the radio to tune to different frequencies as the policeman
move around, the change from channel to channel. The policeman know which
frequency covers approximately what area, when thy don’t get a signal from the
transmitter, they also try other channels until they find one which works the policeman
only speaks one at a time, as invited by the operator (in a sensed TDMA).
4.6.3 Cell Cluster and Frequency Reuse Plan:
A cell cluster is identical cells in which all of the available (frequencies) are evenly
distributed. The most widely used plan is the N=7 cell cluster (Fig 4-4a), where 416
cellular channels are evenly distributed among seven cells having approximately 59
channels per cell, which then repeats itself over and over according to (Fig 4-6-3b).
Because of the limited number of the channels, their reuse must be carefully planned. In
hexagonal geometry, this reuse plan is given by the equation:
D/R = √3N
Where D is the reuse distance, R is the cell Radii and N is the modulus.

D
(a)
(b)
Fig 4-4: (a) A Seven cell Cluster. (b) A Seven cell reuse plan
The increased capacity in a cellular network, compared with a network with a single
transmitter, comes from the fact that the same radio frequency can be reused in a
different area for a completely different transmission. If there is a single plain
transmitter, only one transmission can be used on any given frequency. Unfortunately,
there is inevitably some level of interference from the signal from the other cells which
use the same frequency. This means that, in a standard FDMA system there must be at
least a one cell gap between cells which reuse the same frequency.
The frequency reuse factor is the rate at which the same frequency can be used in the
network. It is 1/n where n is the number of cells which cannot use a frequency for
transmission. A common value for the frequency reuse factor is 7.
Code division multiple access – based systems use a wider frequency band to achieve
the same rate of transmission as FDMA, but this is compensated for by the ability to use
a frequency reuse factor of 1. In other words, every cell use same frequency and the
different systems are separated by codes rather than frequencies.
Depending on the size of the city a police radio system may not have any frequency –
reuse in its own city, but certainly in other nearby cities the same frequency can be
used. In a big city, on the other hand, frequency-reuse could certainly be in use.
6 3
4 1 5
2 7
6 3
4 1 5
2 7
6 3
4 1 5
2 7
6 3
4 1 5
2 7
6 3
4 1 5
2 7
6 3
4 1 5
2 7
6 3
4 1 5
2 7
6 3
4 1 5
2 7

Chapter
Wi-Fi Communication System
5.1 Introduction
The OSI network model illustrates how data and protocol messages from the application
level cascade down through the logical layers and result in a series of data frames to be
transmitted across the physical network medium.
Starting with the RF spectrum, the regulation of spectrum use is briefly described and
spread spectrum techniques are then introduced. This is a key technology that enables
high data link reliability by making RF communications less susceptible to interference.
Multiple access methods that enable many users to simultaneously use the same
communication channel are then discussed. Signal coding and modulation is the step
that encodes the data stream onto the RF carrier or pulse train, and a range of coding
and modulation techniques applied in wireless networking will be covered, from the
simplest to some of the most complex.
The various elements that impact on RF signal propagation will be described, enabling
a calculation of the link budget, the balance of power available to overcome system and
propagation losses to bring the transmitted signal to the receiver at a sufficient power
level for reliable, low error rate reception. The link budget calculation is an essential part
of the toolkit of the wireless network designer in defining the basic power requirements
for a given network installation. This will usually be supplemented by practical
techniques such as site surveys.

5.2 Radio Communication Basics
5.2.1 The RF Spectrum
The radio frequency, or RF, communication at the heart of most wireless networking
operates on the same basic principles as everyday radio and TV signals. The RF
section of the electromagnetic spectrum lies between the frequencies of 9 kHz and 300
GHz (Table 5-1), and different bands in the spectrum are used to deliver different
services.
Recalling that the wavelength and frequency of electromagnetic radiation are related via
the speed of light, so that wavelength (l) = speed of light (c) / frequency ( f ), or
wavelength in metres = 300 / frequency in MHz.
Transmission type Frequency Wavelength
Very low frequency (VLF) 9–30 kHz 33–10 km
Low frequency (LF) 30–300 kHz 10–1 km
Medium frequency (MF) 300–3000 kHz 1000–100 m
High frequency (HF) 3–30 MHz 100–10 m
Very high frequency
(VHF)
30–300 MHz 10–1 m
Ultra high frequency
(UHF)
300–3000 MHz 1000–100 mm
Super high frequency
(SHF)
3–30 GHz 100–10 mm
Extremely high frequency
(EHF)
30–300 GHz 10–1 mm
Table 5-1: Subdivision of the Radio Frequency Spectrum
Beyond the extremely high frequency (EHF) limit of the RF spectrum lies the infrared
region, with wavelengths in the tens of micro-meter range and frequencies in the region
of 30 THz (30,000 GHz).
Virtually every Hz of the RF spectrum is allocated for one use or another (Table 5-2),
ranging from radio astronomy to forestry conservation, and some RF bands have been
designated for unlicensed transmissions.

The RF bands which are used for most wireless networking are the unlicensed ISM or
Instrument, Scientific and Medical bands, of which the three most important lie at 915
MHz (868 MHz in Europe), 2.4 GHz and 5.8 GHz (Table 5-3). As well as these narrow
band applications, new networking standards such as ZigBee (IEEE 802.15.4), will
make use of the FCC spectrum allocation for ultra wideband radio (UWB) that permits
very low power transmission across a broad spectrum from 3.1 to 10.6 GHz.
Fixed Mobile
Space
Research
Space
Operation
Earth Exploration
Space
Research
Fixed Mobile
Amateur
Broadcasting Satellite
Mobile Radio Location
Amateur
Fixed Mobile
Radio-determination
Satellite
Mobile Satellite
Broadcasting Satellite Fixed
Radio
Astronomy
Space
Research
Earth Exploration Satellite
Aeronautical
Radio-
navigation
Meteorological
Aids
Radio Location
2.2
2.3
2.4
2.5
2.7
2.9
GHz
Table 5-2: FCC Spectrum Allocation around the 2.4GHz ISM Band
RF band Wireless networking specification
915/868 MHz ISM ZigBee
2.4 GHz ISM IEEE 802.11b, g, Bluetooth, ZigBee
5.8 GHz IEEE 802.11a
Table 5-3: Radio Frequency Bands in Use for Wireless Networking

5.2.2 Radio Frequency Spectrum Regulation
The use of the radio frequency spectrum, in terms of the frequency bands that can be
used for different licensed and unlicensed services, and the allowable transmission
power levels for different signal formats, are controlled by regulatory authorities in
individual countries or regions.
Although there is an increasing trend towards harmonisation of spectrum regulation
across countries and regions, driven by the International Telecommunications Union’s
World Radio Communication Conference, there are significant differences in spectrum
allocation and other conditions such as allowable transmitter power levels which have
an impact on wireless networking hardware design and interoperability.
Regulator 2.4 GHz ISM specifications
FCC (USA) 1 W maximum transmitted power
2.402–2.472 GHz, 11 × 22 MHz
channels
ETSI (Europe) 100 mW maximum EIRP
2.402–2.483 GHz, 13 × 22 MHz
channels
ARIB (Japan) 100 mW maximum EIRP
2.402–2.497 GHz, 14 × 22 MHz
channels
Table 5-4: 2.4 GHz ISM Band Regulatory Differences by Region
As an example, in the 5.8 GHz ISM band used for IEEE 802.11a networks, the FCC in
the USA allows a maximum transmitted power of 1 W, while in Europe the ETSI permits
a maximum EIRP (equivalent isotropic radiated power) of just 100 mW EIRP or 10
mW/MHz of bandwidth, with variations in other countries. Table 5-4 shows a range of
other regulatory differences that apply to the 2.4 GHz ISM band used for IEEE 802.11
b/g networks.
The pace of regulatory change also differs from region to region. For example, the FCC
developed regulations governing ultra wideband radio in 2002, while in Europe ETSI
Task Group 31 was still working on similar regulations in 2006.
Although the regulatory bodies impose conditions on the unlicensed use of parts of the
RF spectrum, unlike their role in the licensed spectrum, these bodies take no
responsibility for or interest in any interference between services that might result from
that unlicensed use. In licensed parts of the RF spectrum, the FCC and similar bodies
have a role to play in resolving interference problems, but this is not the case in
unlicensed bands. Unlicensed means in effect that the band is free for all, and it is up to
users to resolve any interference problems. This situation leads some observers to
predict that the 2.4 GHz ISM band will eventually become an unusable junk band,
overcrowded with cordless phone, Bluetooth, 802.11 and a cacophony of other

transmissions. This impending “tragedy of the commons” may be prevented by the
development of spectrum agile radios.
5.3 Radio Transmission as a Network Medium
Compared to traditional twisted-pair cabling, using RF transmission as a physical
network medium poses a number of challenges. Security has been a significant concern
since RF transmissions are far more open to interception than those confined to a
cable. Data link reliability, bit transmission errors resulting from interference and other
signal propagation problems, are probably the second most significant challenge in
wireless networks, and one technology that resulted in a quantum leap in addressing
this problem (spread spectrum transmission) is the subject of the next section.
Controlling access to the data transmission medium by multiple client devices or
stations is also a different type of challenge for a wireless medium, where, unlike a
wired network, it is not possible to both transmit and receive at the same time. Two key
situations that have the potential to degrade network performance are the so-called
hidden station and exposed station problems.
Figure 5-1: Hidden and Exposed Station Challenges for Wireless Media Access
Control
The hidden station problem occurs when two stations A and C are both trying to
transmit to an intermediate station B, where A and C are out of range and therefore one
cannot sense that the other is also transmitting (see Figure 5-1). The exposed station
problem occurs when a transmitting station C, prevents a nearby station B from
transmitting although B’s intended receiving station A is out of range of station C’s
transmission.

The later sections of this chapter look at digital modulation techniques and the factors
affecting RF propagation and reception, as well as the practical implications of these
factors in actual wireless network installations.
5.4 Orthogonal Frequency Division Multiplexing
Orthogonal frequency division multiplexing (OFDM) is a variant of frequency division
multiplexing (FDM), in which a number of discrete sub-carrier frequencies are
transmitted within a band with frequencies chosen to ensure minimum interference
between adjacent sub-carriers. This is achieved by controlling the spectral width of the
individual sub-carriers (also called tones) so that the frequencies of sub-carriers
coincide with minima in the spectra of adjacent sub-carriers, as shown in Figure 5-1.
Figure 5-2: Orthogonality of OFDM Sub-carriers in the Frequency Domain
In the time domain, the orthogonality of OFDM tones means that the number of sub-
carrier cycles within the symbol transmission period is an integer, as illustrated in Figure
5-3. This condition can be expressed as:
Ts = ni / ni or ni = ni / Ts
Where, Ts is the symbol transmission period and ni is the frequency of the ith
sub-
carrier. The Sub-carriers are therefore evenly spaced in frequency, with separation
equal to the reciprocal of the symbol period.
Figure 5-3: Orthogonality of OFDM Sub-carriers in the Time Domain

There are a number of ways in which the multiple Sub-carriers of OFDM can be used:
■ OFDM can be used as a multiple access technique (OFDMA), by assigning
single Sub-carriers or groups of Sub-carriers to individual users according to their
bandwidth needs.
■ A serial bit stream can be turned into a number of parallel bit streams each one
of which is encoded onto a separate sub-carrier. All available Sub-carriers are used by
a single user to achieve a high data throughput.
■ A bit stream can be spread using a chipping code and then each chip can be
transmitted in parallel on a separate sub-carrier. Since the codes can allow multiple
user access, this system is known as Multi-Carrier CDMA (MC-CDMA). MC-CMDA is
under consideration by the WIGWAM project as one of the building blocks of the 1 Gbps
wireless LAN.
A significant advantage of OFDM is that, since the symbol rate is much lower when
spread across multiple carriers than it would be if the same total symbol rate were
transmitted on a single carrier, the wireless link is much less susceptible to inter-symbol
interference (ISI). ISI occurs when, as a result of multi-path propagation, two symbols
transmitted at different times arrive together at the receiving antenna after traversing
different propagation paths (Figure 5-4). Although OFDM is inherently less
Figure 5-4: Inter Symbol Interference (ISI)
Susceptible to ISI, most OFDM systems also introduce a guard interval between each
symbol to further reduce ISI.
OFDM radios also use a number of Sub-carriers, called pilot tones, to gather
information on channel quality to aid demodulation decisions. These Sub-carriers are
modulated with known training data at the start of each transmitted data packet.
Decoding this known data enables the receiver to determine and adaptively correct for
the frequency offset and phase noise between the reference oscillators in the
transmitter and the receiver and for fading during propagation.
Figure 5-5 shows a schematic block diagram of a simple OFDM transmitter and
receiver.
Tx Process:
• At the transmitter, the input information (a serial bit or symbol stream) sequence
is first converted into parallel data sequences.
• Then each serial/parallel converter output is multiplied with spreading code (QAM
code modulation).

• Data from all subcarriers is modulated in baseband by inverse fast Fourier
transform (IFFT).
• After the Fourier transform, the data is converted back into serial form.
• The guard interval is inserted between symbols to avoid ISI caused by multipath
fading.
• Finally, the signal is converted to analog form suitable for transmission and
transmitted after RF up-conversion.
Rx Process:
• At the receiver, after down- conversion, the signal is converted to digital form.
• The guard interval is removed from symbols; which was used to avoid ISI caused
by multipath fading.
• The m-subcarrier component corresponding to the received data is first
coherently detected with FFT.
• After the Fourier transform, the data is decoded to retrieve the original parallel
data and is multiplied with gain to combine the energy of the received signal
scattered in the frequency domain.
• Finally, the parallel data is converted to serial data in the parallel to serial
converter.
Figure 5-5: Schematic Block Diagram of an OFDM Transmitter and Receiver
The IEEE 802.11a/g standards uses OFDM in the unlicensed 2.4 and 5 GHz ISM bands
respectively to provide data rates up to 54 Mbps. The system uses 52 Sub-carriers of
which 48 are used to carry data and are modulated using binary or quadrature
phase shift keying (BPSK/QPSK), 16-quadrature amplitude modulation (QAM) or 64-
QAM. The remaining four Sub-carriers are used as pilot tones.
RF down
conversi
on
Noise, multi path and
other interference
Transmission
channel
Sub-carrier phase
and amplitude
Serial to
parallel
Data
coding
(QAM)
Inverse
Fast
Fourier
Transfo
rm
(IFFT)
Guard
interval
insertion
Digital to
analogue
conversion and
filtering
RF up
Conve
rsion
OFDM
M d l ti
Serial bit
or
symbol
t
Parallel
to serial
Channel
equalization
and data
decoding
Fast
Fourier
Transfo
rm
(FFT)
Guard
interval
Analogue to
digital
Serial bit
or
symbol
stream

5.5 Spread Spectrum Transmission
Spread spectrum is a radio frequency transmission technique initially proposed for
military applications in World War II with the intention of making wireless transmissions
safe from interception and jamming. These techniques started to move into the
commercial arena in the early 1980s. Compared to the more familiar amplitude or
frequency modulated radio transmissions, spread spectrum has the major advantage of
reducing or eliminating interference with narrowband transmissions in the same
frequency band, thereby significantly improving the reliability of RF data links.
Figure 5-6: A Simple Explanation of Spread Spectrum
Unlike simple amplitude or frequency modulated radio, a spread spectrum signal is
transmitted using a much greater bandwidth than the simple bandwidth of the
information being transmitted. Narrow band interference (the signal I in Figure 5-6) is
rejected when the received signal is “de-spread”. The transmitted signal also has noise-
like properties and this characteristic makes the signal harder to eavesdrop on.
5.6 Types of Spread Spectrum Transmission
The key to spread spectrum techniques is some function, independent of the data being
transmitted, that is used to spread the information signal over a wide transmitted
bandwidth. This process results in a transmitted signal bandwidth which is typically 20
to several 100 times the information bandwidth in commercial applications, or 1000 to 1
million times in military systems.
Several different methods of spread spectrum transmission have been developed,
which differ in the way the spreading function is applied to the information signal. Two
methods, direct sequence spread spectrum and frequency hopping spread spectrum,
are most widely applied in wireless networking.
In Direct Sequence Spread Spectrum (DSSS) (Figure 5-7), the spreading function is a
code word, called a chipping code, that is XOR’d with the input bit stream to generate a
higher rate “chip stream” that is then used to modulate the RF carrier.

Figure 5-7: A Simple Explanation of DSS
In Frequency Hopping Spread Spectrum (FHSS) (Figure 5-8), the input data stream is
used directly to modulate the RF carrier while the spreading function controls the
specific frequency slot of the carrier within a range of available slots spread across the
width of the transmission band.
Time Hopping Spread Spectrum (THSS) (Figure 5-9), is a third technique in which the
input data stream is used directly to modulate the RF carrier which is transmitted in
pulses with the spreading function controlling the timing of each data pulse.
Figure 5-8: A Simple Explanation of FHSS
Data burst transmitted at a position within the
time slot determined by the spreading function
Figure 5-9: A Simple Explanation of THSS

For example, impulse radio uses pulses that are so short, typically in the region of 1
nanosecond (nS), that the spectrum of the signal is very wide and meets the definition
of an ultra wideband (UWB) system. The spectrum is effectively spread as a result of
the narrowness of transmitted pulses, but time-hopping, with each user or node being
assigned a unique hopping pattern, is a simple technique for impulse radio to allow
multiple user access.
Two other less common techniques are Pulsed FM systems and Hybrid systems (Figure
5-10). In Pulsed FM systems, the input data stream is used directly to modulate the RF
carrier, which is transmitted in frequency modulated pulses. The spreading function
controls the pattern of frequency modulation, which could for example be a linear “chirp”
with frequency sweeping up or down.
Data pulse with FM pattern determined by the spreading function
Figure 5-10: A Simple Explanation of Pulsed FM Systems
Hybrid systems also use combinations of spread spectrum techniques and are designed
to take advantage of specific characteristics of the individual systems. For example,
FHSS and THSS methods are combined to give the hybrid frequency division – time
division multiple access (FDMA/TDMA) technique (see the Section “Wireless
Multiplexing and Multiple Access Techniques).
Of these alternative spread spectrum techniques, DSSS and FHSS are specified in the
IEEE 802.11 wireless LAN standards, although DSSS is most commonly used in
commercial 802.11 equipments. FHSS is used by Bluetooth, and FHSS and chirp
spread spectrum are optional techniques for the IEEE 802.15.4a (ZigBee) specification.

5.7 Chipping, Spreading and Correlating
The spreading function used in DSSS is a digital code, known as a chipping code or
pseudo-noise (PN) code, which is chosen to have specific mathematical properties. One
such property is that, to a casual listener on the broadcast band, the signal is similar to
random noise, hence the “pseudo-noise” label.
Under the IEEE 802.11b standard, the specified PN code for 1 Mbps and 2 Mbps data
rates is the 11-bit Barker code. Barker codes are binary sequences that have low auto-
correlation, which means that the sequence does not correlate with a time-shifted
version of itself. Barker codes of length 2 to 13 are shown in Table 5-5.
Length Code
2 10 and 11
3 110
4 1011 and 1000
5 11101
7 1110010
11 11100010010
13 111100111001
Table 5-5: Barker Codes of Length 2 to 13
Figure 5-11 illustrates the direct sequence encoding of a data stream using this code.
To distinguish a data bit from a code bit, each symbol (each 1 or 0) in the coded
sequence is known as a chip rather than a bit.
Bit stream Data
Chipping code 11-bit Barker sequence
Chip stream Encoded data
Figure 5-11: DSSS Pseudo-noise Encoding
This process results in a chip stream with a wider bandwidth than the original input data
stream. For example, a 2 Mbps input data rate is encoded into a
22 Mbps (Mega chips per second) sequence, the factor of 11 coming about since the
encoded sequence has 11 chips for each data bit. The resulting bandwidth of the

transmitted RF signal will depend on the technique used to modulate the encoded data
stream onto the RF carrier. A simple modulation technique like binary phase-shift keying
(BPSK) results in a modulated carrier with a bandwidth equal to twice the input bit rate
(or in this case, the chip rate).
When the encoded signal is received, a code generator in the receiver recreates the
same PN code and a correlator uses this to decode the original information signal in a
process known as correlating or dispreading. Since the correlator only extracts signals
encoded with the same PN code, the receiver is unaffected by interference from narrow
band signals in the same RF band, even if these signals have a higher power density (in
watts/Hz) than the desired signal.
5.7.1 Chipping Codes
One of the desirable mathematical properties of PN codes is that it enables the
receiver’s PN code generator to very rapidly synchronize with the PN code in the
received signal. This synchronization is the first step in the de-spreading process. Fast
synchronization requires that the position of the code word can be quickly identified in a
received signal, and this is achieved as a result of the low auto-correlation property of
the Barker codes. Another benefit of low auto-correlation is that the receiver will reject
signals that are delayed by more than one chip period. This helps to make the data link
robust against multi-path interference.
A second key property of chipping codes that is important in applications where
interference between multiple transmitters must be avoided, for example in mobile
telephony, is low cross-correlation. This property reduces the chance that a correlator
using one PN code will experience interference from a signal using a different code (i.e.
that it will incorrectly decode a noisy signal that was encoded using a different chipping
code). Ideally codes in use in this type of multiple access application should have zero
cross-correlation, a property of the orthogonal codes used in CDMA.
Code orthogonality for multiple access control is not required for wireless networking
applications, such as IEEE 802.11 networks, as these standards use alternative
methods to avoid conflict between overlapping transmitted signals from multiple users.

5.8 Direct Sequence Spread Spectrum (DSSS) in the
2.4GHz ISM Band
It is the most widely recognized form of spread spectrum. The DSSS process is
performed by effectively multiplying an RF carrier and a pseudo-noise (PN) digital
signal. First the PN code is modulated onto the information signal using one of several
modulation techniques (eg. BPSK, QPSK, etc ). Then, a doubly balanced mixer is used
to multiply the RF carrier and PN modulated information signal. This process causes
the RF signal to be replaced with a very wide bandwidth signal with the spectral
equivalent of a noise signal. The demodulation process (for the BPSK case) is then
simply the mixing/multiplying of the same PN modulated carrier with the incoming RF
signal. The output is a signal that is a maximum when the two signals exactly equal one
another or are "correlated". The correlated signal is then filtered and sent to a BPSK
demodulator.
The signals generated with this technique appear as noise in the frequency domain.
The wide bandwidth provided by the PN code allows the signal power to drop below the
noise threshold without loss of information.
One feature of DSSS is that QPSK may be used to increase the data rate. This
increase of a factor of two bits per symbol of transmitted information over BPSK causes
an equivalent reduction in the available process gain. The process gain is reduced
because for a given chip rate, the bandwidth (which sets the process gain) is halved
due to the two-fold increase in information transfer. The result is that systems in a
spectrally quiet environment benefit from the possible increase in data transfer rate.
As noted above, in DSSS the data signal is combined with a code word, the chipping
code, and the combined signal is used to modulate the RF carrier, resulting in a
transmitted signal spread over a wide bandwidth. For example, in the 2.4 GHz ISM
band, a spread bandwidth of 22 MHz is specified for IEEE 802.11 networks, as shown
in Figure 5-12.
The 2.4 GHz ISM band has a total allowed width of 83.5 MHz and is divided into a
number of channels (11 in the USA, 13 in Europe, 14 in Japan), with 5 MHz steps
between channels.
Figure 5-12: 802.11 DSSS Channels

Figure 5-13 : DSSS Channels in the 2.4 GHz ISM Band (US)
To fit 11 or more 22 MHz wide channels into an 83 MHz wide band results in
considerable overlap between the channels (as shown in Figure 5-13), resulting in the
potential for interference between signals in adjacent channels. The 3 non-overlapping
channels allow 3 DSSS networks to operate in the same physical area without
interference.
5.9 Spread Spectrum in Wireless Networks — Pros
and Cons
The advantages of spread spectrum techniques, such as resistance to interference and
eavesdropping and the ability to accommodate multiple users in the same frequency
band, make this an ideal technology for wireless network applications (Table 1).
Although the good interference performance is achieved at the cost of relatively
inefficient bandwidth usage, the available radio spectrum, such as the 2.4 GHz ISM
band, still permits data rates of up to 11 Mbps using these techniques.
Since speed and range are important factors in wireless networking applications, DSSS
is the more widely used of the two techniques although, because of its simpler and
cheaper implementation, FHSS is used for lower rate, shorter-range systems like
Bluetooth and the now largely defunct Home-RF.

5.10 Different standards for 802.11 according to
technology
IEEE 802.11
The bit rate for the original IEEE 802.11 standard is 2 Mbps using the FHSS
transmission scheme and the S-Band Industrial, Scientific, and Medical (ISM) frequency
band, which operates in the frequency range of 2.4 to 2.5 GHz. However, under less-
than-ideal conditions, a lower bit rate speed of 1 Mbps is used.
802.11b
The major enhancement to IEEE 802.11 by IEEE 802.11b is the standardization of the
physical layer to support higher bit rates. IEEE 802.11b supports two additional speeds,
5.5 Mbps and 11 Mbps, using the S-Band ISM. The DSSS transmission scheme is used
in order to provide the higher bit rates. The bit rate of 11 Mbps is achievable in ideal
conditions. In less-than-ideal conditions, the slower speeds of 5.5 Mbps, 2 Mbps, and 1
Mbps are used.
Note
• 802.11b uses the same frequency band as that used by microwave ovens, cordless
phones, baby monitors, wireless video cameras, and Bluetooth devices.
802.11a
IEEE 802.11a (the first standard to be ratified, but just now being widely sold and
deployed) operates at a bit rate as high as 54 Mbps and uses the C-Band ISM, which
operates in the frequency range of 5.725 to 5.875 GHz. Instead of DSSS, 802.11a uses
OFDM, which allows data to be transmitted by subfrequencies in parallel and provides
greater resistance to interference and greater throughput. This higher-speed technology
enables wireless LAN networking to perform better for video and conferencing
applications. Because they are not on the same frequencies as other S-Band devices
(such as cordless phones), OFDM and IEEE 802.11a provide both a higher data rate
and a cleaner signal. The bit rate of 54 Mbps is achievable in ideal conditions. In less-
than-ideal conditions, the slower speeds of 48 Mbps, 36 Mbps, 24 Mbps, 18 Mbps, 12
Mbps, and 6 Mbps are used.
802.11g
IEEE 802.11g operates at a bit rate as high as 54 Mbps, but uses the S-Band ISM and
OFDM. 802.11g is also backward-compatible with 802.11b and can operate at the
802.11b bit rates and use DSSS. 802.11g wireless network adapters can connect to an
802.11b wireless AP, and 802.11b wireless network adapters can connect to an
802.11g wireless AP. Thus, 802.11g provides a migration path for 802.11b networks to
a frequency-compatible standard technology with a higher bit rate. Existing 802.11b
wireless network adapters cannot be upgraded to 802.11g by updating the firmware of
the adapter — they must be replaced. Unlike migrating from 802.11b to 802.11a (in
which all the network adapters in both the wireless clients and the wireless APs must be
replaced at the same time), migrating from 802.11b to 802.11g can be done
incrementally. Like 802.11a, 802.11g uses 54 Mbps in ideal conditions and the slower
speeds of 48 Mbps, 36 Mbps, 24 Mbps, 18 Mbps, 12 Mbps, and 6 Mbps in less-than-
ideal conditions.

Chapter
Communication Process
6.1 The 802.11 protocol stack
The protocols used by all the 802 variants, including Ethernet, have certain commonality
of structure. A partial view of the 802.11 protocol stack is given in fig 6-1. The physical
layer corresponds to the OSI physical layer fairly well, but the data link layer in all the
802 protocols is split into two or more sub-layer. In 802.11 the MAC (medium access
control) sublayer determines how the channel is allocated, that is, who gets to transmit
next. Above it is the LLC (logical link control) sublayer, whose job it is to hide the
difference between the different 802 variants and make them indistinguishable as far as
the network layer is concerned.

Figure 6-1: Part of the 802.11 Protocol Stack
The 1997 802.11 standard specifies three transmission techniques allowed in the
physical layer. The infrared method uses much the same technology as television
remote controls do. The other two uses short range radio, using techniques called
FHSS and DSSS. Both of these use a part of the spectrum that does not require
listening (the 2.4-GHz ISM band). In 1999, two new techniques were introduced to
achieve higher bandwidth. These are called OFDM and HR-DSSS. They operate at up
to 54 Mbps and 11 Mbps, respectively. In 2001, a second OFDM modulation was
introduced, but in a frequency band different from the first one.
6.2 The 802.11 Sub-layer Protocol (MAC layer)
6.2.1 Process Overview:
The MAC layer controls the traffic that moves through the radio network. It prevents
data collisions and conﬂicts by using a set of rules called ‘Carrier Sense Multiple Access
with Collision Avoidance’ (CSMA/CA), and it supports the security functions specified in
the 802.11b standard. When the network includes more than one access point, the
MAC layer associates each network client with the access point that provides the best
signal quality.
When more than one node in the network tries to transmit data at the same time,
CSMA/CA instructs all but one of the conﬂicting nodes to back off and try again later,
and it allows the surviving node to send its packet. CSMA/CA works like this: when a
network node is ready to send a packet, it listens for other signals first. If it doesn't hear
anything, it waits for a random (but short) period of time and then listens again. If it still
doesn't sense a signal, it transmits a packet. The device that receives the packet
evaluates it, and if it's intact, the receiving mode returns an acknowledgement. But if the
sending node does not receive the acknowledgement, it assumes that there has been a
Logical Link Control
802.11
Infrared
802.11
FHSS
802.11
DSSS
802.11
OFDM
802.11b
HR-DSSS
802.11g
OFDM
MAC
Sublayer
Upper
Layer
Data
Link
Layer
Physical
Layer

collision with another packet, so it waits for another random interval and then tries
again.
CSMA/CA also has an optional feature that sets an access point (the bridge between
the wireless LAN and the backbone network) as a point coordinator that can grant
priority to a network node that is trying to send time-critical data types, such as voice or
streaming media.
The MAC layer can support two kinds of authentication to confirm that a network
device is authorized to join the network: open authentication and shared key
authentication. When configuring the network, all the nodes in the network must use the
same kind of authentication.
The network supports all of these housekeeping functions in the MAC layer by
exchanging (or trying to exchange) a series of control frames before it allows the higher
layers to send data. It also sets several options on the network adapter:
1. Power mode: The network adapter supports two power modes: ‘Continuous Aware
Mode’ and ‘Power Save Polling Mode’. In ‘Continuous Aware Mode’, the radio receiver
is always on and consuming power. In ‘Power Save Polling Mode’, the radio is idle
much of the time, but it periodically polls the access point for new messages. As the
name suggests, ‘Power Save Polling Mode’ reduces the battery drain on portable
devices such as laptop computers and PDAs.
2. Access control: The network adapter contains the access control that keeps
unauthorized users out of the network. An 802.11b network can use two forms of
access control: the SSID (the name of the network), and the MAC address (a unique
string of characters that identifies each network node). Each network node must have
the SSID programmed into it, or the access point will not associate with that node. An
optional table of MAC addresses can restrict access to radios whose addresses are on
the list.
3. WEP encryption: The network adapter controls the ‘Wired Equivalent Privacy’
(WEP) encryption function. The network can use a 64-bit or a 128-bit encryption key to
encode and decode data as it passes through the radio link.
6.2.2 Process Detail:
The 802.11 MAC sub-layer protocol is quite different from that of Ethernet due to the
inherent complexity of the wireless environment compared to that of a wired system.
With Ethernet, a station just waits until the ether goes silent and it starts transmitting. If it
does not receive a noise burst back within the first 64 bytes, the frame has almost
assuredly been delivered correctly. With wireless, this situation does not hold. Since not
all stations are within radio range of each other, transmission going on in one part of a
cell may not be received elsewhere in the same cell. In this example, station C is
transmitting to station B. If station A senses the channel, it will not hear anything and
falsely conclude that it may now start transmitting to station B. In addition, there is the
inverse problem, the exposed station problem (illustrated in fig 6-2).

(a) (b)
Figure 6-2: (a) The hidden station problem. (b) The exposed station problem
Here station B wants to send to station C so listens to the channel. When it hears a
transmission, it falsely concludes that it may not send to station C, even though station
A may be transmitting to station D (not shown). In addition, Most radios are half duplex,
meaning that they cannot transmit listen for noise bursts at the same time on a single
frequency. As a result of this problem, 802.11 does not use CSMA/CD, as Ethernet
does.
To deal with this problem, 802.11 supports two modes of operations. The first, called
DCF (distributed coordination function), does not use any kin of central control (in that
respect, similar to Ethernet). The other, called PCF (point coordination function), uses
the base station to control all activity in its cell. All implementation must support DCF but
PCF is optional. These two terms will be described now.
When DCF is employed, 802.11 uses a protocol called CSMA/CA (CSMA with collision
avoidance). In this protocol, both physical channel sensing and virtual channel sensing
are used. Two methods of operation are supported by CSMA/CA. In the first method,
when a station wants to transmit, it senses the channel. If it is idle, it just starts
transmitting. It does not sense the channel when transmitting but emits it entire frame,
which may well be destroyed at the receiver due to interference there. If the channel is
busy, the sender differs until it goes idle and they start transmitting. If a collision occurs,
the colliding stations wait a random time, using the Ethernet binary exponential backoff
algorithm, and then try again later.
B C A B
B wants to send to
C but mistakenly
thinks the
transmission will
fail.
Range of
C’s radio
C is transmitting
Range of
A’s radio
A is transmitting
A wants to send to
B but can’t hear
that B is busy.

The other method of CSMA/CA operation is based on MACAW and uses virtual channel
sensing, as illustrated in figure 6-3.
A RTS Data
B CTS ACK
C NAV
D NAV
TIME
Figure 6-3: The use of virtual channel sensing using CSMA/CA
In this example, A wants to send to B. C is a station within range of A (and possibly
within range of B, but that does not matter). D is a station within range of B but not
within range of A.
The protocol starts when A decides it wants to send data to B. It begins by sending an
RTS frame to B to request permission to send it a frame. When B receives this request,
it may decide to grant permission, in which case it sends a CTS frame back. Upon
receipt of the CTS, A now sends it frame and starts an ACK timer. Upon correct receipt
of the data frame, B responds with an ACK frame, terminating the exchange. If A’s ACK
timer expires before the ACK gets back to it, the whole protocol is run again.
Now let us consider this exchange from the viewpoints of C and D. C is within range of
A, so it may receive the RTS frame. If it does, it realizes that someone is going to send
data soon, so for the good of all it decides from transmitting anything until the exchange
is completed. From the information provided in the RTS request it can estimate how
long the sequence will take including the final ACK, so it asserts a kind of virtual
channel busy for itself, indicated by NAV (network allocation vector) in figure 5-3. D
does not hear the RTS, but it does hear the CTS, so it also asserts the NAV signal for
itself. Note that the NAV signals are not transmitted they are just internal reminders to
keep quite for a certain period of time.
In contrast to wired networks, wireless networks are noisy and unreliable, in no small
part due to microwave ovens, which also use the unlicensed ISM bands. As a
consequence, the probability of a frame making it through successfully decreases with
frame length. If the probability of any bit being in error is p, then the probability of an n-

bit frame being received entirely correctly is (1-p)n
. For example, for p=10-4
, the
probability of receiving a full Ethernet frame (12,144 bits) correctly is less than 30%. If
p= 10-5
, about 1 frame in 9 will be damaged. Even if p=10-6
, over 1% of the frames will
be damaged, which amounts to almost a dozen per second, and more if frames shorter
than the maximum are used. In summery, if a frame is too long; it has very little chance
of getting through undamaged and will probably have to be retransmitted.
To deal with the problem of noisy channels, 802.11 allows frames to be fragmented into
smaller pieces, each with its own checksum. The fragments are individually numbered
and acknowledged using a top-and-wait protocol. Once the channel has been acquired
using RTS and CTS, multiple fragments can be sent in a row, shown in figure 6-4. The
sequence of fragments is called a fragment burst.
Fragment Burst
A RTS Frag
1
Frag
2
Frag
3
B CTS ACK ACK ACK
C NAV
D NAV
TIME
Figure 6-4: A Fragment Burst
Fragmentation increases the throughput by restricting retransmissions to the bad
fragments rather than the entire frame. The fragment size is not fixed by the standard
but is a parameter of each cell and can be adjusted by the base station. The NAV
mechanism keeps other stations quiet only until the next acknowledgment, but another
mechanism is used to allow whole fragment burst to be sent without interference.
All of the above discussion applies to the 802.11 DCF mode. In this mode there is no
central control, the stations compete for airtime, just as they do with Ethernet. The other
allowed mode is PCF, in which the base stations poll for other stations, asking them is
they have any frames to send. Since transmission order is completely controlled by the
base station in PCF mode, no collision ever occurs. The standard prescribes the

mechanism for polling, but not the polling frequency, polling order or even whether all
stations need to get equal service.
The basic mechanism for the base station is to broadcast a beacon frame periodically
(10-100 times per second). The beacon frame contains system parameters, such as
hopping sequences and dwell times (for FHSS), clock synchronization, etc. it also
invites new stations to sign up for polling service. Once a station has signed up for
polling service at a certain rate, it is effectively guaranteed a certain fraction of the
bandwidth, thus making it possible to give quality-of-service guarantees.
PCF and DCF can coexist within one cell. At first it might seem impossible to have
central control and distributed control operating at the same time, but 802.11 provides a
way to achieve this goal. It works by carefully defining the inter-frame time interval. After
a frame has been sent, a certain amount of dead time is required before any station
may send a frame. Four different intervals are defined, each for specific purpose. The
intervals are depicted in figure 6-5.
SIFS
PIFS
DIFS
EIFS
ACK
TIME
Figure 6-5: Inter frame Spacing in 802.11
The shortest interval is SIFS (Short Inter-Frame Spacing). It is used to allow the parties
in a single dialog the chance to go first. This includes letting the receiver send a CTS to
respond to an RTS, letting the receiver send an ACK for a fragment or full data frame,
and letting the sender of a fragment burst transmit the next fragment without having to
send an RTS again.
Ctrtl frame may be sent
here
PCF frame may be sent
here
DCF frame may be sent
here
Bad frame recovery done
here

There is exactly one station that is entitled to respond after a SIFS interval. If it fails to
make use of its chance and a time PIFS (PCF Inter-Frame Spacing) elapses, the base
station may send a beacon frame or poll frame. This mechanism allows a station
sending a data frame or fragment sequence to finish its frame without anyone else
getting in the way, but gives the base station grab the channel when the previous
sender is done without having to compete with eager users.
If the base station has nothing to say and a time DIFS (DCF Inter-Frame Spacing)
elapses, any station may attempt to acquire the channel to send a new frame. The
usual contention rules apply. And binary exponential back-off may be needed if a
collision occurs.
The last time interval, EIFS (Extended Inter-Frame Spacing), is used only by a station
that has just received a bad or unknown frame to report the bad frame. The idea of
giving this event the lowest priority is that since the receiver may have no idea of what is
going on, it should wait a substantial time to avoid interfering with an ongoing dialog
between two stations.
6.3 Other Control Layers
All of the activity specified in the 802.11 standards takes place at the Physical and MAC
layers. The higher layers control things like addressing and routing, data integrity,
syntax, and the format of the data contained inside each packet. It doesn't make any
difference to these higher layers whether they're moving packets through wires, fiber
optic lines, or radio links. Therefore, an 802.11b network with any kind of LAN or other
network protocol could be used. The same radios can handle TCP/IP, Novell NetWare,
and all the other network protocols built into Windows, Unix, Mac OS, and other
operating systems equally well.
6.4 The 802.11 Frame Structure
The 8002.11 standard defines three different classes of frames on the wire: data,
control, and management. Each of these has a header with a variety of with a variety of
fields used within the MAC sub-layer. In addition, there are some headers used by the
physical layer but these mostly deal with the modulation techniques used.
The format of the data frame is shown in the figure 6-6. First comes the ‘Frame Control’
field. It itself has 11 subfields. The first of these is the ‘Protocol version’, which allows
two versions of the protocol to operate at the same time in the same cell. Then come
the ‘Type’ (data, control, or management) and ‘Subtype’ fields (RTS or CTS). The ‘to
DS’ and ‘from DS’ bits indicate the frame is going to or coming from the intercell
distribution system (Ethernet). The ‘MF’ bit means that more fragments will follow. The
retry bit marks a retransmission of a frame sent earlier. The ‘Power management’ bit is
used by the base station to put the receiver into sleep state or take it out of sleep state.
The ‘More’ bit indicates that the sender has additional frames for the receiver. The ‘W’
bit specifies that the frame body has been encrypted using the WEP (Wired Equivalent

Privacy) algorithm. Finally the ‘O’ bit tells the receiver that a sequence of frames with
this bit must be processed strictly in order.
Byte 2 2 6 6 6 2 6 0-2312 4
Frame
Ctrl
Duration
Address1
Address2
Address3
Seq.
Address4
Data
Check
Sum
Bits 2 2 4 1 1 1 1 1 1 1 1
Version
Type
Sub
Type
ToDS
From
DS
MF
Retry
Pwr
More
W O
Frame Ctrl
Figure 6-6: 802.11 Data Frame
The second field of the data frame, the ‘Duration field’, tells how long the frame and its
acknowledgement will occupy the channel. This field is also present in the control
frames and is how other stations manage the NAV mechanism.
The frame header contains four addresses, all in standard IEEE 802 format. Two
addresses are for the source and destination, which are obviously needed. The other
two are used for the source and destination of base stations for inter-cell traffic,
because frames may enter or leave a cell via a base station.
The ‘Sequence’ field allows fragments to be numbered. Of the 16 bits available, 12 of
them identify the frame and 4 of them identify the fragment.
The ‘Data’ field contains all the data to be sent, up to 2310 bytes, followed by the
‘Checksum’ field. This checksum field contains 4 bytes of error correcting codes, used
to correct the data if it is corrupted or lost.
Management frames have a format similar to that of data frames, except without one of
the base station addresses, because management frames are restricted to a single cell.
Control frames are shorter still, having only one or two addresses, no ‘Data’ field and no
‘Sequence’ field. The key information here is in the ‘Subtype’ field, usually RTS CTS or
ACK.

Chapter
Wireless Network Physical Architecture
7.1 Wired Network Topologies
The topology of a wired network refers to the physical configuration of links between
networked devices or nodes, where each node may be a computer, an end-user device
such as a printer or scanner, or some other piece of network hardware such as a hub,
switch or router.
The building block from which different topologies are constructed is the simple point-to-
point wired link between two nodes, shown in Figure 7-1. Repeating this element results
in the two simplest topologies for wired networks — bus and ring.
For the ring topology, there are two possible variants depending on whether the inter-
node links are simplex (one-way) or duplex (two-way).

Figure 7-1: Point to point, Bus and Ring topologies
In the simplex case, each inter-node link has a transmitter at one end and a receiver at
the other, and messages circulate in one direction around the ring, while in the duplex
case each link has both transmitter and receiver (a so-called transceiver) at each end,
and messages can circulate in either direction.
Bus and ring topologies are susceptible to single-point failures, where a single broken
link can isolate sections of a bus network or halt all traffic in the case of a ring.
The step that opens up new possibilities is the introduction of specialized network
hardware nodes designed to control the flow of data between other networked devices.
The simplest of these is the passive hub, which is the central connection point for LAN
cabling in star and tree topologies, as shown in Figure 7-2. An active hub, also known
as a repeater, is a variety of passive hub that also amplifies the data signal to improve
signal strength over long network connections.
Figure 7-2: Star and Tree Topologies
An active or passive hub in a star topology LAN transmits every received data packet to
every connected device. Each device checks every packet and decodes those identified
by the device’s MAC address. The disadvantage of this arrangement is that the
bandwidth of the network is shared among all devices, as shown in Figure 7-3. For

example, if two PCs are connected through a 10 Mbps passive hub, each will have on
average 5 Mbps of bandwidth available to it.
If the first PC is transmitting data, the hub relays the data packets on to all other devices
in the network. Any other device on the network will have to wait its turn to transmit
data.
Figure 7-3: A Passive Hub in a Physical Star Network
A switching hub (or simply a switch) overcomes this bandwidth sharing limitation by only
transmitting a data packet to the device to which it is addressed. Compared to a non-
switching hub, this requires increased memory and processing capability, but results in
a significant improvement in network capacity.
The first PC (Figure 7-4) is transmitting data stream A to the printer and the switch
directs these data packets only to the addressed device. At the same time, the scanner
is sending data stream B to the second PC.

Figure 7-4: Switching Hub in Physical Star Network.
The switch is able to process both data stream concurrently, so that the full network
bandwidth is available to every device.
7.2 Wireless Network Topologies
7.2.1 Point to Point Connections
The simple point to point connection shown in Figure 7-1 is probably more common in
wireless than in wired networks, since it can be found in a wide variety of different
wireless situations, such as:
• peer-to-peer or ad-hoc Wi-Fi connections
• wireless MAN back-haul provision
• LAN wireless bridging
• Bluetooth
• IrDA
7.2.2 Star Topologies in Wireless Networks
In wireless networks the node at the centre of a star topology (Figure 7-5), whether it is
a WiMAX base station, Wi-Fi access point, Bluetooth Master device or a ZigBee PAN
coordinator, plays a similar role to the hub in a wired network. The different wireless
networking technologies require and enable a wide range of different functions to be
performed by these central control nodes.

Figure 7-5: Star Topologies in Wireless Networks
The fundamentally different nature of the wireless medium means that the distinction
between switching and non-switching hubs is generally not relevant for control nodes in
wireless networks, since there is no direct wireless equivalent of a separate wire to each
device. The wireless LAN switch or controller (Figure 7-6) is a wired network device that
switches data to the access point that is serving the addressed destination station of
each packet.
Figure 7-6: A Tree Topology Using a Wireless Access Point Switch
The exception to this general rule arises when base stations or access point devices are
able to spatially separate individual stations or groups of stations using sector or array
antennas. Figure 3-7 shows a wireless MAN example, with a switch serving four base
station transmitters each using a 90° sector antenna. With this configuration, the overall

wireless MAN throughput is multiplied by the number of transmitters, similar to the case
of the wired switching hub shown in Figure 7-4.
Figure 7-7: Switched Star Wireless MAN Topology
In the wireless LAN case, a similar spatial separation can be achieved using a new
class of device called an access point array which combines a wireless LAN controller
with an array of sector antennas to multiply network capacity. The general technique of
multiplying network throughput by addressing separate spatial zones or propagation
paths is known as space division multiplexing and finds its most remarkable application
in MIMO radio.
7.2.3 Mesh Networks
Mesh networks, also known as mobile ad hoc networks (MANETs), are local or
metropolitan area networks in which nodes are mobile and communicate directly with
adjacent nodes without the need for central controlling devices. The topology of a mesh,
shown generically in Figure 7-8, can be constantly changing, as nodes enter and leave
the network, and data packets are forwarded from node-to-node towards their
destination in a process called hopping.
The data routing function is distributed throughout the entire mesh rather than being
under the control of one or more dedicated devices. This is similar to the way that data
travels around the Internet, with a packet hopping from one device to another until it
reaches its destination, although in mesh networks, the routing capabilities are included
in every node rather than just in dedicated routers.

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