Wi-Fi Technology 1. INTRODUCTION 1.1. NEED FOR A NETWORK: In the world of computers, networking is the practice of linking two or more computing devices together for the purpose of sharing data. Networks are built with a mix of computer hardware and computer software. Networking provides various advantages like sharing of resources, files and information as well as sharing of Internet connection. With the advent of technology, it is possible to share data between computers without physical connections. Inter-networking evolved as a solution to three key problems: isolated LANs, duplication of resources, and a lack of network management. Isolated LANS made electronic communication between different offices or departments impossible. Duplication of resources meant that the same hardware and software had to be supplied to each office or department, as did a separate support staff. This lack of network management meant that no centralized method of managing and troubleshooting networks existed. 1.2. WIRED V/S WIRELESS NETWORKS: The wired network that has been in use till date has certain limitations as follows: There is difficulty in installing the wired network, as it requires the fiber optic cables to be put up, which may not be feasible in all cases. The cost of installation is high and hence is not economic to one and all. One cannot connect the network wherever one wants to do so since it requires space for the cables to run. 1
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Wi-Fi Technology
1. INTRODUCTION
1.1. NEED FOR A NETWORK:
In the world of computers, networking is the practice of linking two or more computing devices
together for the purpose of sharing data. Networks are built with a mix of computer hardware
and computer software. Networking provides various advantages like sharing of resources, files
and information as well as sharing of Internet connection. With the advent of technology, it is
possible to share data between computers without physical connections.
Inter-networking evolved as a solution to three key problems: isolated LANs, duplication of
resources, and a lack of network management. Isolated LANS made electronic communication
between different offices or departments impossible. Duplication of resources meant that the
same hardware and software had to be supplied to each office or department, as did a
separate support staff. This lack of network management meant that no centralized method of
managing and troubleshooting networks existed.
1.2. WIRED V/S WIRELESS NETWORKS:
The wired network that has been in use till date has certain limitations as follows:
There is difficulty in installing the wired network, as it requires the fiber optic cables to
be put up, which may not be feasible in all cases.
The cost of installation is high and hence is not economic to one and all.
One cannot connect the network wherever one wants to do so since it requires space
for the cables to run.
Connecting the laptop to the network becomes difficult.
The traffic increases with everyone trying to use the services at a time.
Mobility is not much and hence productivity declines.
The performance is not that great and can be improved.
The speed of data transfer is less.
Due to the above pitfalls of the wired Ethernet network connection, the wireless networks have
come up.
Wireless LANs provide all the functionality of wired LANs, but without the physical
constraints of the wire itself.
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Wireless LAN configurations include independent networks, offering peer-to-peer
connectivity, and infrastructure networks, supporting fully distributed data
communications.
A wireless LAN-LAN bridge is an alternative to cable that connects LANs in two
separate buildings.
Wireless LANs provide flexibility, improved quality and easy installation.
1.3. VARIOUS WIRELESS TECHNOLOGIES:
In the process of development in technology, a variety of wireless technologies have been
developed. Various application specific technologies have been developed. The various
wireless technologies are listed below:
1. WI-FI (802.11):
It is used for wireless Internet access in devices like laptops, computers. It has a limited
range and hence used for local area network.
2. WI-MAX (802.16):
It is an emerging wireless technology, which will be used for metropolitan area network.
Thus the speed and range is high.
3. BLUETOOTH (802.15):
It is used to connect two peripherals like computers within a range of 33 feet.
4. GSM (GLOBAL SYSTEM FOR MOBILE COMMUNICATION):
It is used in digital cellular telephone system.
5. 3GSM:
It has the same use as GSM but has a higher speed.
6. GPRS (GENERAL PACKET RADIO SERVICE):
It is an interface overlaid on existing GSM networks to allow for Internet access.
7. CDMA (CODE DIVISION MULTIPLE ACCESS):
It is used in digital telephone system mainly in U.S.
8. CDPD (CELLULAR DIGITAL PACKET DATA):
It is used to transmit data over analog cellular networks. In today’s world it is not
applicable.
9. TDMA (TIME DIVISION MULTIPLE ACCESSS):
It is also used in digital telephone system with a speed of 64-120 Kbps.
1.4. WI-FI IN A NUTSHELL.
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IEEE 802.11 is a standard that digresses slightly from the OSI model in that it provides a
standard for wireless data transmission. To do this, the standard defines the MAC and PHY
layers of the OSI model for use of DSSS, FDSS, and OFDM. The MAC layer is responsible for
managing data transfer from higher-level functions to PHY media. This standard details how
data is modulated for transmission and correlated at the receiving end. The topology of wireless
networks is fairly simple. In a BSS, an AP is connected to an existing LAN from which wireless
stations can access the network. An ESS extends this topology to expand the network. Using an
ad hoc topology, stations (PCs) can communicate directly with one another. Mobility measures
permit wireless users to access the wireless network from any point on the network and maintain
their connection regardless of where they roam on the network.
802.11 has a number of built-in measures, including WEP, to protect a network from external
threats. Should the network manager feel that WEP is not adequate to protect the network based
on the previous equation, a number of other measures can be added to the network to heighten
the level of security in the network. With the addition of external security measures, 802.11
networks can be as secure as most wired networks
In summary, 802.11 presents the best of all possible worlds for the small office/home office
(SOHO) subscriber in providing telephony as good or better than the PSTN while delivering an
overwhelming advantage in bandwidth. The spread of broadband Internet access to a majority of
households will probably happen in the form of 802.11. The demand for broadband will have
the effect of bringing different forms of delivery (DSL, cable modem, and 802.11) into the
marketplace. The form of access that is least expensive and most easily deployed will win.
2. IEEE 802.11 ARCHITECTURE
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802.11 supports three basic types of topologies:
1. Ad-Hoc/IBSS
2. BSS (Basic Services set)
3. ESS (Extended services set)
2.1.AD – HOC / IBSS
Ad hoc networking connects a set of PCs with wireless adapters. This arrangement is sometimes
called peer-to-peer networking. Any time two or more wireless adapters within range of each
other can set up an independent network. These on-demand networks typically require no
administration or pre-configuration.
Advantages:
1. Cost savings
2. Gives peer-to-peer networks in some applications a great deal of power.
3. Rapid setup time
4. Can’t provide access to applications and servers on a wired network.
Disadvantages:
1. Covers a very small area
2. Security
FIG1.AD-HOC ARCHITECTURE
2.2. BASIC SERVICE SET / INFRASTRUCTURE.
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A basic service set is made of stationary or mobile wireless stations and a possible central base
station, known as the Access point (AP). There are two types of access points:
i. Dedicated hardware access points (HAP).
ii. Software Access Points that run on a computer equipped with a wireless network
interface card as used in an ad-hoc or peer-to-peer wireless network. It includes features
not commonly found in hardware solutions, such as extensive configuration flexibility,
but may not offer the full range of wireless features defined in the 802.11 standard
Access points can extend the range of independent WLANs by acting as a repeater,
effectively doubling the distance between wireless PCs.
The access point also performs a number of other roles, such as connecting the nodes to the
Internet or other WAN (wide area network), connecting multiple wireless networks, connecting
the wireless nodes to a wired network, and providing management and security functionality.
The access points not only provide communication with the wired network but also mediate
wireless network traffic in the immediate neighborhood. Multiple access points can provide
wireless coverage for an entire building or campus.
Advantages:
1. Connect to the wired network and allow users to efficiently share network resources
2. Provides management and security functionality
3. Covers a large area
FIG2.BSS / INFRASTRUCTURE NETWORK
2.3. EXTENDED SERVICE SET / INFRASTRUCTURE.
Ethernet LAN
Access Point
Laptop
Desktop computer Desktop
Computer
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The Extended service set (ESS) is made up of two or more BSSs with APs. The BSSs are
connected through a distribution system, which is usually a wired LAN. The distribution system
connects the APs in the BSSs. The distribution system can be any IEEE LAN such as Ethernet.
Thus, ESS uses 2 types of stations: mobile and stationary. The mobile stations are normally
inside the BSS; the stationary stations are AP stations that are a part of the wired LAN.
In this network, the stations within reach of one another can communicate without the use of an
AP. However, communication between two stations in two different BSSs usually occurs via
two APs.
Advantages:
1. Covers a larger area
2. Allows sharing of network resources
FIG3.ESS/INFRASTRUCTURE
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3. UNDERLYING TECHNOLOGY
3.1. OPEN SYSTEM INTERCONNECTION (OSI) MODEL.
The Open System Interconnection (OSI) reference model describes how information from a
software application in one computer moves through a network medium to a software
application in another computer. The OSI reference model is a conceptual model composed of
seven layers, each specifying particular network functions. The model was developed by the
International Organization for Standardization (ISO) in 1984, and it is now considered the
primary architectural model for inter computer communications. The OSI model divides the
tasks involved with moving information between networked computers into seven smaller, more
manageable task groups. Each layer is reasonably self-contained so that the tasks assigned to
each layer can be implemented independently. This enables the solutions offered by one layer to
be updated without adversely affecting the other layers.
The seven layers of the OSI reference model can be divided into two categories:
1. Upper layers: The upper layers of the OSI model deal with application issues and
generally are implemented only in software. The highest layer, the application layer, is
closest to the end user. Both users and application layer processes interact with software
applications that contain a communications component. The term upper layer is
sometimes used to refer to any layer above another layer in the OSI model.
2. Lower layers: The lower layers of the OSI model handle data transport issues. The
physical layer and the data link layer are implemented in hardware and software. The
lowest layer, the physical layer, is closest to the physical network medium (the network
cabling, for example) and is responsible for actually placing information on the medium.
Actual communication is made possible by using communication protocols. A protocol is a
formal set of rules and conventions that governs how computers exchange information over a
network medium. A protocol implements the functions of one or more of the OSI layers. A
system that implements protocol behavior consisting of a series of these layers is known as a
'protocol stack' or 'stack'. Protocol stacks can be implemented either in hardware or software, or
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a mixture of both. Typically, only the lower layers are implemented in hardware, with the higher
layers being implemented in software.
FIG4: NETWORK ARCHITECTURE BASED ON OSI MODEL
Description of layers
1. Physical Layer:
The physical layer defines the electrical, mechanical, procedural, and functional specifications
for activating, maintaining, and deactivating the physical link between communicating network
systems. Physical layer specifications define characteristics such as voltage levels, timing of
voltage changes, physical data rates, maximum transmission distances, and physical connectors.
This includes the layout of pins, voltages, and cable specifications. Hubs and repeaters are
physical-layer devices. The major functions and services performed by the physical layer are:
establishment and termination of a connection to a communications medium.
participation in the process whereby the communication resources are effectively shared among
multiple users. For example, conversion between the representation of digital data in user
equipment and the corresponding signals transmitted over a communications channel. These are
signals operating over the physical cabling - copper and fiber optic, for example. SCSI operates
at this level.
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It conveys the bit stream - electrical impulse, light or radio signal -- through the network at the
electrical and mechanical level. It provides the hardware means of sending and receiving data on
a carrier, including defining cables, cards and physical aspects. Fast Ethernet, RS232, and ATM
are protocols with physical layer components.
2. Data link layer:
The Data link layer provides the functional and procedural means to transfer data between
network entities and to detect and possibly correct errors that may occur in the Physical layer.
Different data link layer specifications define different network and protocol characteristics,
including physical addressing, network topology, error notification, sequencing of frames, and
flow control. Physical addressing (as opposed to network addressing) defines how devices are
addressed at the data link layer. Network topology defines how devices are to be physically
connected, such as in a bus or a ring topology. Error notification alerts upper-layer protocols
that a transmission error has occurred, and the sequencing of data frames reorders frames that
are transmitted out of sequence. Finally, flow control moderates the transmission of data so that
the receiving device is not overwhelmed with more traffic than it can handle at one time.
The data link layer is divided into two sublayers: The Media Access Control (MAC) layer and
the Logical Link Control (LLC) layer.
The MAC sublayer controls how a computer on the network gains access to the data and
permission to transmit it. It thus manages protocol access to the physical network medium.
The LLC layer controls frame synchronization, flow control and error checking. The Logical
Link Control (LLC) sublayer of the data link layer manages communications between devices
over a single link of a network.
3. Network layer:
The Network layer provides the functional and procedural means of transferring variable length
data sequences from a source to a destination via one or more networks while maintaining the
quality of service requested by the Transport layer. The Network layer performs network
routing, switching, flow control, segmentation/desegmentation, and error control functions. It
converts the segments into smaller datagrams that the network can handle. The router operates
at this layer -- sending data throughout the extended network and making the Internet possible,
although there are layer 3 (or IP) switches. This is a logical addressing scheme - values are
chosen by the network engineer. The addressing scheme is hierarchical.
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4. Transport layer:
The purpose of the Transport layer is to provide transparent transfer of data between end users,
thus relieving the upper layers from any concern with providing reliable and cost-effective data
transfer. The transport layer controls the reliability of a given link. Some protocols are stateful
and connection oriented. This means that the transport layer can keep track of the packets and
retransmit those that fail. The best known example of a layer 4 protocol is TCP.
5. Session layer:
The session layer establishes, manages, and terminates communication sessions.
Communication sessions consist of service requests and service responses that occur between
applications located in different network devices. These requests and responses are coordinated
by protocols implemented at the session layer. The session layer sets up, coordinates, and
terminates conversations, exchanges, and dialogues between the applications at each end.
6. Presentation layer:
The Presentation layer relieves the Application layer of concern regarding syntactical
differences in data representation within the end-user systems. It provides a variety of coding
and conversion functions that are applied to application layer data. These functions ensure that
information sent from the application layer of one system would be readable by the application
layer of another system. Encoding, encryption and similar manipulation of the presentation of
data is done at this layer.Presentation layer implementations are not typically associated with a
particular protocol stack.
7. Application layer:
This layer supports application and end-user processes. This layer interfaces directly to and
performs common application services for the application processes.
This layer interacts with software applications that implement a communicating component.
Such application programs fall outside the scope of the OSI model. Application layer functions
typically include identifying communication partners, determining resource availability, and
synchronizing communication.
When identifying communication partners, the application layer determines the identity and
availability of communication partners for an application with data to transmit.
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When determining resource availability, the application layer must decide whether sufficient
network resources for the requested communication exist. In synchronizing communication, all
communication between applications requires cooperation that is managed by the application
layer.
The above OSI model is the same for both wired and wireless transmission. The only difference
comes in the protocol used at the physical layer and the data link layer, which is different for
both wireless and wired transmissions.
Thus here we now concentrate on the protocols used at the physical layer and data link layer
while communicating using Wi-Fi technology.
3.2. PHYSICAL LAYER
IEEE 802.11 defines specifications for the conversion of bits to a signal in the physical layer.
The 1997 802.11 standard specifies three transmission techniques allowed in the physical layer.
One specification is in infrared and the other two are short-range radio frequency.
INFRARED TECHNOLOGY:
The infrared option uses line of sight transmission at .85 or .95 microns. Two speeds are
permitted: 1Mbps and 2Mbps. Infrared signals cannot penetrate walls, so cells in different
rooms are well isolated from each other. Nevertheless, due to the low bandwidth (and the fact
that sunlight swamps infrared signals), this is not a popular option.
RADIO FREQUENCY TECHNOLOGY:
The other basic technology is Spread spectrum radio. The fundamental concept of spread
spectrum radio is the use of a wider frequency bandwidth than that needed by the information
that is transmitted. Using extra bandwidth would seem to be wasteful, but it actually results in
several benefits, including reduced vulnerability to jamming, less susceptibility to interference,
and coexistence with narrowband transmissions. Several spread spectrum techniques are
available, out of which FHSS and DSSS are most widely used.
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FHSS (FREQUENCY-HOPPING SPREAD SPECTRUM):
IEEE 802.11 FHSS describes the frequency-hopping spread spectrum (FHSS) method for signal
generation in a 2.4-GHz ISM band. FHSS is the method in which the sender sends on one
carrier frequency for a short period of time, then hops to another carrier frequency for the same
amount of time, hops again to still another for the same amount of time, and so on. After N
hoppings, the cycle is repeated .If the bandwidth of the original signal is B, then the allocated
bandwidth of the spread spectrum is N*B. In FHSS the sender and receiver agree on the
sequence of the allocated bands. Hence spreading makes it difficult for unauthorized persons to
make sense of transmitted data. FHSS uses a 2 .4 GHz industrial, scientific, and medical band.
The modulation technique `in this specification is FSK at 1 Mbands/s. The system allows 1 or 2
bits/baud, which results in a data rate of 1 or 2 Mbps.
DSSS (THE DIRECT SEQUENCE SPREAD SPECTRUM):
IEEE 802.11 DSSS describes the direct sequence spread spectrum (DSSS) method for signal
transmission in a 2.4 GHz ISM band. In DSSS, each bit sent by the sender is replaced by a
sequence of bits called a chip code. To avoid buffering, however, the time needed to send one
chip must be the same as the time needed to send one signal bit. If N is the no. of bits in each
chip code, then the data rate for sending chip codes is N times the data rate of the original bit
stream. DSSS uses 2.4 GHz ISM band. The bit sequence uses the entire band. The modulation
technique in this specification is PSK at 1 Mbaud/s. The system allows 1 or 2 bits/baud, which
results in a data rate of 1 or 2 Mbps.
MEDIUM ACCESS CONTROL (MAC) SUBLAYER
The MAC sublayer is the sublayer of data link layer. It controls how a computer on the network
gains access to the data and permission to transmit it. It thus manages protocol access to the
physical network medium. Thus the MAC layer is responsible for managing data transfer from
higher-level functions to the physical media.
Devices using the IEEE 802.11 PHY and MAC as part of a WLAN are called stations. Stations
can be endpoints or APs. APs are stations that act as part of the DS and facilitate the distribution
of data between endpoints. The MAC provides nine logical services: authentication,