maulana azad national institute of technology,bhopal department of computer science &...

Maulana Azad National Institute of Technology,Bhopal

Department of Computer Science & Engineering

IEEE 802.11 WLAN Guided By:

Mrs. NAMITA TIWARI

Presented By: Manisha BhadoriaManisha Bhadoria

082112111082112111

Technology Tree for Wireless LAN

HomeRFHomeRFBluetoothBluetooth

What is unique about wireless?Difficult mediaDifficult media

interference and noisequality varies over space and timeshared with Unwanted 802.11 devicesshared with non-802 devices (unlicensed

spectrum: microwave ovens, bluetooth, etc.,microwave ovens, bluetooth, etc.,)Full connectivity cannot be assumed

Hidden node problemHidden node problemMultiple international regulatory

requirements

Uniqueness of Wireless (continued)Mobility

variation in link reliabilitybattery usage: requires power managementpower managementwant seamless connections

Securityno physical boundariesoverlapping LANs

Wireless LANs: CharacteristicsTypes

Infrastructure basedAd-hoc

AdvantagesFlexible deploymentMinimal wiring difficultiesMore robust against disasters (earthquake etc)Historic buildings, conferences, trade shows,…

DisadvantagesLow bandwidth compared to wired networks (1-10

Mbit/s)Proprietary solutionsNeed to follow wireless spectrum regulations

IEEE 802 Protocol Layers

Protocol ArchitectureFunctions of media access control (MAC) layer:

On transmission, assemble data into a frame with address and error detection fields

On reception, disassemble frame and perform address recognition and error detection

Govern access to the LAN transmission medium

Functions of logical link control (LLC) Layer:Provide an interface to higher layers and perform flow and

error control

Components/ArchitectureStation (STA) - Mobile nodeAccess Point (AP) - Stations are connected

to access points.Basic Service Set (BSS) - Stations and the AP

with in the same radio coverage form a BSS.Extended Service Set (ESS) - Several BSSs

connected through APs form an ESS.

infrastructure network

ad-hoc network

APAP

AP

wired network

AP: Access Point

Source: Schiller

IEEE 802.11 Architecture (model)

Distribution system (DS) – the network backboneAccess point (AP) – a bridge or relay Basic service set (BSS)

Stations competing for access to shared wireless mediumIsolated or connected to backbone DS through APThe entity in which the stations are within range of each

other although BSSs can easily overlapExtended service set (ESS)

Two or more basic service sets interconnected by DS usually a wired LAN

802.11 Architecture Model

DS

ESS

Wireless LAN: MotivationCan we apply media access methods from

fixed networks?Example CSMA/CD

Carrier Sense Multiple Access with Collision Detection

send as soon as the medium is free, listen into the medium if a collision occurs (original method in IEEE 802.3)

Medium access problems in wireless networks

signal strength decreases proportional to the square of the distance

sender would apply CS and CD, but the collisions happen at the receiver

sender may not “hear” the collision, i.e., CD does not work

CS might not work, e.g. if a terminal is “hidden”

Difference Between Wired and Wireless

If both A and C sense the channel to be idle at the same time, they send at the same time.

Collision can be detected at sender in Ethernet.Half-duplex radios in wireless cannot detect

collision at sender.

A B C

A

B

C

Ethernet LAN Wireless LAN

FragmentationWireless LANs have high bit error rates.The probability of erroneous frame is much

higher for wireless links802.11 uses fragmentation to reduce the

frame error rate.

MAC managementSynchronization - finding and

staying with a WLAN - synchronization functions

Power Management - sleeping without missing any messages - power management functions

Roaming - functions for joining a network - changing access points - scanning for access points

Management information base

ScanningScanning is required for many

functions - finding and joining a network - finding a new access point during roaming

Passive scanning- find networks simply by

listening for beacons Active scanning

- on each channel send a probe and wait for probe response

802.11 Protocol ArchitectureMAC Entity

basic access mechanismfragmentation/defragmentationencryption/decryption

MAC Layer Management Entitysynchronizationpower managementroamingMAC MIB (management information base)

Physical Layer Convergence Protocol (PLCP)PHY-specific, supports common PHY SAPprovides Clear Channel Assessment signal

(carrier sense)

802.11 Protocol Architecture (cont.)

Physical Medium Dependent Sublayer (PMD)modulation and encoding

PHY Layer Managementchannel tuning (channel switching delay : 224us224us in

802.11b)PHY MIB

Station Management interacts with both MAC Management and PHY

Management

Commercial Products : WLAN Cards

One pieceTwo pieces

Commercial Products : AP

Outdoor Application - Antenna

Outdoor Application

Wi-Fi Protected Access (WPA) - Authentication

Open system authenticationExchange of identities, no security benefits

Shared Key authenticationShared (secret) Key assures authenticationWEP (wired equivalent privacy) is easily broken since key is

used to encrypt every transmissionImmediate solution for WEP was to increase key length from

40, to 128, to 156 bits where 128 bits is the minimum used today.

IEEE 802.11a, 802.11b and 802.11g

IEEE 802.11a5-GHz band with data rates of 6, 9 , 12, 18, 24, 36, 48, 54 MbpsUses orthogonal frequency division multiplexing (OFDM)Subcarrier modulated using BPSK, QPSK, 16-QAM or 64-QAM

IEEE 802.11bProvides data rates of 5.5 and 11 Mbps at 2.4 GHzComplementary code keying (CCK) modulation scheme

IEEE 802.11g2.4 GHz, up to 54 Mbps, OFDM .11g and .11b can operate simultaneously but with an .11b user in the cell

the wireless network will degrade the .11g performance (AP must do translation for .11b) but still much faster than .11b alone.

802.11a : 802.11a The 802.11a standard uses the same data link layer protocol and frame format as the original standard, but an OFDM based air interface (physical layer). It operates in the 5 GHz band with a maximum net data rate of 54 Mbit/s, plus error correction code, which yields realistic net achievable throughput in the mid-20 Mbit/s[citation needed]. Since the 2.4 GHz band is heavily used to the point of being crowded, using the relatively un-used 5 GHz band gives 802.11a a significant advantage. However, this high carrier frequency also brings a disadvantage: The effective overall range of 802.11a is less than that of 802.11b/g; 802.11a signals cannot penetrate as far as those for 802.11b because they are absorbed more readily by walls and other solid objects in their path due to their smaller wavelength.

802.11b : 802.11b 802.11b has a maximum raw data rate of 11 Mbit/s and uses the same media access method defined in the original standard. 802.11b products appeared on the market in early 2000, since 802.11b is a direct extension of the modulation technique defined in the original standard. The dramatic increase in throughput of 802.11b (compared to the original standard) along with simultaneous substantial price reductions led to the rapid acceptance of 802.11b as the definitive wireless LAN technology. 802.11b devices suffer interference from other products operating in the 2.4 GHz band. Devices operating in the 2.4 GHz range include: microwave ovens, Bluetooth devices, baby monitors and cordless telephones.

802.11-2007 : 802.11-2007 In 2003, task group TGma was authorized to "roll up" many of the amendments to the 1999 version of the 802.11 standard. REVma or 802.11ma, as it was called, created a single document that merged 8 amendments (802.11a,b,d,e,g,h,i,j) with the base standard. Upon approval on March 08, 2007, 802.11REVma was renamed to the current standard IEEE 802.11-2007.[4] This is the single most modern 802.11 document available that contains cumulative changes from multiple sub-letter task groups.

Useful Web Sites from Stallings

IEEE 802.11 Wireless LAN Standard

IEEE 802.11 Wireless LAN Working Group: Contains working group documents plus discussion archives.

Wi-Fi Alliance: An industry group promoting the interoperabiltiy of 802.11 wireless products with each other.

ReferencesBrian P Crow, Indra Widjaja, J G Kim,

Prescott T Sakai. IEEE 802.11 Wireless Local Area Networks. IEEE Communications Magazine

www.breezecom.comJochen H. Schiller, Mobile Communications