802.11b Wireless LAN

Presentation bySrividya Srinivasan

What is unique about wireless

• Interference and Noise• Full connectivity cannot be assumed

– Hidden terminal problem• Battery Usage

– Requires power management• Security

– No physical boundaries– Overlapping LANs

Requirements of a wireless standard

• Single MAC to support multiple PHY mediums• Robust to interference• Need to deal with the hidden terminal problem• Need provision for time bounded services• Support for power management to save battery

power• Ability to operate world wide

– Hence uses the 2.4GHz ISM band

System Architecture

• Two basic system architectures – Ad hoc– Infrastructure based

Ad hoc wireless LANs

• A group of stations using the same radio frequency – Basic Service Set

Infrastructure based Wireless LANs

• Stations and Access Points

• Distribution system connects cells via access points to form a single network

• Extends wireless area coverage

Access Points

• Stations select an AP and “associate” with it• Support roaming• Provide other functions

– time synchronization (beaconing)– power management support– point coordination function

• Traffic typically (but not always) flows through AP– direct communication possible

Distribution system

• Used to interconnect multiple wireless cells – Multiple BSS are connected to form an ESS

Extended Service Set– Allows mobile stations to access fixed resources

• Not part of the 802.11 standard

MAC layer

• Controls medium access– Distributed coordinated function

• Mandatory access mechanism – CSMA/CA• Optional RTS/CTS extension to deal with the hidden

terminal problem– Point Coordinated function

• Contention free polling method to support time bounded services

MAC Services

• Synchronization– Specifies the Timing Synchronization function (TCF) – Within a BSS (quasi) periodic transmission of

beacon signals – contains timestamp – all local nodes adjust their local timer according to the timestamp.

MAC Services

• Infrastructure based networks – Access point relays the beacon signal

• Ad hoc networks – each node maintains own synchronization timer and starts the transmission of a beacon signal. All stations adjust their clocks to the beacon which wins and suppress their beacons for this cycle

MAC Services

• Power management– Two states for a station –sleep and awake– Transceiver is switched off whenever it is not needed– Transceiver wakes up periodically and checks

whether there are any data frames for it buffered

MAC Services

• Infrastructure based networks– Access point buffers all frames destined for stations

operating in power saving mode– With every beacon signal it transmits a TIM – traffic

indication map – list of stations for which data frames are buffered in the access point

MAC Services

• Ad hoc networks– No access point to buffer the data– Each station needs to have the ability to buffer data if

it wants to communicate with a power saving station– All stations now announce a list of buffered frames

during a period when they are all awake – the ATIM window

MAC Services

• Roaming– Station starts scanning for a new access point if

current link quality is very poor– Passive scanning and Active scanning– Selects best access point based on signal strength

and sends Association request to it– Selected AP informs DS which updates its database

PHY layer

• Supports 3 different physical layers– Infrared– Radio transmission

• Frequency hopping spread spectrum• Direct sequence spread spectrum• These two do not interoperate with one another

• The data rates achieved are 1 and 2 Mbps using radio waves

802.11b Enhancements

• 802.11b supports two new speeds 5.5 and 11 Mbps

• DSSS is the sole physical layer technique• 802.11b systems will interoperate with 1 Mbps

and 2 Mbps 802.11 DSSS systems but not with FHSS systems

802.11b Enhancements

• Uses Complementary Code keying ( CCK ) to encode 4 bits per career to get 5.5 Mbps and 8 bits per career to get 11 Mbps.