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IEEE 802.11 Standard

Why we study this standard:• overall architecture• physical layer spec.

– direct sequence– frequency hopping

• MAC layer spec.– channel access– mobility support

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802.11 Architecture

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802.11 Features

• CSMA/CA based MAC protocol• support for both time-critical and non-

critical traffic• support multiple priority levels• spread spectrum technology (no

licensing)• power management allows a node to

doze off

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802.11 Protocol Entities• MAC entity

– basic access mechanism– fragmentation & encryption

• MAC layer management entity– synchronization– power management– roaming

• Physical layer convergence protocol (PLCP)– PHY-specific, common PHY SAP support– provides carrier sense

• Physical medium dependent sublayer (PMD)– modulation & coding

• PHY layer management– channel tuning & PHY MIB

MAC Sublayer

MAC layerManagement

PLCP sublayer

PMD sublayer

PHY layerManagement

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PHY spec• Infrared PHY

– diffuse infrared– 1 and 2Mbps

• Frequency hopping PHY• Direct Sequence PHY• CCA: how to sense a channel is clear:

– energy level is above a threshold– can detect a signal– use both

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Frequency Hopping Spread Spectrum• Pseudo-random frequency hopping• spreads the power over a wide

spectrum ->spread spectrum• narrowband interference cannot jam• developed initially for military• 2.4Ghz ISM band, 1-2Mbps; 2GFSK,

4GFSK; hop over 79 channels

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Frequency Hopping

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Direct Sequence Spread Spectrum• Spreading factor = code bits/data bit,

10-100 commercial (min 10 by FCC).• Signal bandwidth > 10*data bandwidth• code sequence synchronization• correlation between codes ->

interference -: orthogonal• 2.4Ghz band, 1,2Mbps; DBPSK, DQPSK;

11 chip barker sequence

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Direct Sequence Spread Spectrum

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802.11b Frequency Channels• In US, the 2.4ISM band is from 2400MHz to

2483.5MHz– Divided up to 11 “channels” from

2412~2462MHz, spaced 5MHz apart– Each 802.11b channel is 22MHz

• Channel 1: centered at 2412MHz, 2400~2423MHz• Channel 2: centered at 2417MHz, • Channel 6: centered at 2437MHz, 2426~2448MHz • Channel 11: centered at 2462MHz, 2451~2473MHz

– 3 channels (e.g., Channels 1, 6, 11) are safe to use simultaneously

• 3MHz of buffer zone between channels

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Basic MAC Features• Carrier sense multiple access with collision

avoidance (CSMA/CA) based– based on carrier sense function in PHY called Clear

Channel Assessment (CCA)– CSMA/CA+ACK for unicast frames, with MAC level

recovery– parameterized use of RTS/CTS to protect against hidden

nodes– frame formats to support both infrastructure and ad-hoc

networks

• supports time-critical traffic in addition to traditional data traffic

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CSMA/CA+ACK: 4-way handshake

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Frame Format

• MAC headers format differs per type– control frames: RTS, CTS, ACK– management frames, e.g. beacon, probe/probe

response, (re)-association request/response,– data frames

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802.11 frame priorities

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CSMA/CA+ACK explained

• Reduce collision probability where mostly needed– stations are waiting for medium to become free– select random backoff after a defer, resolving contention to avoid collision

• efficient backoff algorithm stable at high loads• defer access based on carrier sense

–CCA from PHY and virtual carrier sense state• direct access when medium is sensed free longer than DIFS, otherwise defer and backoff• receiver of directed frames to return ACK when CRC correct

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4-Way Handshake

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Time-critical service via PCF

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PCF Access Procedure• PC senses the medium at the beginning of each CFP

• PC transmits a beacon containing “CF parameter set element” when idle > PIFS

• each station presets its NAV to the CFPMaxDuration from the CF Parameter Set Element in beacons from the PC.

• after a SIFS period, PC sends one of the following: a data frame, CF-Poll frame, Data+CF-Poll frame, CF-end frame (when no traffic buffered & no polls to send at the PC)

• PC maintains a polling list to select stations that are eligible to receive CF-Polls during contention-free periods.

• A CF-Pollable station always responds to a CF-Poll: if no data from the station, responds with a Null Frame or a CF-ACK (no data) frame (when ACK is required);

• “piggyback” ACK or Poll in the data frame whenever possible

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Further details

• Alternating Contention free and contention operations under PCF control• NAV prevents contention traffic until reset by the last PCF transfer ->

variable length contention free period per interval• both PCF and DCF defer to each other causing PCF burst start variations• CF-burst by polling bit in CF-down frame• immediate response by station on a CF_Poll

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Synchronization in 802.11• All stations maintain a local timer• Timing synchronization function (TSF)

– keeps timers from all stations in synch– AP controls timing in infrastructure networks

• timing conveyed by periodic beacons– beacons contain timestamp for the entire BSS– timestamp from beacons to calibrate local clocks– not required to hear every beacon to stay in synch

• used for power management– beacons sent at well known intervals– all station timers in BSS are synchronized

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Roaming in 802.11

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Roaming Approach• Station decides that link to its current AP is poor• station uses scanning function to find another AP• station sends Reassociation Request to new AP• if Reassociation Response is successful

– then station has roamed to the new AP– else station scans for another AP

• if AP accepts Reassociation Request– AP indicates Reassociation to the Distribution System– Distribution System information is updated– normally old AP is notified thru distributation system

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Scanning• Scanning required for many functions

– finding and joining a network– finding a new AP while roaming– initializing an ad hoc network

• 802.11 MAC uses a common mechanism– passive or active scanning

• Passive scanning– by listening for Beacons

• Action Scanning– probe + response

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Active scanning

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Power Management

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802.11a Standard• PHY layer

– 12 nonoverlapping channels in 5GHz band– OFDM– Offers rate up to 54Mbps

• MAC– Roughly the same as 802.11b

• 802.11g– Backward compatible with 802.11b, operating

at 2.4Ghz, fall back to 11Mbps with 802.11b AP– OFDM based