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IEEE 802.11 Standard

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. 802.11 Architecture. 802.11 Features. CSMA/CA based MAC protocol

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IEEE 802.11 Standard

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  1. 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 CS211

  2. 802.11 Architecture CS211

  3. 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 CS211

  4. 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 layer Management PLCP sublayer PHY layer Management PMD sublayer CS211

  5. 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 CS211

  6. 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 CS211

  7. Frequency Hopping CS211

  8. 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 CS211

  9. Direct Sequence Spread Spectrum CS211

  10. 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 CS211

  11. 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 CS211

  12. CSMA/CA+ACK: 4-way handshake CS211

  13. 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 CS211

  14. 802.11 frame priorities CS211

  15. 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 CS211

  16. 4-Way Handshake CS211

  17. Time-critical service via PCF CS211

  18. 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 CS211

  19. 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 CS211

  20. 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 CS211

  21. Roaming in 802.11 CS211

  22. 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 CS211

  23. 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 CS211

  24. Active scanning CS211

  25. Power Management CS211

  26. 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 CS211

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