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802.11 Specification overview

This overview covers the 802.11 specifications, including MAC and PHY layers, system architecture, control, and MAC protocol details for wireless networking. Learn about the basic service set (BSS), extended service set (ESS), frequency hopping spread spectrum (FHSS), direct sequence spread spectrum (DSSS), and more.

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802.11 Specification overview

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  1. 802.11 Specification overview P. Bhagwat

  2. 802.11 Specifications LLC MAC Mgmt Service Interface MAC Service Interface LLC MAC sublayer MAC Layer Management MAC Mgmt WEP MAC MIB PHY Service Interface PHY Mgmt Service Interface PLCP Sublayer PHY PHY layer Management FH IR DSSS OFDM PMD Sublayer P. Bhagwat

  3. 802.11 Specifications LLC MAC Service Interface (clause 6) MAC Mgmt Service Interface (clause 10) MAC sublayer MAC Management MAC framing (clause 7) MAC operation (clause 9) WEP (clause 8) State Machines (Annex C) Protocols (clause 11) State Machines (Annex C) MIBs (Annex D) PHY Service Interface (clause 12) PHY Mgmt Service Interface (clause 13) PHY Layer PHY Management FH (clause 14) DSSS (clause 15) Infrared (clause 16) OFDM (clause 17) High rate DSSS (clause 18) MIBs (Annex D) P. Bhagwat

  4. 802.11 System Architecture Infrastructure Basic Service Set (BSS) • AP provides • connection to wired network • relay function • stations not allowed to communicate directly Basic Service Set (BSS): a set of stations which communicate with one another Independent Basic Service Set (IBSS) • only direct communication possible • no relay function P. Bhagwat

  5. Extended Service Set ESS: a set of BSSs interconnected by a distribution system (DS) • ESS and all of its stations appear to be a single MAC layer • AP communicate among themselves to forward traffic • Station mobility within an ESS is invisible to the higher layers P. Bhagwat

  6. 802.11 Specifications Control • Specification of layers below LLC • Associated management/control interfaces Applications LLC MAC Mgmt WEP MAC MIB PHY FH IR DSSS OFDM P. Bhagwat

  7. 802.11 PHY Control Applications LLC WEP MAC Mgmt MAC MIB PHY FH IR DSSS OFDM P. Bhagwat

  8. 802.11 PHY Direct Sequence Spread Spectrum (DSSS) PHY 1,2 Mbps Frequency Hopping Spread Spectrum (FHSS) PHY 1, 2 Mbps Infrared (IR) PHY 1,2 Mbps Orthogonal Frequency Division Multiplexing (OFDM) PHY 6,9,12,18,24,36,48,54 Mbps 802.11a Higher rate (DSSS) PHY 20+ Mbps 802.11g High rate (DSSS) PHY 11, 5.5 Mbps 802.11b 5.7 GHz 2.4 GHz Sender Receiver MAC Protcol Data Unit (MPDU) MAC Protcol Data Unit (MPDU) MAC PHY PLCP header MAC Protcol Data Unit (MPDU) PLCP header MAC Protcol Data Unit (MPDU) Physical Media Dependent (PMD) layer PMD layer P. Bhagwat

  9. DSSS PHY • Baseband signal is spread using Barker word (10 dB processing gain) • Spread signal occupies approximately 22 Mhz bandwidth • Receiver recovers the signal by applying the same Barker word • DSSS provides good immunity against narrowband interferer • CDMA (multiple access) capability is not possible Preamble Header MPDU Preamble Header MPDU 1 Mbps 1 Mbps 1, 2 Mbps 1, 2 Mbps DPSK modulation DPSK de-modulation Spread the signal using Barker word (11 bits) +1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1 Received signal after despreading Transmitter baseband signal Transmitted signal after spreading P. Bhagwat

  10. DSSS PHY Ch 11 Ch 6 Ch 1 • Direct sequence spread spectrum • Each channel is 22 Mhz wide • Symbol rate • 1 Mb/s with DBPSK modulatio • 2 Mbps with DQPSK modulation • 11, 5.5 Mb/ps with CCK modulation • Max transmit power • 100 Mw 22 Mhz . . . 83.5 Mhz P. Bhagwat

  11. 802.11 MAC Control Applications LLC WEP MAC Mgmt MAC MIB PHY FH IR DSSS OFDM P. Bhagwat

  12. 802.11 MAC : Design goals • Single MAC to support multiple PHYs • Support multiple channel PHYs • Robust against interference • Cope with hidden nodes • Support for time bounded service, QoS • Should be scalable and stable at high loads • Need provisions for Power Saving Modes • Need provisions for Privacy and Access Control P. Bhagwat

  13. 802.11 MAC • Carrier sensing (CSMA) • Rules: • carrier ==> do not transmit • no carrier ==> OK to transmit • But the above rules do not always apply to wireless. • Solution: RTS/CTS • Collision detection (CD) • Does not work over wireless • Therefore, use collision avoidance (CA) • random backoff • priority ack protocol P. Bhagwat

  14. 802.11 - MAC layer • Priorities • defined through different inter frame spaces • no guaranteed, hard priorities • SIFS (Short Inter Frame Spacing) • highest priority, for ACK, CTS, polling response • PIFS (PCF IFS) • medium priority, for time-bounded service using PCF • DIFS (DCF, Distributed Coordination Function IFS) • lowest priority, for asynchronous data service DIFS DIFS PIFS SIFS medium busy contention next frame t direct access if medium is free  DIFS P. Bhagwat

  15. 802.11 MAC protocol: CSMA/CA contention window slot time • Use CSMA with collision Avoidance • Based on carrier sense function in PHY called Clear Channel Assessment (CCA) • Reduce collision probability where mostly needed • Efficient backoff algorithm stable at high loads • Possible to implement different fixed priority levels DIFS Next Frame Busy medium Defer access P. Bhagwat

  16. 802.11 MAC : Contention window 1023 CW max For DSSS PHY Slot time = 20 s 511 255 127 63 31 CW min Fifth retransmission Fourth retransmission Third retransmission Second retransmission First retransmission Initial attempt P. Bhagwat

  17. Backoff procedure DIFS DIFS DIFS DIFS defer Frame Frame Frame Frame DIFS • Immediate access when medium is free >= DIFS • When medium is not free, defer until the end of current frame trasnsmission + DIFS period • To begin backoff procedure • Choose a random number in ( 0, Cwindow) • Use carrier sense to determine if there is activity during each slot • Decrement backoff time by one slot if no activity is detected during that slot • Suspend backoff procedure if medium is determined to be busy at anytime during a backoff slot • Resume backoff procedure after the end of current frame transmission CWindow CWindow Frame A B defer C defer D P. Bhagwat

  18. CSMA/CA + ACK protocol DIFS • Defer access based on carrier sense • Direct access when medium is sensed free longer than DIFS • Receiver of directed frames to return an ACK immediately when CRC is correct • When no ACK received then retransmit frame after a random backoff Data Src SIFS ACK Dest contention window DIFS Next Frame Other P. Bhagwat

  19. Fragments transmission SIFS SIFS Fragment burst DIFS • Fragment transmission supported to improve transmission reliability under noisy environments • Transmitter holds the channel until the end of fragment transmission burst • If the source does not receive and ACK frame, it will transmit the failed MPDU after performing the backoff procedure and the contention process • Receiver may receive duplicate fragments and is responsiblefor detecting and discarding duplicate fragments Backoff window SIFS SIFS SIFS Source SIFS Fragment 0 Fragment 1 Fragment 2 ACK 0 ACK 1 ACK 2 Destination P. Bhagwat

  20. Problems with carrier sensing Exposed terminal problem Z W Z is transmitting to W X Y Y will not transmit to X even though it cannot interfere / Presence of carrier ===> hold off transmission P. Bhagwat

  21. Problems with carrier sensing Hidden terminal problem Y Z W W finds that medium is free and it transmits a packet to Z no carrier ===> OK to transmit / P. Bhagwat

  22. Solving Hidden Node problem with RTS/CTS CTS RTS - listen RTS - wait long enough for the requested station to respond with CTS - if (timeout) then ready to transmit - listen CTS - wait long enough for the transmitter to send its data listen RTS ==> transmitter is close to me listen CTS ==> receiver is close to me Y Z X W Note: RTS/CTS does not solve exposed terminal problem. In the example above, X can send RTS, but CTS from the responder will collide with Y’s data. P. Bhagwat

  23. RTS/CTS exchange example SIFS • RTS + CTS + Frame + ACK exchange invoked when frame size is large • Overhead estimation • RTS -- 18 bytes (PCLP Preamble) + 6 bytes (PCLP Header) + 20 bytes (RTS) • 192 µs + 160 µs = 352 µs • CTS -- 18 bytes (PCLP Preamble) + 6 bytes (PCLP Header) + 14 bytes (RTS) • 192 µs + 112 µs = 304 µs • SIFS – 10 µs • NAV (Network Allocation Vector) • NAV maintains prediction of future traffic on the medium based on duration information that is announced in RTS/CTS frames prior to actual exchange of data DIFS Frame RTS Src ACK CTS Dest 8192 s 352 µs 304 µs 304 µs 10 µs 10 µs 10 µs Dest NAV (RTS) NAV (CTS) P. Bhagwat

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