802.11 Specification overview Pravin Bhagwat 802.11 Specification overview P. Bhagwat Tutorial: 802.11

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

802.11 Specifications LLC MAC sublayer MAC Management PHY Layer 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

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

ESS: a set of BSSs interconnected by a distribution system (DS) 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

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

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

802.11 PHY Sender Receiver 802.11a 802.11g 802.11b MAC PHY Pravin Bhagwat 802.11 PHY 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 Direct Sequence Spread Spectrum (DSSS) PHY 1,2 Mbps Frequency Hopping Spread Spectrum (FHSS) PHY 1, 2 Mbps Infrared (IR) PHY Orthogonal Frequency Division Multiplexing (OFDM) PHY 6,9,12,18,24,36,48,54 Mbps 802.11a 5.7 GHz Mention that over SCO link you cannot carry any other real-time traffic. There is no protocol-id field in the SCO header/payload. Is this really true? High rate (DSSS) PHY 11, 5.5 Mbps 802.11b Higher rate (DSSS) PHY 20+ Mbps 802.11g 2.4 GHz P. Bhagwat Tutorial: 802.11

DSSS PHY 1 Mbps 1, 2 Mbps 1 Mbps 1, 2 Mbps Pravin Bhagwat DSSS PHY Preamble Header MPDU Preamble Header MPDU 1 Mbps 1, 2 Mbps 1 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 Transmitter baseband signal Transmitted signal after spreading Received signal after despreading 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 Mention that over SCO link you cannot carry any other real-time traffic. There is no protocol-id field in the SCO header/payload. Is this really true? P. Bhagwat Tutorial: 802.11

DSSS PHY Direct sequence spread spectrum Symbol rate Ch 1 Ch 6 Ch 11 22 Mhz . . . 83.5 Mhz 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 P. Bhagwat

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

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

802.11 MAC Carrier sensing (CSMA) Collision detection (CD) 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

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

802.11 MAC protocol: CSMA/CA Busy medium Next Frame contention window slot time DIFS Busy medium Next Frame Defer access 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 P. Bhagwat

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

Backoff procedure A B C D DIFS DIFS DIFS DIFS DIFS CWindow CWindow Frame Frame defer B Frame defer C Frame defer D Frame 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 P. Bhagwat

CSMA/CA + ACK protocol Defer access based on carrier sense DIFS Data Src SIFS ACK Dest DIFS contention window Next Frame Other 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 P. Bhagwat

Fragments transmission Fragment burst DIFS SIFS SIFS SIFS SIFS Backoff window SIFS Source SIFS Fragment 0 Fragment 1 Fragment 2 ACK 0 ACK 1 ACK 2 Destination 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 responsible for detecting and discarding duplicate fragments P. Bhagwat

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

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

Solving Hidden Node problem with RTS/CTS listen RTS ==> transmitter is close to me listen CTS ==> receiver is close to me - listen RTS - wait long enough for the requested station to respond with CTS - if (timeout) then ready to transmit - listen CTS for the transmitter to send its data 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

RTS/CTS exchange example SIFS DIFS RTS Frame Src CTS ACK Dest 352 µs 304 µs 8192 s 304 µs 10 µs 10 µs 10 µs Dest NAV (RTS) NAV (CTS) 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 P. Bhagwat