1. 110/15/2003CS211 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

2. 210/15/2003CS211 802.11 Architecture

3. 310/15/2003CS211 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

4. 410/15/2003CS211 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 PMD sublayer PHY layer Management

5. 510/15/2003CS211 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

6. 610/15/2003CS211 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

7. 710/15/2003CS211 Frequency Hopping

8. 810/15/2003CS211 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

9. 910/15/2003CS211 Direct Sequence Spread Spectrum

10. 1010/15/2003CS211 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

11. 1110/15/2003CS211 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

12. 1210/15/2003CS211 CSMA/CA+ACK: 4-way handshake

13. 1310/15/2003CS211 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

14. 1410/15/2003CS211 802.11 frame priorities

15. 1510/15/2003CS211 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

16. 1610/15/2003CS211 4-Way Handshake

17. 1710/15/2003CS211 Time-critical service via PCF

18. 1810/15/2003CS211 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

19. 1910/15/2003CS211 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

20. 2010/15/2003CS211 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

21. 2110/15/2003CS211 Roaming in 802.11

22. 2210/15/2003CS211 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

23. 2310/15/2003CS211 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

24. 2410/15/2003CS211 Active scanning

25. 2510/15/2003CS211 Power Management

26. 2610/15/2003CS211 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