110/15/2003CS211 IEEE Standard Why we study this standard: overall architecture physical layer spec. –direct sequence –frequency hopping MAC layer spec. –channel access –mobility support

210/15/2003CS Architecture

310/15/2003CS 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

410/15/2003CS 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

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

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

710/15/2003CS211 Frequency Hopping

810/15/2003CS211 Direct Sequence Spread Spectrum Spreading factor = code bits/data bit, 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

910/15/2003CS211 Direct Sequence Spread Spectrum

1010/15/2003CS b Frequency Channels In US, the 2.4ISM band is from 2400MHz to MHz –Divided up to 11 “channels” from 2412~2462MHz, spaced 5MHz apart –Each b 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

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

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

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

1410/15/2003CS frame priorities

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

1610/15/2003CS211 4-Way Handshake

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

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

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

2010/15/2003CS211 Synchronization in 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

2110/15/2003CS211 Roaming in

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

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 MAC uses a common mechanism –passive or active scanning Passive scanning –by listening for Beacons Action Scanning –probe + response

2410/15/2003CS211 Active scanning

2510/15/2003CS211 Power Management

2610/15/2003CS a Standard PHY layer –12 nonoverlapping channels in 5GHz band –OFDM –Offers rate up to 54Mbps MAC –Roughly the same as b g –Backward compatible with b, operating at 2.4Ghz, fall back to 11Mbps with b AP –OFDM based