L1.p.80211standard.pptVIP

CS219/Fall99 IEEE 802.11 Standard Why we study this standard: Showcase issues with WIRELESS and MOBILITY As a case study to tell you WHY MAC layer spec. Wireless channel access mobility support HOW to apply ADAPTION principle in one case study? 802.11 Architecture 802.11 Top Goal Act as wireless Ethernet ! It is all about WIRELESS DATA, DATA, DATA Not the same as voice, which is #1 goal in cell phone service Other goals are also there, but secondary Voice support, security, … 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 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 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 Direct Sequence Spread Spectrum Question: How to Design Wireless MAC in 802.11? Goal: support DATA Condition: single shared, wireless channel in an ad-hoc setting Why not use CSMA/CD MAC for wired Ethernet??? CSMA/CA+ACK: 4-way handshake Time-critical service via PCF 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 bea