Wireless Ad HocSensor Networks From IEEE 802 11

Wireless Ad Hoc/Sensor Networks: From IEEE 802. 11 to Berkeley Motes and Beyond Ten-Hwang Lai, OSU

Introduction to IEEE 802. 11 Ten-Hwang Lai Ohio State University

Standards for Wireless LANs l l l IEEE 802. 11 Bluetooth Hiper. Lan (Europe)

History of IEEE 802. 11 l 802. 11 standard first ratified in 1997 – – l 802. 3 LAN emulation 1 & 2 Mbps in the 2. 4 GHz band Two high rate PHY’s ratified in 1999 – – 802. 11 a: 6 to 54 Mbps in the 5 GHz band 802. 11 b: 5. 5 and 11 Mbps in the 2. 4 GHz band

The Beat Goes On l l l l 802. 11 c: support for 802. 11 frames 802. 11 d: new support for 802. 11 frames 802. 11 e: Qo. S enhancement in MAC 802. 11 f: Inter Access Point Protocol 802. 11 g: 2. 4 GHz extension to 22 Mbps 802. 11 h: channel selection and power control 802. 11 i: security enhancement in MAC 802. 11 j: 5 GHz globalization

IEEE 802 Standards 802. 1 MANAGEMENT 802. 2 LOGICAL LINK CONTROL DATA LINK LAYER 802. 1 BRIDGING 802. 3 802. 4 802. 5 802. 11 802. 12 MEDIUM ACCESS MEDIUM ACCESS (WLAN) (Gigabit LAN) PHYSICAL (Ethernet) (token bus) (token ring) PHYSICAL . . . PHYSICAL LAYER

802. 11 MAC 802. 11 a 802. 11 b FHSS DSSS OFDM DSSS

802. 11 BSS Basic Service Set (BSS) --- a basic LAN l Infrastructure BSS l Independent BSS (Ad Hoc LAN) Access point

802. 11 ESS l Extended Service Set (ESS) Distributed System

Major Protocols l l MAC Management Operations – – – l l Scanning Authentication Association Power Saving Timing Synchronization WEP (wired equivalent privacy) Physical Layer

Power Saving Beacon interval sleep Beacon window ATIM window time

When to stay awake? l ATIM: Announcement traffic indication map. l IBSS: If a node has an outgoing data frame for B, it sends B a traffic announcement in the ATIM window. l A node stays awake for an entire beacon interval if it has incoming and/or outgoing traffic.

Beacons l Beacons carry information about the BSS. – – l l l To allow new stations to join in Timing synchronization Every station must listen to Beacons. Infrastructured BSS: AP sends beacons. IBSS: every station contends for beacon generation in the beacon window.

Beacon Contention/Generation l Each station: – – – l determines a random number k; waits for exactly k idle slots to pass; transmits a beacon (if no one else has done so). Beacon: several slots in length. beacon interval window

Timing Sync Needed for Power Saving Beacon interval sleep Beacon window ATIM window time

Timing Sync Needed for Frequency Hopping f 1 f 2 f 3 f 4 f 5

802. 11 Timers (Clocks) l l l Timer: 64 bits, ticking in microseconds. Accuracy: within + 0. 01%, or +100 ppm. Time synchronization needed for: – – l Frequency hopping Power management ∆ = max tolerable difference between clocks.

802. 11’s Time Sync Function l l l Beacon contains a timestamp. On receiving a beacon, STA adopts beacon’s timing if T(beacon) > T(STA). Clocks move only forward. 12: 01 12: 00 12: 01 12: 02 faster adopts slower not adopts

Is IEEE 802. 11 TSF Scalable? Why or Why Not?