Introduction to IEEE 802.11

IEEE 802.11

A standard for wireless LANs

An excellent book:– Matthew S. Gast, Mike Loukides,

802.11 Wireless Networks: The Definitive Guide , O’Reilly & Associates, Inc.

History of IEEE 802.11

802.11 standard first ratified in 1997– 802.3 LAN emulation– 1 & 2 Mbps in the 2.4 GHz band

Two high rate PHY’s ratified in 1999– 802.11a: 6 to 54 Mbps in the 5 GHz band– 802.11b: 5.5 and 11 Mbps in the 2.4 GHz band

The Beat Goes On

802.11c: support for 802.11 frames 802.11d: new support for 802.11 frames 802.11e: QoS enhancement in MAC 802.11f: Inter Access Point Protocol 802.11g: 2.4 GHz extension to 22 Mbps 802.11h: channel selection and power control 802.11i: security enhancement in MAC 802.11j: 5 GHz globalization

IEEE 802 Standards

802.3 MEDIUMACCESS

(Ethernet)

PHYSICAL

802.4 MEDIUMACCESS

(token bus)

PHYSICAL

802.5 MEDIUMACCESS

(token ring)

PHYSICAL

802.11 MEDIUMACCESS

(WLAN)

PHYSICAL

802.12MEDIUMACCESS

(Gigabit

LAN)PHYSICAL

. . .

802.1 BRIDGING

802.2 LOGICAL LINK CONTROL

PHYSICAL

LAYER

DATA

LINK

LAYER

802.

1 M

AN

AG

EM

EN

T

802.11

802.11 MAC

802.11FHSS

802.11DSSS

802.11aOFDM

802.11bDSSS

802.11 BSS

Basic Service Set (BSS) --- a basic LAN Infrastructure BSS Independent BSS (Ad Hoc LAN)

Access point

802.11 ESS

Extended Service Set (ESS)

Distributed System

Major Protocols

MAC Management Operations

– Scanning– Authentication– Association– Power Saving– Timing Synchronization

Power Saving

sleep

Beacon window ATIM window

Beacon interval

timesleep

When to stay awake?

ATIM: Announcement traffic indication map.

IBSS: If a node has an outgoing data frame for B, it sends B a traffic announcement in the ATIM window.

A node stays awake for an entire beacon interval if it has incoming and/or outgoing traffic.

Beacons

Beacons carry information about the BSS.– To allow new stations to join in– Timing synchronization

Every station must listen to Beacons. Infrastructure BSS: AP sends beacons. IBSS: every station contends for beacon

generation in the beacon window.

Beacon Contention/Generation

Each station:– 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.

window

beacon interval

Timing Sync Needed for Power Saving

sleep

Beacon window ATIM window

Beacon interval

timesleep

Timing Sync Needed for Frequency Hopping

f1

f2

f4

f3

f5

802.11 Timers (Clocks)

Timer: 64 bits, ticking in microseconds. Accuracy: within + 0.01%, or +100 ppm. Time synchronization needed for:

– Frequency hopping– Power management

∆ = max tolerable difference between clocks.

– Desired value: 25 s

802.11’s Time Sync Function

Beacon contains a timestamp. On receiving a beacon, STA adopts beacon’s

timing if T(beacon) > T(STA). Clocks move only forward.

faster adopts

12:01 12:00

slower not adopts

12:01 12:0212:01

Is IEEE 802.11 TSF Scalable?

Why or Why Not?