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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

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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
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
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

802.1 MANAGEMENT

802.2 LOGICAL LINK CONTROL

DATA

LINK

LAYER

802.1 BRIDGING

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

. . .

PHYSICAL

LAYER

IEEE 802 Standards
802 11
802.11

802.11 MAC

802.11

FHSS

802.11

DSSS

802.11a

OFDM

802.11b

DSSS

802 11 bss
802.11 BSS

Basic Service Set (BSS) --- a basic LAN

Infrastructure BSS

Independent BSS (Ad Hoc LAN)

Access point

802 11 ess
802.11 ESS

Extended Service Set (ESS)

Distributed System

major protocols
Major Protocols
  • MAC
  • Management Operations
    • Scanning
    • Authentication
    • Association
    • Power Saving
    • Timing Synchronization
power saving
Power Saving

Beacon interval

sleep

sleep

time

Beacon window ATIM window

when to stay awake
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
  • 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
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.

beacon interval

window

timing sync needed for power saving
Timing Sync Needed for Power Saving

Beacon interval

sleep

sleep

time

Beacon window ATIM window

802 11 timers clocks
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
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.

12:01

12:02

12:01

12:00

12:01

faster

slower

adopts

not adopts