Introduction to IEEE 802.11

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Introduction to IEEE 802.11
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IEEE 802.11

• A standard for wireless LANs
• An excellent book
• Matthew S. Gast, Mike Loukides, 802.11 Wireless
  Networks The Definitive Guide , OReilly
  Associates, Inc.

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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 PHYs 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

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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

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IEEE 802 Standards
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802.11
802.11 MAC
802.11 FHSS
802.11 DSSS
802.11a OFDM
802.11b DSSS
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802.11 BSS

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

Access point
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802.11 ESS

• Extended Service Set (ESS)

Distributed System

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Major Protocols

• MAC
• Management Operations
• Scanning
• Authentication
• Association
• Power Saving
• Timing Synchronization

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Power Saving
Beacon interval
sleep
sleep
time
Beacon window ATIM window
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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.

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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.

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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
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Timing Sync Needed for Power Saving
Beacon interval
sleep
sleep
time
Beacon window ATIM window
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Timing Sync Needed for Frequency Hopping
f1
f2
f3
f4
f5
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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

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802.11s Time Sync Function

• Beacon contains a timestamp.
• On receiving a beacon, STA adopts beacons timing
  if T(beacon) gt T(STA).
• Clocks move only forward.

1201
1202
1201
1200
1201
faster
slower
adopts
not adopts
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Is IEEE 802.11 TSF Scalable?

• Why or Why Not?

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