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Introduction to IEEE 802.11

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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Introduction to IEEE 802.11

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  1. Introduction to IEEE 802.11

  2. 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.

  3. 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

  4. 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

  5. 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

  6. 802.11 802.11 MAC 802.11 FHSS 802.11 DSSS 802.11a OFDM 802.11b DSSS

  7. 802.11 BSS Basic Service Set (BSS) --- a basic LAN Infrastructure BSS Independent BSS (Ad Hoc LAN) Access point

  8. 802.11 ESS Extended Service Set (ESS) Distributed System

  9. Major Protocols • MAC • Management Operations • Scanning • Authentication • Association • Power Saving • Timing Synchronization

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

  11. 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.

  12. 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.

  13. 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

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

  15. Timing Sync Needed for Frequency Hopping f1 f2 f3 f4 f5

  16. 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

  17. 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

  18. Is IEEE 802.11 TSF Scalable? Why or Why Not?

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