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Introduction to Wireless LAN

Introduction to Wireless LAN. Outline. Architecture MAC Management Distributed Coordination Function (DCF). Architecture. Station Access Point (AP) Basic Service Set (BSS) Distribution System (DS) Extended Service Set (ESS). Access Point (AP). Basic Service Set (BSS).

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Introduction to Wireless LAN

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  1. Introduction to Wireless LAN

  2. Outline • Architecture • MAC Management • Distributed Coordination Function (DCF)

  3. Architecture • Station • Access Point (AP) • Basic Service Set (BSS) • Distribution System (DS) • Extended Service Set (ESS)

  4. Access Point (AP)

  5. Basic Service Set (BSS) • Ad-hoc Mode(非常態): 又稱為peer-to-peer,由Station組成Independent Basic Service Set (IBSS) • Infrastructure Mode: 由Station和Access Point (AP) 組成Basic Service Set (BSS)

  6. Extended Service Set (ESS) • Extended Service Set (ESS) = BSSs+ DS • 常見的DS是Ethernet • Station可以在ESS下，不同的BSS 之間漫游(Roaming)

  7. Example

  8. B1 A1 Gateway to Internet portal Server Distribution System portal AP1 AP2 A2 B2 BSS A BSS B Example

  9. Hidden Node Problem A can communicate only with B B can communicate with A and B C can communicate only with B The frame from A to B can be corrupted by transmission from C

  10. 4-Way Handshake • RTS: Request To Send • CTS: Clear To Send • Frame length <RTS Threshold: Exchange without RTS ,CTS A B RTS CTS Data ACK

  11. MAC Architecture 免競爭式服務 (具時限傳輸) 競爭式服務 (非同步傳輸) Point Coordination Function (PCF) MAC Extent Distributed Coordination Function (DCF)

  12. Distributed Coordination Function (DCF):CSMA/CA • Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) • Net Allocation Vector (NAV): • Prediction of future traffic • Operation like a timer • NAV value=Duration field of last MAC frame,counting down to zero • After NAV=0,the station can transmit if PHY indicates a clear channel

  13. CSMA/CA

  14. Time Intervals • SIFS:Short InterFrame Space(e.g. ACK or CTS) • PIFS:Priority InterFrame Space used in PCF • DIFS:Distributed InterFrame Space used in DCF

  15. DFC Operation Timing Immediate access when medium is free >= DIFS DIFS PIFS Contention Window DIFS SIFS Back off- Window Busy Medium Next Frame Slot time Defer Access

  16. Example of CSMA/CA DIFS A Frame Backoff 9 us Frame B 19 us 4 us 2 us C Frame 10 us D Frame 15 us 5 us E Frame 7 us 2 us

  17. Reference • Wireless Local Area Network by Dr.Morris Chang

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