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Wireless LANs

Wireless LANs. Wireless LAN. Basic structure: Stations plus an access point Stations talk to the access point, then to outside Access point talks to stations Stations talk to stations Problem: If more than one station talk at the same time, the packets will be corrupted Design goal:

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Wireless LANs

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  1. Wireless LANs

  2. Wireless LAN • Basic structure: • Stations plus an access point • Stations talk to the access point, then to outside • Access point talks to stations • Stations talk to stations • Problem: • If more than one station talk at the same time, the packets will be corrupted • Design goal: • A MAC protocol to determine who talks next

  3. Wireless communications • Signal decays according to a power law with the distance, at least to the power of -2 with distance • When a station is sending, not all stations can hear the medium is busy.

  4. Wireless communications • When a station is sending, he cannot hear other stations – cannot decide if there is a collision. No collision detection in wireless LAN.

  5. Wireless communications • Being able to sense the carrier does not mean that you can decode the data • If received signal having power P means that you can decode the data, it may be true that at power P/2 you can realize that there is something going on

  6. Wireless communication • The received signal can be decoded if the signal to noise ratio is larger than a certain threshold. Whether there is a collision depends on the signal to noise ratio at the receiver. • You may allow two transmissions at the same time without collision. A B C D A B C D A->B, D->C A->B, C->D

  7. Wireless communications • Hidden terminal, A->B, C->D. C did not hear A. A B C D • Exposed terminal. A->B, C->D. C hears A. B A C D

  8. MAC • What if we just let everyone talk when he wants to? • There will be collisions.

  9. MAC • When medium is free, what if we say just let people who has been waiting to talk? • More than one station could start at the same time.

  10. MAC • How to make sure that a packet is delivered? • Use ACK.

  11. DCF • Simplified 802.11 DCF operation for unicast. (Automating Cross-Layer Diagnosis of Enterprise Wireless Networks, Sigcomm 2007)

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