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Designing Efficient MAC Protocols for Wireless LANs

Learn about the basic structure of Wireless LANs, station communication with access points, signal decay, collision detection challenges, and the importance of MAC protocols for delivering packets efficiently in wireless communications.

josephine-phelps
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Designing Efficient MAC Protocols for 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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