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802.11 Opersating Mode

802.11 Opersating Mode. DCF: distributed coordination function basic access mode, CSMA/CA, RTS/CTS, Backoff Algorithm PCF: Point coordination function Centralized MAC Support collision free and time bounded service. RTS/CTS Handshake.

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802.11 Opersating Mode

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  1. 802.11 Opersating Mode • DCF: distributed coordination function • basic access mode, CSMA/CA, • RTS/CTS, Backoff Algorithm • PCF: Point coordination function • Centralized MAC • Support collision free and time bounded service

  2. RTS/CTS Handshake • Random access MAC protocols for ad hoc networks support reliable unicast.

  3. 802.11 DCF Basic Mode • CSMA/CA: • No handshake • Carrier Sense Multiple Access with Collision Avoidance • All the DATA transmissions must wait one DCF Inter Frame Space (DIFS). • ACKs need to wait one Short Inter Frame Space (SIFS)

  4. Assumptions • The RTS/CTS mechanism is not considered • Only consider the basic 802.11 DCF function (i.e., the CSMA/CA mechanism). • Asymptotic conditions: All M stations have packets ready for transmission

  5. Distributed Coordination Function • Before transmission, a station senses the medium. • If the medium is sensed idle for DIFS, the station is allowed to transmit upon backoff timer expiration. • After each unsuccessful transmission, the station exponentially increases the maximum contention window size, CW, chooses randomly the value of its backoff timer from [0,CW-1]. • CWmin=32 and CWmax=256. • The backoff timer is decreased as long as the channel is sensed idle, and stopped when transmission is in progress.

  6. Distributed Coordination Function • When the backoff timer expires, the station attempts transmission at the beginning of the next slot time. • If the packet is successfully received, the receiver sends an acknowledgement after SIFS (<DIFS). • If an ACK is not received, the data packet is assumed lost and a retransmission is scheduled. (Up to 7 retransmissions are allowed before the packet is dropped.)

  7. Average CW Estimation

  8. Protocol Capacity (Analytical and Simulation) P-persistent Well approximated

  9. How to switch between basic mode and RTS/CTS mode? • Basic mode: can’t have contention • RTS/CTS mode: overhead of RTS/CTS • Use a RTS threshold • Switch to RTS/CTS mode when packet size is big enough.

  10. Throughput simulation in Paper # 14 Fig 2: N=25, lamda=0.0001 packets/slot/node

  11. Recommendation • Always use RTS/CTS • However, … • In another study, the data rate was fixed at 11Mbps. The results were the opposite to the above. • for a low data rate, of 1 or 2Mbps, the RTS/CTS is superior to the basic mechanism in almost all cases. • But for a high data rate such as 11Mbps, the RTS/CTS mechanism can achieve a better performance only when there are many STAs transmitting large packets. • This is mainly because the overhead ratio increases when the data rate increases since the physical header and preamble are always transmitted at 1Mbps. • Optimal threshold depends on: active station number, contention degree.

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