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• Random access MAC protocols for ad hoc

networks support reliable unicast.

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)

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

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.

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.)

Average CW Estimation

Protocol Capacity (Analytical and Simulation)

Well approximated

P-persistent

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.

Fig 2: N=25, lamda=0.0001 packets/slot/node

Throughput simulation in Paper # 14

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.