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

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

4. 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)

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

6. 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.

7. 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.)

8. Average CW Estimation

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

10. 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.

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

15. 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.