On the Performance Behavior of IEEE 802.11 Distributed Coordination Function

1. On the Performance Behavior of IEEE 802.11 Distributed Coordination Function M.K.Sidiropoulos, J.S.Vardakas and M.D.Logothetis Wire Communications Laboratory, Department of Electrical & Computer Engineering, University of Patras, 265 00 Patras, Greece E-mail: m-logo@wcl.ee.upatras.gr

2. Outline • Purpose of the paper. • DCF-An Example. • Mathematical Analysis ( Assumptions ). • 802.11 DCF: Markov Chain Model and Steady State Analysis leading to a Saturation Throughput formula. • Simulation results ( IEEE 802.11b network). • Conclusion.

3. Purpose of the paper • We propose a new Markov model for the DCF of IEEE 802.11 based on Bianchi’s, Wu’s and Ziouva’s models. • and derive an analytical formula for the Saturation Throughput for both Basic and RTS/CTS access schemes. • Simulation Study: • Validation of our new Markov model based on throughput results by the NS-2. • Average end-to-end packet delay for both access schemes.

4. DCF-An Example • DCF employs 2 mechanisms: • Basic access scheme: A 2-way handshaking technique. • Note that: • After a DIFS time interval each station defers for an additional random backoff time. • The backoff counter is frozen if a transmission is detected on the channel BACKOFF SUSPENSION

5. DCF-An Example (cont.) • RTS/CTS: • Request-To-Send / Clear-To- Send. • It is a 4-way handshaking technique. • Introduced to tackle the hidden terminal problem. • Improve throughput performance in case of long packets.

6. Mathematical Analysis Assumptions: • Ideal channel conditions ( error-free channel). • Finite number of stations, each of which has always a packet available for transmission. (saturation conditions) • Constant and independent collision probability p. • Probability pb independent of the backoff procedure.

7. 802.11 DCF: Markov Chain Model

8. Saturation Throughput model. Normalized Throughput: ( fraction of the channel time used for payload transmissions) • Ps : a successful transmission in a slot. • Ptr : at least one transmission in a slot. • E[P]: average packet payload. • σ : duration of an empty slot time • Ts: average time of a successful transmission • Tc : average duration of a collision .

9. Steady State Analysis Stationary Distribution of the chain ( Steady State ): From the chain we have:

10. Steady State Analysis (cont.) Normalization condition : Contention Window :

11. Steady State Analysis (cont.) Channel access probability : Collision probability : Probability of channel being busy :

12. Saturation Throughput model. Normalized Throughput: ( fraction of the channel time used for payload transmissions) • Ps : a successful transmission in a slot. • Ptr : at least one transmission in a slot. • E[P]: average packet payload. • σ : duration of an empty slot time • Ts: average time of a successful transmission • Tc : average duration of a collision .

13. Simulation Study • Performance metrics measured by simulation: • Saturation throughput. • End-to-end average packet delay. • Simulations in NS-2 • IEEE 802.11b single-hop network. • Network Topology: • No hidden stations, all have LOS. • CBR traffic over UDP links towards the AP. • No mobility.

14. Model Validation: Simulation vs. Analysis: 1Mbps. Basic and RTS/CTS • Close match of analytical model and simulation results. • Our model is closer to simulation than Wu’s. • The RTS/CTS gives higher throughput than Basic due to the short RTS frames. ( Only exception for n =5).

15. Model Validation: Simulation vs. Analysis: 5.5 and 11 Mbps. • In both cases analysis and simulation are in satisfactory agreement. • Basic access scheme gives higher throughput than RTS/CTS when channel bit rate ↑. ( RTS, CTS packets are transmitted at 1Mbps). • Throughput ↓ as bit rate↑ ( DIFS,SIFS, Backoff delay remain unchanged)

16. Average Delay Simulation • As network size ↑delay ↑ for both access schemes. • As channel bit rate ↑delay ↓. • RTS/CTS delay is lower than Basic delay only for 1Mbps. Not efficient to use RTS/CTS for high data rates

17. We have developed an analytical model to enhance Bianchi’s and Wu’s analytical model for the saturation throughput of the DCF of the IEEE 802.11 protocol. Our model gives greater throughput results than Wu’s model for both access schemes, Basic and RTS/CTS. Via numerous simulations with NS-2 we have shown that our model is close to simulation, for all network sizes . As channel bit rate increases: throughput decreases Average delay decreases. Basic vs. RTS/CTS: In low rates RTS is better than Basic. In higher rates Basic is preferable than RTS ( gives greater throughput and lower delay). Conclusion

18. THANK YOU!