Delay Analysis of IEEE 802.11 in Single-Hop Networks

1. Delay Analysis of IEEE 802.11 in Single-Hop Networks Marcel M. Carvalho, J.J.Garcia-Luna-Aceves

2. Outline • The Distributed coordination function mechanism • Service time characterization • Channel probabilities • Model validation • Performance evaluation of DSSS and FHSS • Conclusion

3. The Distributed coordination function(DCF) backoff • DCF describes two techniques for packet transmissiom • Basic access mechanism • RTS/CTS access mechanism • The backoff time counter is decremented if the channel is sensed idle • Otherwise it is frozen in its current state until the channel is sensed idle more than a DIFS

4. Notation • Three possible events a node can sense during its backoff • = {successful transmission} , • ={idle channel} , • ={collision} , • Let denote the backoff stage, • Let , , and denote the number of idle slots, collision slots and successful transmission slots respectively, and , and denote their probabilities respectively. • and is the average time the channel is sensed busy due a collision and a successful transmission respectively. is the time used when the channel is sensed idle (one backoff time) • The average backoff step size is

5. Service time characterization • At k-th backoff stage, the number of backoff steps is chosen uniformly from . The average number of backoff steps is • The average time a node spends at k-th stage is • Let the probability of collision. The probability of successful transmission at k-th stage is • The cumulative delay of a node at k-th backoff stage is

6. Service time characterization • The average single-hop delay considering the frame retry limit is given by the following expression Where

7. Channel Probabilities • This model is applicable whenever , , and are known • These probabilities are computed for saturated, single hop ad hoc network under ideal channel condition (i.e, no hidden terminals and capture), with fixed number of nodes • This analysis is based on Bianchi framework, the probability that a node transmits in a randomly chosen slot time is Where p is the probability of a collision experienced by a transmitted packet on the channel

8. Channel Probabilities • Let then • Using the Taylor series expansion of at p=0 , the first order approximation of is • is approximated the following

9. Channel Probabilities

10. Channel Probabilities

11. Channel Probabilities • Probability that there is one transmission in the considered time slot is • Let is the probability that a transmission occurring on the channel is successful • The probability of collision is • The probability of successful transmission is • The probability of idle slot is

12. Model Validation • Ns-2 simulator • Network size from 8 to 56 nodes • Packet size 1500 bytes • Nodes randomly distributed • Network area 20*20 m • No mobility • Performance metrics: service time and jitter

13. Model Validation

14. Performance Evaluation

15. Performance evaluation

16. Performance Evaluation

17. Conclusion • DSSS performs better than FHSS in term of delay and jitter • The higher the initial contention window size, the smaller the average service time and jitter are, especially for large networks • The binary exponential backoff algorithm has negative impact if both the maximum backoff stage and the number of nodes in the network are large