1. of the IEEE 802.11 Distributed Coordination Function
Performance Analysis
of the IEEE Distributed Coordination Function
(By Giuseppe Bianchi, 2000)
Sunghwa Son

2. Outline Introduction 802.11 DCF
Maximum & Saturation Throughput Performance
Throughput Analysis
Model Validation
Maximum Saturation Throughput
Performance Evaluation
Conclusions

3. Introduction Distribution Coordination Function (DCF)
Fundamental mechanism to access medium
Based on CSMA/CA
Wireless LAN uses half-duplex
Describes two techniques
Basic Access Mechanism
RTS/CTS Mechanism

4. Basic Access Mechanism
Channel Idle for a period of time Distribution Inter Frame Space (DIFS)
Station generates random backoff interval
Employs discrete-time backoff scale
Transmission take at beginning of time slot
Backoff time chosen between (0,w-1)
Backoff time counter decremented to 0

5. Basic Access Mechanism

6. RTS/CTS Waits until channel sensed idle
Transmits short frame called RTS
Receiving station send CTS
Carry information of length of packet
Solves hidden terminal problems

7. RTS/CTS

8. Saturation Throughput
Limit reached by system throughput as offered load increases
Maximum load that the system can carry in stable conditions

9. Throughput Analysis Analytical Evaluation of Saturation Throughput
Assumption
Finite number of terminals and ideal channel conditions
Fixed number of stations, each always having packet for transmission

10. Throughput Analysis Two cases Packet Transmission Probability
Markov Chain model
Throughput
System throughput

11. Packet Transmission Probability
Each packet collide with constant and independent probability p
p will be referred to as conditional collision probability
Model bidimensional process {s(t) , b(t)} with discrete-time Markov chain
Stochastic Process b(t) representing backoff time counter
Stochastic Process s(t) representing backoff stage

12. Markov Chain Model

13. Markov Chain Model
Closed-form solution for Markov chain

14. Markov Chain Model
Probability τ that a station transmits in a randomly chosen slot time

15. Markov Chain Model
When m =0, i.e., no exponential backoff is considered, probability τ results to be independent of p
In general τ depends on conditional collision probability p

16. Throughput Normalized system throughput S
Probability that there is at least one transmission in the considered slot time
Probability that exactly one station transmits on the channel, conditioned on the fact that at least one station transmits

17. Throughput Normalized system throughput
E[P] is the average packet payload size
Specify Ts and Tc to compute throughput for DCF access mechanism

18. Throughput Considering System via Basic Access mechanism
Packet header H = PHYhrd +MAChrd
Propagation delay δ

19. Throughput
Packet transmission via RTS/CTS Access mechanism

20. Model Validation

21. Maximum Saturation Throughput
τ depends on n (network size), W, and m. As n is not a directly controllable variable, the only way to achieve optimal performance is to employ adaptive techniques to tune the values m and W
In the standard, the values W and m are hardwired in the PHY layer details, and thus they cannot be made dependent on n

22. Maximum Saturation Throughput
For n sufficiently large
Results to be independent of n

23. Performance Evaluation
Throughput of Basic Access mechanism depends on W
Optimal value of W depends on n
High value of W gives excellent throughput performance in case of 50 contending stations

24. Performance Evaluation
Throughput obtained with RTS/CTS mechanism is almost independent of the value

25. Performance Evaluation
Number of transmissions per packet increases as W reduces & network size n increases.

26. Conclusion
Model suited for both Basic Access and RTS/CTS Access mechanisms
Performance of Basic Access method depends on W and n
Performance is only marginally dependent on the system parameters when the RTS/CTS mechanism is considered
RTS/CTS scheme solved hidden terminal problem