1. IEEE Journal on Selected Areas in Communications
Performance Analysis of the IEEE Distributed Coordination Function
Giuseppe Bianchi
IEEE Journal on Selected Areas in Communications
March 2000

2. Outline Introduction 802.11 Distributed Coordination Function
Maximum & Saturation Throughput Performance
Throughput Analysis
Model Validation
Maximum Saturation Throughput
Performance Evaluation
Conclusion

3. Introduction 802.11 Distributed Coordination Function
The fundamental mechanism to access the medium
Based on CSMA/CA
Two techniques
Basic Access Mechanism
RTS/CTS Mechanism

4. 802.11 DCF Two access techniques Basic mechanism: 2 way handshaking
RTS/CTS mechanism: 4 way handshaking
RTS
DATA
CTS
DESt
Source
Dest
Source
DATA
ACK
ACK

5. DCF

6. DCF

7. DCF

8. Maximum and Saturation Throughput Performance
Maximum throughput performance
Saturation throughput performance
Maximum load in stable condition

9. Throughput Analysis Assumption Fixed # of stations
Always having a packet available for transmission
Transmission queues are always nonempty
Two parts of analysis
Study the behavior of single station with a Markov model
Study the events that occur within a generic slot time & expressed throughput for both Basic & RTS/CTS access method
Obtain the stationary probability

10. Throughput Analysis n stations b(t) W = 𝐶𝑊 𝑚𝑖𝑛 ; 𝐶𝑊 𝑚𝑎𝑥 = 2 𝑚 W s(t)
Each station always has a packet available for transmission
b(t)
Stochastic Process representing backoff time counter
W = 𝐶𝑊 𝑚𝑖𝑛 ; 𝐶𝑊 𝑚𝑎𝑥 = 2 𝑚 W
s(t)
Stochastic Process representing backoff stage
(0,m)

11. Throughput Analysis
Each packet collide with constant and independent probability p
Model bi-dimensional process {s(t) , b(t)} with discrete-time Markov chain

12. Markov Chain model

13. Markov Chain model Stationary distribution of the chain
𝑏 𝑖,𝑘 = lim 𝑡→∞ 𝑃{𝑠 𝑡 =𝑖, 𝑏 𝑡 =𝑘}
i ϵ ( 0, m ) , k ϵ ( 0, 𝑊 𝑖 -1 )

14. Markov Chain model 𝑏 𝑖,𝑘 = 𝑊 𝑖 − 𝑘 𝑊 𝑖 𝑏 𝑖,0 1= 𝑖=0 𝑚 𝑘=0 𝑊 𝑖 −1 𝑏 𝑖,𝑘
𝑏 𝑖,𝑘 = 𝑊 𝑖 − 𝑘 𝑊 𝑖 𝑏 𝑖,0
1= 𝑖=0 𝑚 𝑘=0 𝑊 𝑖 −1 𝑏 𝑖,𝑘
𝑏 0,0 = 2(1−2𝑝)(1−𝑝) 1−2𝑝 𝑊+1 +𝑝𝑊(1− (2𝑝) 𝑚 )
Probability τ
a station transmits in randomly chosen slot time

15. Markov Chain model Some note In general, τ depends on p
If m = 0 , 𝜏= 2 𝑊+1
Independent of p
In general, τ depends on p
𝑝=1− (1−𝜏) 𝑛−1

16. Throughput Normalized system throughput S
Probability of transmission 𝑃 𝑡𝑟
At least one transmission in the slot time
𝑃 𝑡𝑟 =1− 1−𝜏 𝑛
Probability of successful transmission 𝑃 𝑠
Transmit successfully
𝑃 𝑠 = 𝑛𝜏 (1−𝜏) 𝑛−1 𝑃 𝑡𝑟 = 𝑛𝜏 (1−𝜏) 𝑛−1 1− (1−𝜏) 𝑛

17. Throughput
𝑆= 𝐸[𝑝𝑎𝑦𝑙𝑜𝑎𝑑 𝑖𝑛𝑓𝑜𝑟𝑚𝑎𝑡𝑖𝑜𝑛 𝑡𝑟𝑎𝑛𝑠𝑚𝑖𝑡𝑡𝑒𝑑 𝑖𝑛 𝑎 𝑠𝑙𝑜𝑡 𝑡𝑖𝑚𝑒] 𝐸[𝑙𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝑎 𝑠𝑙𝑜𝑡 𝑡𝑖𝑚𝑒]
E[P]: average packet payload size
𝑇 𝑠 : average time the channel is sensed busy because of a successful transmission
𝑇 𝑐 : average time the channel is sensed busy by each stationi during a collusion

18. Throughput

19. Throughput

20. Maximum Saturation Throughput
Optimal 𝜏≈ 1 𝑛 𝑇 𝑐 /2𝜎
𝑆 𝑚𝑎𝑥 = 𝐸[𝑃] 𝑇 𝑠 +𝜎𝐾+ 𝑇 𝑐 (𝐾 𝑒 1 𝐾 −1 −1)
K= 𝑇 𝑐 /2𝜎

21. Model Validation

22. Performance Evaluation
Basic
RTS/CTS

23. Performance Evaluation
Basic
RTS/CTS

24. Performance Evaluation

25. Performance Evaluation

26. Performance Evaluation

27. Conclusion Evaluated the 802.11 DCF throughput performance
Model suited for both Basic Access and RTS/CTS Access mechanisms
The model is extremely accurate in predicting the system throughput
Basic Access strongly depends on n and w
RTS/CTS is better in large network scenarios