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Performance Analysis of IEEE 802.11 Distributed Coordination Function (DCF)

Performance Analysis of IEEE 802.11 Distributed Coordination Function (DCF). (By Giuseppe Bianchi, 2000) Presented By: Sumegha Bagga 01/18/08. Outline. Introduction 802.11 DCF Maximum & Saturation Throughput Performance Throughput Analysis Model Validation Maximum Saturation Throughput

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Performance Analysis of IEEE 802.11 Distributed Coordination Function (DCF)

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  1. Performance Analysis of IEEE 802.11 Distributed Coordination Function (DCF) (By Giuseppe Bianchi, 2000) Presented By: Sumegha Bagga 01/18/08

  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 • Describes two techniques • Basic Access Mechanism • RTS/CTS Mechanism • Key Approximation • Constant and Independent collision probability regardless of retransmission

  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 system can carry in stable conditions

  9. Throughput Analysis • Analytical Evaluation of Saturation Throughput • Fixed number of stations having packet for transmission • Analysis divided into two parts: • 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

  10. Throughput Analysis • Two cases • Packet Transmission Probability • Stochastic Process b(t) representing backoff time counter • Stochastic Process s(t) representing backoff stage • Markov Chain model • Throughput

  11. Packet Transmission Probability • Each packet collide with constant and independent probability • Results more accurate as long as W gets larger • Model bi-dimensional process {s(t) , b(t)} with discrete-time Markov chain

  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 randomly chosen slot time

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

  16. Throughput • Normalized system throughput S • Probability of transmission Ptr • Probability of successful transmission Ps

  17. Throughput Normalized system throughput 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 • Compared analytical results with that obtained by means of simulation • Analytical model extremely accurate • Analytical results (lines) coincide with simulation results (symbols) in both Basic Access & RTS/CTS cases Saturation throughput analysis vs. simulation

  21. Maximum Saturation Throughput τ depends on n, W, and m

  22. Maximum Saturation Throughput • Analytical model determines maximum achievable saturation throughput

  23. Performance Evaluation • Greater the network size lower is the throughput for basic access Saturation throughput analysis vs. simulation

  24. Performance Evaluation • Throughput of Basic Access mechanism depends on W • W depends on number of terminals • High value of W gives excellent throughput performance Saturation throughput vs. initial window size for Basic Access mechanism

  25. Performance Evaluation • Throughput obtained with RTS/CTS mechanism • Independent of value of W Saturation throughput vs. initial window size for RTS/CTS mechanism

  26. Performance Evaluation • Number of transmissions per packet increases as W reduces & network size n increases. Average number of transmissions per packet

  27. Conclusion • Model suited for both Basic Access and RTS/CTS Access mechanisms • Evaluated the 802.11 throughput performance • Performance of Basic Access method depends on W and n • RTS/CTS scheme solved hidden terminal problem

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