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By Muhamad Khaled Alhamwi 260212

COE-541 Research Presentation Saturation Throughput Analysis for Different Backoff Algorithms in IEEE802.11. By Muhamad Khaled Alhamwi 260212. Outline. Introduction Backoff Algorithms Markov Models Analysis Algorithms Throughput Simulation Results Conclusions References Q & A.

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By Muhamad Khaled Alhamwi 260212

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  1. COE-541 Research PresentationSaturation Throughput Analysis for Different Backoff Algorithms in IEEE802.11 By Muhamad Khaled Alhamwi 260212

  2. Outline • Introduction • Backoff Algorithms • Markov Models • Analysis • Algorithms • Throughput • Simulation Results • Conclusions • References • Q & A

  3. Introduction • Backoff periods are used to minimize collision by deferring transmission in CSMA protocols • Different existing backoff algorithms • BEB • EIED • EILD

  4. BEB Algorithm (1) • Binary Exponential Backoff algorithm: • Transmit using CSMA/CA protocol • If transmission was unsuccessful • Double the backoff window • Otherwise (successful) • Reset the window to its minimum value

  5. BEB Algorithm (2) • This aggressive reduction in backoff period can result in more collisions • After successful transmission more stations will try to transmit • Higher probability of collision

  6. EIED Algorithm • Exponential Increase Exponential Decrease algorithm • Transmit using CSMA/CA protocol • If transmission was unsuccessful • Double the backcoff window • Otherwise (successful) • Halve the backoff window

  7. EILD Algorithm • Exponential Increase Linear Decrease algorithm • Transmit using CSMA/CA protocol • If transmission was unsuccessful • Double the backcoff window • Otherwise (successful) • Subtract one from the backoff window

  8. Markov Model of BEB • P is the probability of collision • Assumed constant (does not depend on the state) • 1-P is the probability of successful transmission • Wi (W0 to Wm) is the backoff window size • Assuming NO limit on retransmission trials

  9. Markov Model of EIED (1) • Double when unsuccessful • ri = 2 • Halve when successful • rd = 2

  10. Markov Model of EIED (2) • Multiply by 4 when unsuccessful • ri = 4 • Halve when successful • rd = 2

  11. Markov Model of EILD • Double when unsuccessful • Subtract one when successful • W is the minimum Window size = W0 • (20+21+22+… +2m-1)W+1 = 2mW-W+1 states

  12. Analysis Approach • Markov Model • Steady-state probabilities • Transmission probability • Success probability • Collision probability • Throughput

  13. BEB Analysis (unlimited retransmission) • Steady-state probability • Solve for q0, we get

  14. BEB Analysis, cont’ • The steady-state probabilities can be expressed by • The average number of slots E[Z] spent in each state between transitions, averaged over all states is given by

  15. BEB Analysis, cont’ • Since only one slot is used for transmission between state transitions, the probability of backlogged station to transmit in a random slot is given by • This is the same result obtained by [2] that uses 2-D Markov model

  16. BEB Analysis (limited ret’) • Markov model for limited retransmission • Maximum number of transmissions per packet is M+1 • All states (Wm,i) have the same maximum deference time of 2mW-1

  17. BEB Analysis (limited ret’), cont • Steady-state probabilities are given by • Solving for q0, we get

  18. EIED Analysis (ri=2, rd=2) • Similarly, steady-state probabilities qi: • Solving for q0, we get • Transmission probability in a random slot

  19. EILD Analysis • Steady-state probabilities

  20. Saturation Throughput Analysis • At steady-state, each transmission sees • τ*(P) is continuous and monotone increasing function, τ*(0)=0, and τ*(1)=1 • For BEB case, τ is given by (monotone decreasing), τ(0) > τ*(0), and τ(1) < τ*(1)

  21. Saturation Throughput Analysis, cont’ • Solve for P, obtaining P* and τ*= τ(P*) • Where ‘cycle’ is the time between two consecutive ends of DIFS/EIFS

  22. Saturation Throughput Analysis, cont’ • Probability of successful transmission in a cycle is a probability of one station transmitting given that one transmitted in a slot • Where n is number of backlogged stations • Transmission cycle • Idle (backoff) period following DIFS/EIFS • Busy period (one or more transmissions), and followed by • SIFS, ACK, and DIFS in case of success • EIFS in case of collision

  23. Saturation Throughput Analysis, cont’ • Idle period length is a product of a geometric random variable and the slot length • Busy Period length for basic mode (ignore propagation delay • Different values of Ts, and Tc for RTS/CTS mode

  24. Saturation Throughput Analysis, cont’ • Throughput is given by • Where X is either • Basic Access Mode • RTS/CTS Mode

  25. Saturation Throughput (Basic Mode)

  26. Saturation Throughput (RTS/CTS)

  27. Average Maximum Backoff Window

  28. Conclusions • New Markov chain models were used to analyze BEB, EIED, EILD algorithms • EIED can provide a slight improvement over BEB in a small network and using basic access mode • EILD can provide significant improvement in a large network • The algorithms provide only slight improvement for RTS/CTS mode

  29. References • [1] Vukovic, I.N.; Smavatkul, N., “Saturation throughput analysis of different backoff algorithms in IEEE802.11,” Personal, Indoor and Mobile Radio Communications, 2004. PIMRC 2004. 15th IEEE International Symposium on , vol.3, no., pp. 1870-1875 Vol.3, 5-8 Sept. 2004 • [2] G. Bianchi, “Performance Analysis of The IEEE802.11 Distributed Coordination Function”, IEEE Journal on Selected Areas in Communications, pp. 535-547, Vol. 18, March 2000.

  30. Thank you • Q & A

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