Idle Sense:

1. Idle Sense: An Optimal Access Method for High Throughput and Fairness in Rate Diverse Wireless LANs Presented by Nikki Benecke, October 10th, 2006 for CS577 Martin Heusse Franck Rosseau Romaric Guillier Andrzej Duda

2. Objective: “Define an access methodoptimized for throughput and fairness, able to dynamically adapt to physical channel conditions, to operate near optimum for a wide range of error rates, and to provide equal time shares when hosts use different bit rates.”

3. Access method • Way of deciding who can access the media at a given time • In WLANs, CSMA/CA as implemented by DCF (required) or PCF

4. Optimized for throughput and fairness • Throughput • Not goodput • Expect to maintain, not increase, with IS • Fairness in Idle Sense: • Jain Index • Time Fairness

5. Dynamic channel adaptation • Physical conditions vary wildly with time • Frames received in error -> sender’s bitrate lowered to reduce error rate • Idle Sense tries to intelligently decide when lowering the bitrate is worthwhile

6. Supporting a wide range of error rates • Sort of superfluous – handled by dynamic channel adaptation

7. Equal time shares • Step away from min-max fairness • Keep slow senders from unnecessarily restricting fast senders

8. Motivation • 802.11 currently requires DCF as the access method • Also allows PCF • Idle Sense addresses some key problems with DCF

9. DCF: Operation • [dcf in a nutshell] Martin Heusse’s presentation at SIGCOMM ‘05

10. DCF: Backoff

11. DCF: “Bad Day” Problem Bad transmission conditions -> host will lose many frames High error rate -> frequent backoffs CW is increased -> transmission attempt probability is lower So the host will try to send less often and may eventually starve!

12. DCF: Physical Layer Capture [physical layer capture] Martin Heusse’s presentation at SIGCOMM ‘05

13. Two Modifications to DCF • No exponential backoff • All senders have equal CW

14. Ideal channel contention - number of hosts - probability of one host successfully transmitting - probability of one host attempting to transmit

15. Channel contention For a transmission to occur, one host must try to send and all others must be idle, so:

16. Channel contention - probability that no hosts are sending - probability of a collision

17. Channel contention - average number of consecutive idle slots

18. Channel contention Since all CW are equal:

19. Channel contention Throughput as a function of :

20. Channel contention Goal: maximize the throughput by minimizing the time spent in collisions and contention

21. Channel contention Maximizing is equivalent to minimizing defined as:

22. Channel contention Define as Take the first derivative of the cost function to get: is the unique solution for this equation that is in [0,1]

23. Formulas

24. value based on variant This is because it is based on the ts/tc ratio, which varies by flavor of 802.11 = 0.1622 for 802.11b (important for later use)

25. Idle Sense: Principles Each host estimates and uses it to compute its CW By adjusting CW, a host makes converge to (common across hosts)

26. vs requires knowing N (# of hosts), which we would like to avoid

27. vs quickly approaches , though, so we can use this value as with little penalty

28. Channel adaptation

29. Channel adaptation

30. Adaptation example 802.11b (nitarget = 5.68) N = 5 CW = 60 = 1.2 = 0.001

31. Adaptation example • CW = 60 -> = 0.033, Pi = 0.847, = 5.53 Ni < 5.68, so increase CW CW = = 1.2 x 60 = 72 (Multiplicative decrease)

32. Adaptation example 2. This increase leads to = 6.71 6.71 > 5.68, so decrease CW CW = CW = 69 (Additive Increase)

33. Adaptation example • Ni is now 6.41, still greater than 5.68. So decrease again, CW gets value 67. As this continues, we will oscillate around (Additive Increase)

34. Time-fairness Argument: using min-max fairness restricts fast hosts by making them send as slowly as slow hosts

35. Time-fairness Using time-fairness eliminates two problematic situations: • Slower hosts limit the throughput of faster hosts • Slow hosts suffer starvation because an access point will not switch to a slower rate

36. Time-fairness They say TF is better for both slow hosts and fast hosts.

37. Time-fairness In Idle Sense, accomplish TF by controlling the access probability for the hosts Slow host transmitting at bit rate rcurr receives a modified CW, CW’ = CW x , so scales down.

38. Miscellaneous • Determining idle slots is easy • Efficiency may be slightly lower for small num. of hosts (N) • Near optimal utilization for certain ratios of -- real traffic may behave differently and have lower utilization

39. Idle Sense: Performance • Developed discrete-event simulator that implements 802.11 DCF and Idle Sense • No source code provided • Three variants of 802.11 • 802.11b, 802.11g, theoretical 100Mb/s 802.11 • Idle Sense parameters: = 68.17 for 802.11b = 31.0 for 802.11g = 19.3 for 100Mb/s 802.11

40. Performance • Simulations run for 106 transmissions • Experimentally determined parameters = 0.001 = 1.2 Ntrans = 5 (see figure 6)

41. Throughput Compare 802.11b DCF, Slow Decrease, Asymptotically Optimal Backoff (AOB) and Idle Sense for an increasing number of hosts Throughput is the average of the throughput for all hosts active in the network

42. Throughput • [figure 7]

43. Throughput

44. Throughput: Conclusion • Authors expected throughput to stay around that of DCF • Throughput actually improves slightly

45. Fairness: Jain Index Much better than DCF!

46. Delay K = # of intertransmissions between other hosts between two transmissions of a given host For larger K, hosts experience greater delays

47. Delay [table 5] Hosts should experience significantly less delay!

48. Collision Overhead • [table 3] of DCF

49. Convergence Speed Start out with 5 greedy hosts, add another 5 at 2000, drop 5 at 3000 Stabilizes quickly after every change

50. Time fairness • One 1Mb/s host and N-1 11Mb/s hosts • By TF, fast hosts should get 11 x the throughput of the slow host in Idle Sense