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1 of 56 Idle Sense: An Optimal Access Method for High Throughput and Fairness in Rate Diverse Wireless LANs Martin HeusseFranck Rosseau Romaric GuillierAndrzej Duda Presented by Nikki Benecke, October 10 th, 2006 for CS577

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2 of 56 Objective: “Define an access method optimized 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.”

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3 of 56 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

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4 of 56 Optimized for throughput and fairness Throughput –Not goodput –Expect to maintain, not increase, with IS Fairness in Idle Sense: –Jain Index –Time Fairness

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5 of 56 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

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6 of 56 Supporting a wide range of error rates Sort of superfluous – handled by dynamic channel adaptation

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7 of 56 Equal time shares Step away from min-max fairness Keep slow senders from unnecessarily restricting fast senders

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8 of 56 Motivation currently requires DCF as the access method –Also allows PCF Idle Sense addresses some key problems with DCF

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9 of 56 DCF: Operation [dcf in a nutshell] Martin Heusse’s presentation at SIGCOMM ‘05

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10 of 56 DCF: Backoff

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11 of 56 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!

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12 of 56 DCF: Physical Layer Capture [physical layer capture] Martin Heusse’s presentation at SIGCOMM ‘05

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13 of 56 Two Modifications to DCF 1.No exponential backoff 2.All senders have equal CW

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14 of 56 Ideal channel contention - number of hosts - probability of one host successfully transmitting - probability of one host attempting to transmit

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15 of 56 Channel contention For a transmission to occur, one host must try to send and all others must be idle, so:

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16 of 56 Channel contention - probability that no hosts are sending - probability of a collision

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17 of 56 Channel contention - average number of consecutive idle slots

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18 of 56 Channel contention Since all CW are equal:

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19 of 56 Channel contention Throughput as a function of :

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20 of 56 Channel contention Goal: maximize the throughput by minimizing the time spent in collisions and contention

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21 of 56 Channel contention Maximizing is equivalent to minimizing defined as:

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22 of 56 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]

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23 of 56 Formulas

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24 of 56 value based on variant This is because it is based on the ts/tc ratio, which varies by flavor of = for b (important for later use)

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25 of 56 Idle Sense: Principles Each host estimates and uses it to compute its CW By adjusting CW, a host makes converge to (common across hosts)

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26 of 56 vs requires knowing N (# of hosts), which we would like to avoid

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27 of 56 vs quickly approaches, though, so we can use this value as with little penalty

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28 of 56 Channel adaptation

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29 of 56 Channel adaptation

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30 of 56 Adaptation example b (nitarget = 5.68) N = 5 CW = 60 = 1.2 = 0.001

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31 of 56 Adaptation example 1.CW = 60 -> = 0.033, Pi = 0.847, = 5.53 Ni < 5.68, so increase CW CW = = 1.2 x 60 = 72 (Multiplicative decrease)

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32 of 56 Adaptation example 2. This increase leads to = > 5.68, so decrease CW CW = CW = 69 (Additive Increase)

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33 of 56 Adaptation example 3.Ni is now 6.41, still greater than So decrease again, CW gets value 67. As this continues, we will oscillate around (Additive Increase)

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34 of 56 Time-fairness Argument: using min-max fairness restricts fast hosts by making them send as slowly as slow hosts

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35 of 56 Time-fairness Using time-fairness eliminates two problematic situations: (i)Slower hosts limit the throughput of faster hosts (ii)Slow hosts suffer starvation because an access point will not switch to a slower rate

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36 of 56 Time-fairness They say TF is better for both slow hosts and fast hosts.

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37 of 56 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.

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38 of 56 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

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39 of 56 Idle Sense: Performance Developed discrete-event simulator that implements DCF and Idle Sense –No source code provided Three variants of –802.11b, g, theoretical 100Mb/s Idle Sense parameters: = for b = 31.0 for g = 19.3 for 100Mb/s

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40 of 56 Performance Simulations run for 10 6 transmissions Experimentally determined parameters = = 1.2 N trans = 5 (see figure 6)

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41 of 56 Throughput Compare b 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

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42 of 56 Throughput [figure 7]

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43 of 56 Throughput

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44 of 56 Throughput: Conclusion Authors expected throughput to stay around that of DCF Throughput actually improves slightly

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45 of 56 Fairness: Jain Index Much better than DCF!

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46 of 56 Delay K = # of intertransmissions between other hosts between two transmissions of a given host For larger K, hosts experience greater delays

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47 of 56 Delay [table 5] Hosts should experience significantly less delay!

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48 of 56 Collision Overhead [table 3] of DCF

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49 of 56 Convergence Speed Start out with 5 greedy hosts, add another 5 at 2000, drop 5 at 3000 Stabilizes quickly after every change

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50 of 56 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

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51 of 56 Time fairness In DCF, all hosts get the slow host’s throughput – tput much lower than in IS!

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52 of 56 Conclusions Throughput equivalent (sometimes better) than in DCF Better short-term fairness Shorter delay Less collision overhead Converges quickly Is time fair/doesn’t cripple fast senders

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53 of 56 Possible weaknesses? No info provided on actual simulation tool/we can’t recreate experiments Some “experimentally derived” parameters How is this actually implemented? What happens if some senders have IS and others don’t? Is time-fairness really “fair”?

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54 of 56 Oddities Figure 9 never referenced in text (second Jain fairness figure)

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55 of 56 Questions/Comments

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56 of 56 Acknowledgements Most figures taken directly from the paper Slide 10 is from the technical specifications (Section 9.2, DCF) Slides 9 & 12 and the “Bad Day” and physical capture ideas are from Martin Heusse’s presentation of Idle Sense at SIGCOMM ‘05

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