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doc.: IEEE 802.11-01/011r1 Submission January, 2001 Wim Diepstraten, Agere Systems Slide 1 Comparing V-DCF with other EDCF proposals Wim DiepstratenAgere Systems Menzo WentinkIntersil Maarten Hoeben Intersil Greg ChessonAtheros Harold TeunissenLucent Technologies

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doc.: IEEE 802.11-01/011r1 Submission January, 2001 Wim Diepstraten, Agere Systems Slide 2 Agenda Discuss new Retry Backoff approach in V-DCF. Main Characteristics of the proposals –where they are similar –where they are different Comparison of V-DCF with P-DCF –delay variance comparison Comparison of V-DCF with TCMA Conclusion

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doc.: IEEE 802.11-01/011r1 Submission January, 2001 Wim Diepstraten, Agere Systems Slide 3 New Retry Backoff for V-DCF Based on simulation and further analyses we came to the conclusion to change our Retry backoff approach, to better work in a QoS environment. Target functionality is: –We need a distributed mechanism that can stabilize the throughput for high contention situations, preventing collision avelange effects when the load and number of contenders increase. –An other function of the Retry backoff mechanism is to circumvent repeated interference situations at the receiver caused by hidden stations contending or collisions with overlapping BSS traffic. Simulation analyses show that the pure exponential backoff behavior is too aggressive for contention stabilization, while its characteristics are ok for the hidden station case. Therefore the Retry Backoff procedure is slightly modified.

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doc.: IEEE 802.11-01/011r1 Submission January, 2001 Wim Diepstraten, Agere Systems Slide 4 Retry Backoff The new retry Backoff procedure: –Start exponential backoff by CW doubling after the first retry, on all Qs, so per station. In an AP CW doubling per Q is more effective.In an AP CW doubling per Q is more effective. This decreases the effect of exponential backoff on delay performance. –The probability of a double retry is the square of the individual collision probability. –So for situations where the collision probability is in the 5-10% range, the probability for a double retry is in the.25-1% range, which is significantly lower. If traffic in the next Q is likely going to the same destination then the CW doubling should apply per station, else it is more effective to apply CW doubling only for the Q experiencing the Retry (AP).

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doc.: IEEE 802.11-01/011r1 Submission January, 2001 Wim Diepstraten, Agere Systems Slide 5 Main characteristics There are a lot of similarities among the proposals. –They all are based on CSMA/CA using contention resolution mechanisms based on random access procedures after a busy medium becomes available again. –Main Priority differentiation mechanism is to force differentiation in average access delay. And / or controlling the idle time after contention resolution is started. –They all assume a Differentiation Control feedback mechanism that is targeted to increase the delay differentiation between priority levels when the load increases. However TCMA seems to favor a local rather then centralized feedback approach. Roughly equivalent behavior can be achieved by the different mechanisms by using a equivalent set of control parameters. There seems to be agreement about the fairness criteria between the proposals (Also TCMA??) This paper intends to focus on the main differences in characteristics. –Mainly between V-DCF and P-DCF –Because TCMA is based on / combined with either of the two mechanisms.

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doc.: IEEE 802.11-01/011r1 Submission January, 2001 Wim Diepstraten, Agere Systems Slide 6 Probability Distribution Effect of different probability distributions: –V-DCF uses a uniform distributed backoff mechanism resulting in a low backoff variance gives higher priority to older frames which have already counted down part of their backoff. allows for immediate access if CCA>DIFS, which is advantageous for high priority frames which Qs are not backlogged. –P-DCF uses a geometric access distribution mechanism resulting in high backoff variance (which is undesirable for QoS) is memoryless, so does not favor frames that are already backing off for some time. –frequent updates makes this worse Frames always go through backoff (no immediate access). Conclusion: Significant difference in backoff variance

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doc.: IEEE 802.11-01/011r1 Submission January, 2001 Wim Diepstraten, Agere Systems Slide 7 Stability Control / Retry Approach Stability control mechanisms. –V-DCF uses an exponential backoff behavior after the first retry. Its stability control mechanism is fully distributed, and does not depend on a centralized congestion control mechanism. It prevents for collision avalanches due to congestion. And is targeted to avoid effect of hidden nodes and overlap interference, by avoiding overlap with the hidden message. –Relates to interference situation at the receiver not visible to the transmitter –P-DCF stability control mechanisms are not clear. Either an autonomous decrease of the PPC based on retry event Or a centralized control mechanism to control the PPC. –If this is the main mechanism then it does fully depend on a centralized entity for stability. Conclusion: A stability control mechanism is needed that is fully distributed and does not depend on other stations.

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doc.: IEEE 802.11-01/011r1 Submission January, 2001 Wim Diepstraten, Agere Systems Slide 8 Complexity Considerations V-DCF: –V-DCF is very similar to the legacy DCF approach. Can be implemented as n*DCF (parallel DCFs). Or a Scheduler function combined with a Delta backoff DCF as presented during the Tampa meeting, and described in doc 00/399 –This scheduler is also effective to select candidate for CF-polled TxOp. –Minimal complexity increase compared to plain DCF. –Roughly the same number of computations per frame needed as in legacy DCF. P-DCF: –In its simplest form it requires a random number generation per slot This is a major computational increase for FW based solutions. Conclusion: V-DCF approach is less computation intensive.

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doc.: IEEE 802.11-01/011r1 Submission January, 2001 Wim Diepstraten, Agere Systems Slide 9 P-DCF Motivation? What is the motivation that justifies the change to P-DCF? –Properties that are claimed to be important: Better analyses possible (memoryless, no exponential backoff) Faster update from one parameter set to another allows better optimization control No internal collisions Better performance due to faster control How different are they and how important are these factors? –It is our opinion that these factors are insignificant, as the controllability of both approaches are equivalent. –A disadvantage of P-DCF is the large variance of the geometric distribution.

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doc.: IEEE 802.11-01/011r1 Submission January, 2001 Wim Diepstraten, Agere Systems Slide 10 Backoff Jitter Analyses The following figures show the difference in backoff jitter between the uniform and geometric distribution. –Clearly jitter is not desirable for QoS. Contention Offset (CO) and CW provide separation of functions –CO provides for differentiation between priorities CO with geometric backoff will break the memoryless property, as will UAT differentiation –CW provides randomness to account for presence of other contenders CO allows the CW to be as small as necessary, which will reduce the delay variance VDCF introduces randomness only when necessary –Important for low jitter –In general, less variance increases resource usage efficiency

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doc.: IEEE 802.11-01/011r1 Submission January, 2001 Wim Diepstraten, Agere Systems Slide 11 Backoff Jitter Simulations The backoff is calculated with different control parameters per priority level (two TC system example). –Top priority example CO=0, CW=15 equivalent to PP=2/17 The high priorities have the same average backoff of 7.5 slots, but the geometric process has a significantly higher backoff variance –The next priority level use CO=10, CW=15 equivalent to PP=2/37 Again same average backoff of 17.5 slots, but but the geometric backoff variance has incresed even more –The backoff variance is the same for both TCs in V-DCF, because they use the same CW In general, lower priorities will have higher backoff jitter in P-DCF The backoff variance will increase the frame delivery variance when both processes have the same collision rate

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doc.: IEEE 802.11-01/011r1 Submission January, 2001 Wim Diepstraten, Agere Systems Slide 12

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doc.: IEEE 802.11-01/011r1 Submission January, 2001 Wim Diepstraten, Agere Systems Slide 13 V-DCF Throughput Optimisation V-DCF throughput can be optimized by dynamically adapting CW based on load measurements. This is similar to P-DCF. Steady-state V-DCF analysis is straightforward when no exponential backoff is considered (see references below) –The steady-state transmission probability is P(transmission)=2/(CW+1), when queues are always backlogged (see ref. 1) –The optimization algorithm introduced by Jin-Meng Ho in paper 00/467, which equalizes collision time and idle time, can be applied to V-DCF by taking CW=2/PP-1 (or CWi=2/TCPP-1 when more queues are present) Selection of Papers 1.G. Bianchi, Performance Analysis of the IEEE 802.11 Distributed Coordination Function, IEEE J. Select. Areas Comm., vol 18, March 2000, pp 535-547. 2.G. Bianchi, IEEE 802.11 – Saturation Throughput Analysis, IEEE Comm. Let., vol 2, pp. 318-320, Dec. 1998. 3.T.S. Ho and K.C. Chen, Performance Evaluation and Enhancement of the CSMA/CA Protocol for 802.11 Wireless LANs, Proc. IEEE PIMRC, Taipei, Taiwan, Oct. 1996, pp. 392-396.

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doc.: IEEE 802.11-01/011r1 Submission January, 2001 Wim Diepstraten, Agere Systems Slide 14 TCMA comparison The only difference of TCMA as suggested is to use PIFS for high priority and DIFS for other priorities. We think that this is NOT a good idea, as the PIFS can better be reserved for the AP. –Giving AP a slight EDCF access advantage by starting its backoff downcount at PIFS rather then DIFS. –While the PIFS access priority can also be used during Burst access as in the V-DCF and HCF proposals. It is unclear how the TCMA approach which claims that it does NOT like Retry backoff increase is assuring a stable throughput. –Further a centralized monitoring feedback is assumed to perform better then individual station monitoring: –Because station does not see all traffic in BSS, and it can lead to unequal priority differentiation within a BSS.

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doc.: IEEE 802.11-01/011r1 Submission January, 2001 Wim Diepstraten, Agere Systems Slide 15 Conclusion V-DCF has better QoS properties then P-DCF. –V-DCF has lower backoff variance then P-DCF, particularly in combination with a contention offset –In V-DCF, frames that are in backoff for a while will have higher relative priorities then new arrivals –Stability of Retry Backoff is achieved in a distributed way without the need for a centralized control entity for congestion control. The new Retry Backoff approach is better suited for QoS than exponential backoff –V-DCF has lower implementation complexity –V-DCF is very similar to the legacy DCF approach that everybody is familiar with. –There is no clear motivation to change to a P-DCF approach. –The PIFS tier can better be used to give an AP preferential access rather then for the Top priority, to prevent breaking (HCF like) bursting assumptions. Conclusion: –We should adopt the V-DCF for inclusion in the baseline proposal.

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