Comparing V-DCF with other EDCF proposals

1. Comparing V-DCF with other EDCF proposals Wim Diepstraten Agere Systems Menzo Wentink Intersil Maarten Hoeben Intersil Greg Chesson Atheros Harold Teunissen Lucent Technologies Wim Diepstraten, Agere Systems

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 Wim Diepstraten, Agere Systems

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. Wim Diepstraten, Agere Systems

4. Retry Backoff • The new retry Backoff procedure: • Start exponential backoff by CW doubling after the first retry, on all Q’s, so per station. • 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). Wim Diepstraten, Agere Systems

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. Wim Diepstraten, Agere Systems

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 Q’s 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 Wim Diepstraten, Agere Systems

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. Wim Diepstraten, Agere Systems

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. Wim Diepstraten, Agere Systems

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. Wim Diepstraten, Agere Systems

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 Wim Diepstraten, Agere Systems

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 TC’s 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 Wim Diepstraten, Agere Systems

12. Wim Diepstraten, Agere Systems

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 • G. Bianchi, “Performance Analysis of the IEEE 802.11 Distributed Coordination Function”, IEEE J. Select. Areas Comm., vol 18, March 2000, pp 535-547. • G. Bianchi, “IEEE 802.11 – Saturation Throughput Analysis”, IEEE Comm. Let., vol 2, pp. 318-320, Dec. 1998. • T.S. Ho and K.C. Chen, “Performance Evaluation and Enhancement of the CSMA/CA Protocol for 802.11 Wireless LAN’s”, Proc. IEEE PIMRC, Taipei, Taiwan, Oct. 1996, pp. 392-396. Wim Diepstraten, Agere Systems

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. Wim Diepstraten, Agere Systems

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. Wim Diepstraten, Agere Systems