doc.: IEEE /409r0 Submission March 2002 Mathilde BenvenisteSlide 1 Persistence Factors in EDCF Mathilde Benveniste

doc.: IEEE /409r0 Submission March 2002 Mathilde BenvenisteSlide 2 PF is the multiplier used to adjust the contention window after transmission failure, for retrial –The present standard uses binary exponential backoff; i.e. PF=2 -- the contention window doubles with each failed transmission –Now, PF can assume any value Traffic-adaptive contention window (CW) –Before, a fixed contention window was used when a new frame arrived; binary exponential backoff helped open contention window quickly in congestion –With an initial contention window that adapts to traffic, a smaller PF should be used. Benefit –With lower PF values, older packets get a better chance at the channel –Delay and jitter is reduced Different traffic categories –Delay/jitter-sensitive traffic uses smaller PF values, even without good CW adaptation –Lower priority categories use larger PF values in order to adapt to congestion Persistence Factors (PFs)

doc.: IEEE /409r0 Submission March 2002 Mathilde BenvenisteSlide 3 Proposed solutions for delay/jitter reduction and their attributes Persistence Factors (PF - optional feature in TGe D1 draft proposal) –Instead of 2 (binary exponential backoff), AP-specified coefficients, PFs, multiply CW for each TC –PFs are flexible; they can be set at any value, including PF= 2 (i.e. binary exponential backoff) –smaller value for higher priority traffic (e.g., PF 2) –useful when CWmin is adjusted to traffic (reduce delay and jitter) –PFs are optional; values like 0.5, 1, 1.5, and 2 are easy to implement –The persistence factors may remain constant over time (PF need not adapt to traffic) CWMax Method (Proposed in July 2001, 01/390r1 ) –CWmax varies with TC; smaller values for higher priority classes –This is equivalent to setting PF=2 initially. When CW reaches Cwmax(TC), PF=1 (that is, growth stops ) –The correct choice of Cwmax(TC) must adapt to traffic load –If not and Cwmax(TC) is too small, delay and jitter increases (backoff growth stops too soon) –If too large a Cwmax(TC) does not nothing. There is greater delay and jitter since PF=2.  continuous adaptation to traffic is required for Cwmax(TC) –Frequent Cwmax(TC) updates add to channel overhead

doc.: IEEE /409r0 Submission March 2002 Mathilde BenvenisteSlide 4 Comparison of methods

doc.: IEEE /409r0 Submission March 2002 Mathilde BenvenisteSlide 5 Appendix: Simulation Results for class differentiation by PF Traffic 9 Voice calls Kbps per call Fixed arrivals Frame size* bytes 1 Low Priority session- 3,318 Kbps Frame size* - 1,728 bytes Fixed arrivals 12 ms ON/88 ms OFF WLAN Parameters DS, 11 Mbps channel buffer size=2.024 Mbits; no fragmentation RTS/CTS suppressed; max retry limit=7 * Includes 192 microsec PHY overhead Call Duration Call Time (sec)

doc.: IEEE /409r0 Submission March 2002 Mathilde BenvenisteSlide End to End Delay (sec) End to End Delay (sec) TCMA ( AIFS Differentiation only) ) DCF End to End Delay (sec) TCMA with Persistence Factors: (0.5, 2) Average delay by traffic category Simulation Time (sec) Voice Calls Low Priority

doc.: IEEE /409r0 Submission March 2002 Mathilde BenvenisteSlide 7 DCF Delay and jitter for a single call (sec) TCMA ( AIFS Differentiation only) TCMA with Persistence Factors: (0.5, 2) ) Jitter End to End Delay Simulation Time (sec)