EDCA Backoff Rules Mathilde Benveniste benveniste@ieee.org doc.: IEEE 802.11-00/457 March 2004 EDCA Backoff Rules Mathilde Benveniste benveniste@ieee.org Avaya Labs - Research Mathilde Benveniste, Avaya - Research

The D8.0 backoff rules diminish the potential effectiveness of EDCF March 2004 INTRODUCTION The current TGe draft, D8.0, contains backoff rules that are different from the existing standard, 802.11-1999 The D8.0 backoff rules diminish the potential effectiveness of EDCF A legacy station performs better than EDCF best-effort (EDCF BE) stations This provides disincentive to replace/retire legacy stations The presence of legacy stations in a QBSS severely limits the potential QoS performance of 802.11e stations A simple remedy exists Adopt backoff rules of TGe D3.0; they are the same as 802.11-1999 Mathilde Benveniste, Avaya - Research

The original AIFS 1 – also in D3.0 March 2004 The original AIFS 1 – also in D3.0 The original proposal Used the same backoff countdown rules as in 802.11-1999 Generated an access category exactly equivalent to legacy (more details appear in the Appendix I ) AIFS (Arbitration inter-frame space) = a priority-dependent IFS is the deferral time for access & backoff countdown, used for QoS differentiation aAIFS = AIFS slot count (lowest value for stations is 1) Example: If aAIFS[1] = 1 (for the top-priority classes), AIFS[1] = PIFS If aAIFS[0] = 2 (for legacy-equivalent behavior), AIFS[0] = DIFS ____________________________________________ 1 Called the UAT in the TCMA proposal; see -00/375; -00/456; -00/457; -01/002; -01/004; -01/019; -01/117; -01/144) For stations with AC= 0,1,… AIFS[AC] = aSIFSTime + aAIFS[AC] x aSlotTime Mathilde Benveniste, Avaya - Research

D8.0 modifies AIFS and uses new Backoff Rules March 2004 D8.0 modifies AIFS and uses new Backoff Rules AIFS* is a modification of the original AIFS aAIFSN = AIFS slot count + 1 (lowest possible value for stations is 2) New backoff timing rules are introduced, which are different from the existing 802.11-1999 rules On slot boundaries, the station will perform only one of the following functions: Count down backoff or Transmit if backoff is 0 Backoff is drawn from the range [0, CW[AC]] For stations with AC= 0,1,… AIFS*[AC] = aSIFSTime + aAIFN[AC] x aSlotTime Mathilde Benveniste, Avaya - Research

March 2004 Problems with D8.0 In the presence of legacy stations, the performance of EDCF(BE) is worse than legacy Mathilde Benveniste, Avaya - Research

For D8.0, Minimum AIFSN = 2 So far so good! Medium Busy HC transmits March 2004 For D8.0, Minimum AIFSN = 2 D8.0 Rules: If AIFSN[AC] is 2 for EDCF ACs=3, …, collisions with the HC are avoided HC transmits after a PIFS idle PIFS SIFS Medium Busy HC transmits AIFS for AIFSN=2 EDCF[3] with backoff = 0 transmits after a DIFS idle PIFS PIFS SIFS SIFS Medium Busy HC transmits EDCF[3] transmits Slot time So far so good! Mathilde Benveniste, Avaya - Research

Legacy vs D8.0 EDCF behavior when Backoff =0 March 2004 Legacy vs D8.0 EDCF behavior when Backoff =0 Transmit after DIFS idle DIFS PIFS SIFS Lowest Legacy backoff = 0 Busy Legacy] transmits AIFS for AIFSN=2 Transmit after DIFS +1 idle AIFS for AIFSN=3 DIFS DIFS PIFS PIFS SIFS SIFS Lowest EDCF[0] backoff = 0 Busy Busy EDCF[0] transmits Slot time EDCF BE transmits after Legacy. Not good! Mathilde Benveniste, Avaya - Research

Legacy vs D8.0 EDCF behavior when Backoff =1 March 2004 Legacy vs D8.0 EDCF behavior when Backoff =1 Countdown after DIFS +1 idle and transmit DIFS SIFS PIFS Next Legacy backoff = 1 Busy Legacy] transmits Countdown after DIFS +1 idle Transmit after DIFS +1 idle AIFS for AIFSN=3 DIFS DIFS PIFS PIFS SIFS SIFS Next EDCF[0] backoff = 1 Busy Busy EDCF[0] transmits Slot time EDCF BE transmits after Legacy. Not good! Mathilde Benveniste, Avaya - Research

Legacy vs D8.0 EDCF behavior when Backoff =2 March 2004 Countdown after DIFS +1 idle and transmit DIFS DIFS PIFS PIFS SIFS Next Legacy backoff = 2 SIFS Busy Legacy] transmits Busy Transmit after DIFS +1 idle Countdown after DIFS +1 idle AIFS for AIFSN=3 DIFS DIFS DIFS PIFS PIFS PIFS SIFS SIFS SIFS Next EDCF[0] backoff = 2 Busy Busy EDCF[0] transmits Busy Slot time EDCF BE transmits after Legacy. Not good! Mathilde Benveniste, Avaya - Research

Are there any remedies for this problem? March 2004 Are there any remedies for this problem? It has been suggested that adjustment of QoS parameters might remedy the problem Reducing AIFSN for EDCF BE would improve its performance Reducing the AIFSN for EDCF BE, however, causes either collisions with the HC (if the AIFSN for top priority is also reduced), or eliminates prioritized access Reducing contention window for EDCF BE would lead to smaller backoff values Smaller backoff values can increase collision rate with heavy traffic There is no fix for the problem Mathilde Benveniste, Avaya - Research

Adopt EDCF backoff procedure in D3.0 (described on Slide 3) March 2004 Proposed Solution Adopt EDCF backoff procedure in D3.0 (described on Slide 3) Mathilde Benveniste, Avaya - Research

Avoiding collisions with HCs in D3.0 (original proposal) March 2004 Avoiding collisions with HCs in D3.0 (original proposal) To avoid collisions of stations with HCs, the random backoff must be 1 or greater, for classes i with AIFS[AC]=PIFS To this end, The backoff value is adjusted by a binary constant X, whose value depends on the priority class This is equivalent to modifying the contention window as follows: the backoff value is a random integer drawn from the range Backoff Time = (Random() + X) * aSlotTime where X = 1 for QSTAs with AIFS[AC]=PIFS X = 0 for QSTAs with AIFS[AC]>PIFS [1,CW [AC]+1] for QSTAs with AIFS[AC]=PIFS [0,CW[AC] ] for QSTAs with AIFS[AC]>PIFS Mathilde Benveniste, Avaya - Research

Inter-frame Spaces and EDCF Backoff Rules same as in 802.11-1999 March 2004 Inter-frame Spaces and EDCF Backoff Rules same as in 802.11-1999 New Frame arrives DIFS/AIFS PIFS/AIFS Backoff Delay SIFS Medium Busy Transmit Frame Slot time Earliest Tx time for HCs is PIFS Earliest Tx time for QSTAs with aAIFS = 1 (backoff=1)* Earliest Tx time for QSTAs with aAIFS = 2 (backoff=1) ___________________________________ * Backoff of 0 not allowed when aAIFS = 1 (i.e., when AIFS=PIFS), per slide 12 Mathilde Benveniste, Avaya - Research

For D3.0, Minimum aAIFS = 1 Good! HC goes first. March 2004 For D3.0, Minimum aAIFS = 1 D3.0 Rules: If aAIFS[AC] is 1 for EDCF ACs=3, …, collisions with the HCs are avoided, given that the smallest backoff = 1; i.e. given that backoff in range [1, CW[AC]+1] HC transmits after a PIFS idle PIFS SIFS Busy Medium HC transmits AIFS for aAIFS=1 EDCF[3] lowest possible backoff = 1; it transmits after a DIFS idle PIFS PIFS SIFS SIFS Busy Medium HC transmits EDCF[3] transmits Slot time Good! HC goes first. Mathilde Benveniste, Avaya - Research

Legacy vs D3.0 EDCF behavior when Backoff =0 March 2004 Transmit after DIFS idle DIFS PIFS SIFS Lowest Legacy backoff = 0 Medium Busy Legacy] transmits AIFS for aAIFS=2 DIFS PIFS SIFS Lowest EDCF[0] backoff = 0 Medium Busy EDCF[0] transmits Slot time EDCF BE transmits the same as Legacy. Good! Mathilde Benveniste, Avaya - Research

Legacy vs D3.0 EDCF behavior when Backoff =1 March 2004 Countdown after DIFS +1 idle and transmit DIFS SIFS PIFS Next Legacy backoff = 1 Medium Busy Legacy] transmits AIFS for aAIFS=2 DIFS SIFS PIFS Next EDCF[0] backoff = 1 Medium Busy EDCF[0] transmits Slot time EDCF BE transmits the same as Legacy. Good! Mathilde Benveniste, Avaya - Research

Legacy vs D3.0 EDCF behavior when Backoff =2 March 2004 Countdown after DIFS +1 idle and transmit DIFS DIFS PIFS PIFS SIFS SIFS Next Legacy backoff = 2 Busy Legacy] transmits Busy AIFS for aAIFS=2 DIFS DIFS PIFS PIFS SIFS SIFS Next EDCF[0] backoff = 2 Busy Busy EDCF[0] transmits Slot time EDCF BE transmits same as Legacy. Good! Mathilde Benveniste, Avaya - Research

D3.0 results in EDCF BE behavior that is identical to legacy March 2004 Results from D3.0 D3.0 results in EDCF BE behavior that is identical to legacy Mathilde Benveniste, Avaya - Research

D8.0 provides incentive to continue using the legacy protocol! March 2004 Simulation Results D8.0 provides incentive to continue using the legacy protocol! Mathilde Benveniste, Avaya - Research

D8.0 provides incentive to use legacy protocol for BE! March 2004 Wireless VoIP softphone EDCF[BE] VoIP softphone Legacy D8.0 provides incentive to use legacy protocol for BE! Mathilde Benveniste, Avaya - Research

Best Effort Traffic Parameters March 2004 Best Effort Traffic Parameters Mathilde Benveniste, Avaya - Research

Softphone Traffic Parameters March 2004 Softphone Traffic Parameters Mathilde Benveniste, Avaya - Research

Why should EDCF perform better than legacy? March 2004 Why should EDCF perform better than legacy? D8.0 causes EDCF BE to perform worse than legacy stations This offers incentive for BE applications to use the legacy DCF protocol (802.11-1999)  Discourages retirement of the legacy protocol EDCF performs better in the absence of legacy (see next slide) With legacy stations, all priority EDCF classes must use a single AIFS value Without legacy stations, AIFS can assume a range of different values to get best priority differentiation  Potential QoS performance of EDCF is diminished in the presence of legacy Mathilde Benveniste, Avaya - Research

Stronger EDCF priority differentiation without legacy stations March 2004 Stronger EDCF priority differentiation without legacy stations (Illustration uses D3.0 AIFS definition. For more details, see slides 13 and 14..) The presence of legacy stations requires aAIFS=2 for EDCF BE category All higher-priority EDCF categories must have aAIFS=1  Weak priority differentiation In the absence of legacy, each EDCF class can have a different aAIFS value  Strong priority differentiation AIFS[j] AIFS[i] DIFS/AIFS PIFS Backoff Delay SIFS Medium Busy Next Frame Slot time Earliest Tx time for QSTAs with aAIFS = 2 (backoff=1) Earliest Tx time for HCs is PIFS Earliest Tx time for QSTAs with aAIFS = 1 (backoff=1) Earliest Tx time for QSTAs with aAIFS = 3 (backoff=1) Retirement of legacy is desirable! Earliest Tx time for QSTAs with aAIFS = 7 (backoff=1) Mathilde Benveniste, Avaya - Research

doc.: IEEE 802.11-00/457 March 2004 Conclusions D8.0 introduces greater implementation complexity by introducing new & different backoff rules Unlike D3.0, D8.0 places EDCF best effort traffic at a disadvantage w.r.t. legacy stations D8.0 provides disincentive to use EDCF for BE applications; thus delays retirement of legacy DCF By prolonging the presence of legacy stations, D8.0 reduces potential effectiveness of EDCF Mathilde Benveniste, Avaya - Research

March 2004 Appendix I History Mathilde Benveniste, Avaya - Research

Evolution of the Backoff Rules in TGe March 2004 Evolution of the Backoff Rules in TGe D1.0 (which is cont’ed in D2.0) was voted out of the draft after extensive technical discussions and with support over 75% The backoff rules in D1.0 were introduced without any technical discussion It was shown (01/408) that the D1.0 method causes EDCF stations to perform worse than legacy stations when sending Best Effort traffic D3.0 contains correct backoff rules that are the same as those used in legacy stations (802.11-1999) D4.0 uses different backoff countdown rules from those used in legacy stations (802.11-1999) D4.0 was introduced without any technical discussion It was shown (03/149r1) that the D4.0 method causes EDCF stations to perform worse than legacy stations when sending Best Effort traffic D5.0 (which is cont’ed in D6.0, D7.0, and D8.0) results in identical behavior as D4.0, using a different description Mathilde Benveniste, Avaya - Research