AIFS – Revisited Mathilde Benveniste benveniste@ieee.org doc.: IEEE 802.11-00/457 February 2003 AIFS – Revisited Mathilde Benveniste benveniste@ieee.org Avaya Labs - Research Submission addresses D4.0 ballot comments: 11, 12, 15, 153, 374, 412, 413, 642, 643, 991, 1796 Mathilde Benveniste, Avaya - Research

We explain the problems in D4.0 February 2003 INTRODUCTION D4.0 uses backoff countdown rules that are different from those in 802.11-1999 (used in legacy stations) The backoff countdown rules in D4.0 are the same as in D1.0 (cont’ed in D2.0), which was voted out of the draft following extensive technical discussion The method in D3.0 was voted in with more than 75% support The method in both D1.0 and D4.0 was introduced without any technical discussion It has been shown that the D1.0 method causes ‘Best Effort’ (BE) EDCF stations to perform worse than legacy stations (01/408) We explain the problems in D4.0 Mathilde Benveniste, Avaya - Research

The original AIFS – also in D3.0* February 2003 The original AIFS – also in D3.0* AIFS (Arbitration-time inter-frame space) is the deferral time for access & backoff countdown used for QoS differentiation AIFS uses the same backoff countdown rules as in 802.11-1999, for DIFS AIFS is a priority-dependent IFS The original proposal (details given in the Appendix) generates an access category exactly equivalent to legacy allows for priority classes ____________________________________________ *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 classification i= 0,1,… AIFSi = aSIFSTime + aAIFSi x aSlotTime where aAIFSi is the AIFS slot count for class i Mathilde Benveniste, Avaya - Research

February 2003 D4.0 Modified AIFS AIFSD (which appears in D4.0) is a modification of the original AIFS AIFSD[AC] = AIFSN[AC] × aSlotTime + aSIFSTime (changed notation for clarity; in D4.0 the above appears as follows: AIFSD[AC] = AIFS[AC] × aSlotTime + aSIFSTime Minor editorial changes appear in D4.1 and D4.2) An EDCF TXOP is granted when the channel is idle, and has been idle for a time greater than or equal to AIFSD[AC] + aSlotTime and the backoff has expired Backoff is drawn from the range [1, CW[AC]+1] AIFSN assumes values from 0, …, 10 New backoff timing rules are introduced; different from the existing 802.11-1999 rules Mathilde Benveniste, Avaya - Research

The timing relationships are different in 802.11-1999 and D4.0 February 2003 The timing relationships are different in 802.11-1999 and D4.0 802.11-1999 decrements backoff D4.0 Original AIFS proposal & D3.0 decrement backoff Figure 62.2 – EDCF Timing Relationships Mathilde Benveniste, Avaya - Research

Problems with D4.0 We show how February 2003 Problems with D4.0 We show how D4.0 can cause EDCF stations to collide with the HC D4.0 puts EDCF[0], BE traffic using EDCF, at a disadvantage relative to legacy stations Mathilde Benveniste, Avaya - Research

February 2003 D4.0 Rules: Allow AIFSN[AC] to be 0 This causes EDCF stations to collide with own and neighbor HC HC transmits after a PIFS idle PIFS SIFS Medium Busy HC transmits AIFSD for AIFSN=0 EDCF[3] with backoff = 1 transmits after a PIFS idle! PIFS SIFS Medium Busy EDCF[3] transmits Slot time Not good! Mathilde Benveniste, Avaya - Research

February 2003 D4.0 Rules: If AIFSN[AC] is 1 for EDCF ACs=3, 2, 1, as in D3.0, collisions with the HC are avoided HC transmits after a PIFS idle PIFS SIFS Medium Busy HC transmits AIFSD for AIFSN=1 EDCF[3] with backoff = 1 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 and EDCF BE transmit at the same time. So far so good! February 2003 D4.0 Rules: If AIFSN[0] is set to 2, as in D3.0, priority access is possible, and EDCF[0] (BE) behaves like legacy DCF, given the same backoff value Example: backoff = 1 DIFS PIFS Legacy backoff = 1 SIFS Medium Busy Legacy transmits AIFSD for AIFSN=2 DIFS PIFS SIFS EDCF[0] backoff = 1 Medium Busy EDCF[0] transmits Slot time Legacy and EDCF BE transmit at the same time. So far so good! Mathilde Benveniste, Avaya - Research

February 2003 D4.0 Rules: Backoff is drawn from the range [1, CW[AC]+1] Different backoff range used by legacy stations Legacy stations have a lower backoff than ECDF[0] stations We compare legacy and EDCF[0] performance for the end cases … (0, WC) Legacy Backoff delay pdf … (1, WC+1) EDCF[0] Backoff delay pdf Mathilde Benveniste, Avaya - Research

D4.0 Rules: Comparing the lowest possible backoff values February 2003 D4.0 Rules: Comparing the lowest possible backoff values DIFS PIFS SIFS Lowest Legacy backoff = 0 Medium Busy Legacy transmits AIFSD for AIFSN=2 DIFS DIFS PIFS PIFS SIFS Lowest EDCF[0] backoff = 1 SIFS Medium Busy Legacy transmits EDCF[0] transmits Slot time EDCF BE transmits after Legacy. Not good! Mathilde Benveniste, Avaya - Research

D4.0 Rules: Comparing the highest possible backoff values February 2003 D4.0 Rules: Comparing the highest possible backoff values DIFS PIFS SIFS Nslots Highest Legacy backoff = N Medium Busy Legacy transmits ≀≀ AIFSD for AIFSN=2 DIFS DIFS PIFS PIFS SIFS N slots Highest EDCF[0] backoff = N+1 SIFS Medium Busy ≀≀ Legacy transmits EDCF[0] transmits Slot time EDCF BE transmits after Legacy. Not good! Mathilde Benveniste, Avaya - Research

D4.0 provides incentive to continue using the legacy protocol! February 2003 Simulation Results D4.0 provides incentive to continue using the legacy protocol! Mathilde Benveniste, Avaya - Research

D4.0 provides incentive to choose legacy protocol for BE! February 2003 Wireless VoIP softphone EDCF[BE] VoIP softphone Legacy D4.0 provides incentive to choose legacy protocol for BE! Mathilde Benveniste, Avaya - Research

Best Effort Traffic Parameters February 2003 Best Effort Traffic Parameters Mathilde Benveniste, Avaya - Research

Softphone Traffic Parameters February 2003 Softphone Traffic Parameters Mathilde Benveniste, Avaya - Research

Why should EDCF Best Effort perform no worse than legacy? February 2003 Why should EDCF Best Effort perform no worse than legacy? D4.0 causes EDCF BE traffic to perform worse than legacy stations This offers incentive for BE applications to use the legacy DCF protocol (802.11-1999)  Retirement of the legacy protocol would thus be delayed 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  By delaying legacy retirement, D4.0 limits prioritization benefits otherwise possible with EDCF Mathilde Benveniste, Avaya - Research

Stronger EDCF priority differentiation without legacy stations February 2003 Stronger EDCF priority differentiation without legacy stations The presence of legacy stations requires aAIFS=2 for the lowest priority EDCF category Higher-priority EDCF categories must have aAIFS=1  Weak EDCF class differentiation In the absence of legacy, each EDCF class can have a different aAIFS value  Strong EDCF class differentiation (Illustration uses D3.0 AIFS definition) 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 February 2003 Conclusions AIFSD in D4.0 causes collisions between EDCF stations and the HC Unlike AIFS in D3.0, AIFSD in D4.0 places EDCF best effort traffic at a disadvantage w.r.t. legacy stations AIFSD in D4.0 provides disincentive to use EDCF for BE applications; thus delays retirement of legacy DCF By prolonging the presence of legacy stations, AIFSD in D4.0 reduces potential effectiveness of EDCF Mathilde Benveniste, Avaya - Research

Proposed Solution Disallow AIFSN of 0 February 2003 Proposed Solution Disallow AIFSN of 0 Adopt EDCF backoff procedure in D3.0 (For details see Appendix) Mathilde Benveniste, Avaya - Research

Motion Adopt normative text changes in 03/279r0 February 2003 Mathilde Benveniste, Avaya - Research

Appendix The Method in D3.0 February 2003 Mathilde Benveniste, Avaya - Research

D3.0 (also the original proposal) February 2003 D3.0 (also the original proposal) Use the same backoff rules as 802.11-1999 An AIFS=PIFS does not cause collisions with the HC as a result of special provisions For stations with classification i= 0,1,… AIFSi = aSIFSTime + aAIFSi x aSlotTime where aAIFSi is the AIFS slot count for class i Example: aAIFSi = 1 for the top-priority classes, AIFSi = PIFS aAIFS0 = 2 for legacy-equivalent behavior, AIFS = DIFS Mathilde Benveniste, Avaya - Research

D3.0 -- Avoiding collisions with HCs February 2003 D3.0 -- Avoiding collisions with HCs To avoid collisions with HCs, the random backoff is required to be 1 or greater, for classes i with AIFS[i]=PIFS In D3.0 the backoff value was adjusted by a binary constant X, whose value depends on the priority class Backoff Time = (Random() + X) * aSlotTime where X = 1 for QSTAs with AIFS=PIFS X = 0 for QSTAs with AIFS>PIFS or, equivalently, a modified contention window is used: Backoff is drawn from the range [1,CW [i ]+1] for classes i with aAIFS[i]=1; [0,CW[i] ] for all other classes (i.e., aAIFS[i]>1) Mathilde Benveniste, Avaya - Research

Inter-frame Spaces and EDCF Backoff Rules same as in 802.11-1999 February 2003 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) Mathilde Benveniste, Avaya - Research

February 2003 D3.0 Rules: aAIFS[AC] must be 1 for EDCF ACs=3, 2, 1, for priority access. Lowest backoff delay must be 1 in order to avoid collisions with HC; i.e. backoff in range [1, CW[AC]+1] HC transmits after a PIFS idle PIFS SIFS Busy Medium HC transmits AIFS for aAIFS=0 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

Good! EDCF BE and legacy stations behave the same way. February 2003 D3.0 Rules: aAIFS[0] must be 2 for EDCF[0] (BE) to behave like legacy DCF, when channel becomes idle after a busy period Stations ready to transmit after countdown interruption have backoff = 1 DIFS PIFS Legacy backoff = 1 SIFS Busy Medium Legacy transmits AIFS for aAIFS=2 DIFS PIFS SIFS EDCF[0] backoff = 1 Busy Medium EDCF[0] transmits Slot time Good! EDCF BE and legacy stations behave the same way. Mathilde Benveniste, Avaya - Research

Good! EDCF BE and legacy stations behave the same way. February 2003 D3.0 Rules: EDCF[0] backoff must be drawn from range [0, CW[0]] in order to ensure that legacy stations and EDCF BE behave identically Comparing lowest possible backoff DIFS PIFS SIFS Lowest Legacy backoff = 0 Busy Medium Legacy transmits AIFS for aAIFS=2 DIFS PIFS SIFS Lowest EDCF[0] backoff = 0 Busy Medium EDCF[0] transmits Good! EDCF BE and legacy stations behave the same way. Mathilde Benveniste, Avaya - Research

D3.0 Rules: Comparing highest possible values February 2003 D3.0 Rules: Comparing highest possible values EDCF BE transmits just like Legacy Comparing highest possible backoff DIFS PIFS SIFS Nslots Highest Legacy backoff = N Busy Medium Legacy transmits ≀≀ AIFS for aAIFS=2 DIFS PIFS SIFS N slots Highest EDCF[0] backoff = N Busy Medium ≀≀ EDCF[0] transmits Great! EDCF BE and legacy stations behave the same always. Mathilde Benveniste, Avaya - Research