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AIFS – Revisited

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. INTRODUCTION. D4.0 uses different backoff countdown rules from those used in legacy stations (802.11-1999)

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AIFS – Revisited

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  1. 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

  2. INTRODUCTION • D4.0 uses different backoff countdown rules from those used in legacy stations (802.11-1999) • In that regard, D4.0 is similar to D1.0 (cont’ed in D2.0), which was voted out of the draft after extensive technical discussions and over 75% support • The method in D1.0 and D4.0 was introduced without any technical discussion • It has been shown (01/408) that the D1.0 method causes ‘Best Effort’ (BE) EDCF stations to perform worse than legacy stations • We explain the problems with D4.0 Mathilde Benveniste, Avaya - Research

  3. The original AIFS * – also in D3.0 AIFS(Arbitration inter-frame space)-- a priority-dependent IFS • Is the deferral time for access & backoff countdown used for QoS differentiation The original proposal (details given in the Appendix I ) • uses the same backoff countdown rules as in 802.11-1999 for DIFS • generates an access category exactly equivalent to legacy ____________________________________________ *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 • whereaAIFSiisthe AIFS slot count for class i Mathilde Benveniste, Avaya - Research

  4. 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 of presentation) • An EDCF TXOP is granted when the channel is idle, and has been idle for a time greater than or equal to AIFSD[AC] + aSlotTimeand the backoff has expired • Backoff is drawn from the range[1, CW[AC]+1] • AIFSN assumes values from0, …, 10 • New backoff timing rules are introduced; different from the existing 802.11-1999 rules Mathilde Benveniste, Avaya - Research

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

  6. Problems with D4.0 New backoff rules make implementation harder, as design reuse not possible D4.0 can cause EDCF stations to collide with the HC D4.0 puts EDCF[BE] at a disadvantage to legacy stations, leading to less efficient QoS performance of EDCF Mathilde Benveniste, Avaya - Research

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

  8. Collisions with HC - 2 D4.0 Rules:If AIFSN[AC] is 1 for EDCF ACs=3, 2, 1, as in D3.0, collisions with the HC are avoided PIFS HC transmits after a PIFS idle SIFS Medium Busy HC transmits AIFSD for AIFSN=1 EDCF[3] with backoff = 1 transmits after a DIFS idle PIFS PIFS SIFS SIFS EDCF[3] transmits Medium Busy HC transmits Slot time So far so good! Mathilde Benveniste, Avaya - Research

  9. EDCF[BE] vs Legacy behavior - 1 D4.0 Rules:If AIFSN[0] is set to 2, priority access is possible, and EDCF[0] (BE) behaves like legacy DCF, given the same backoff value, e.g. Backoff=1 DIFS PIFS SIFS Legacy backoff = 1 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

  10. EDCF[BE] vs Legacy behavior - 2 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, CW) Legacy Backoff delay pdf … (1, CW+1) EDCF[0] Backoff delay pdf Mathilde Benveniste, Avaya - Research

  11. Slot time EDCF[BE] vs Legacy behavior - 3 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 SIFS Lowest EDCF[0] backoff = 1 EDCF[0] transmits Medium Busy Legacy transmits EDCF BE transmits after Legacy. Not good! Mathilde Benveniste, Avaya - Research

  12. Slot time EDCF[BE] vs Legacy behavior - 4 D4.0 Rules: Comparing the highest possible backoff values DIFS PIFS Nslots SIFS Highest Legacy backoff = N Medium Busy Legacy transmits ≀≀ AIFSD for AIFSN=2 DIFS DIFS PIFS PIFS N slots SIFS SIFS Highest EDCF[0] backoff = N+1 EDCF[0] transmits Medium Busy Legacy transmits ≀≀ EDCF BE transmits after Legacy. Not good! Mathilde Benveniste, Avaya - Research

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

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

  15. Best Effort Traffic Parameters Mathilde Benveniste, Avaya - Research

  16. Softphone Traffic Parameters Mathilde Benveniste, Avaya - Research

  17. 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) 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

  18. Stronger EDCF priority differentiation without legacy stations The presence of legacy stations requiresaAIFS=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 Next Frame Medium Busy 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) Earliest Tx time for QSTAs with aAIFS = 7 (backoff=1) Retirement of legacy is desirable! Mathilde Benveniste, Avaya - Research

  19. Conclusions • AIFSD in D4.0 introduces greater implementation complexity by introducing new & different backoff rules • 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

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

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

  22. Appendix I The Method in D3.0 Mathilde Benveniste, Avaya - Research

  23. 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 • whereaAIFSiisthe 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

  24. 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

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

  26. D3.0 Rules:aAIFS[AC] must be 1 for EDCF ACs=3, 2, 1,for priority access. Lowest backoff delay must be 1in order to avoid collisions with HC; i.e.backoff in range [1, CW[AC]+1] PIFS HC transmits after a PIFS idle 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 HC transmits EDCF[3] transmits Busy Medium Slot time Good! HC goes first. Mathilde Benveniste, Avaya - Research

  27. 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 SIFS Legacy backoff = 1 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

  28. 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 EDCF[0] transmits Busy Medium Good! EDCF BE and legacy stations behave the same way. Mathilde Benveniste, Avaya - Research

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

  30. Appendix II Hypothetical Fix Mathilde Benveniste, Avaya - Research

  31. Hypothetical fix: Suppose the range for backoff delay were changed in D4.0 from (1, CW[AC]+1) to (0, CW[AC]) … (0, CW) Problems: • ECDF[0] stations would draw the same backoff as legacy stations, but they would not realize the same backoff delay • EDCF[0] stations would still have a disadvantage relative to legacy stations • The realizedbackoff delay for all categories would have a non-uniform pdf; the collision rate would be higher Legacy Backoff delay pdf (drawn and realized) … (0, CW) Pdf of drawn EDCF[.] Backoff Delay … (1, CW) Pdf of realized EDCF[.] Backoff Delay Not a good fix! Mathilde Benveniste, Avaya - Research

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