1. AIFS – Revisited Mathilde Benveniste benveniste@ieee.org
doc.: IEEE /457
February 2003
AIFS – Revisited
Mathilde Benveniste
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. We explain the problems in D4.0
February 2003
INTRODUCTION
D4.0 uses backoff countdown rules that are different from those in (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

3. 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 , 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

4. 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 rules
Mathilde Benveniste, Avaya - Research

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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

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

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

17. 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 ( )
 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

18. 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

19. doc.: IEEE /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

20. 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

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

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

23. D3.0 (also the original proposal)
February 2003
D3.0 (also the original proposal)
Use the same backoff rules as
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

24. D3.0 -- Avoiding collisions with HCs
February 2003
D 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
February 2003
Inter-frame Spaces and EDCF Backoff Rules same as in
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

26. 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

27. 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

28. 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

29. 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