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

2. 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,
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 e stations
A simple remedy exists
Adopt backoff rules of TGe D3.0; they are the same as
Mathilde Benveniste, Avaya - Research

3. 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
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

4. 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 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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
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

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. Best Effort Traffic Parameters
March 2004
Best Effort Traffic Parameters
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22. Softphone Traffic Parameters
March 2004
Softphone Traffic Parameters
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23. 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 ( )
 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

24. 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

25. doc.: IEEE /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

26. March 2004
Appendix I
History
Mathilde Benveniste, Avaya - Research

27. 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 ( )
D4.0 uses different backoff countdown rules from those used in legacy stations ( )
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