1. MAC improvement using random AIFSN
May 2006
doc.: IEEE /0657r0
May 2006
MAC improvement using random AIFSN
Date:
Authors:
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Todor Cooklev
Todor Cooklev

2. May 2006
doc.: IEEE /0657r0
May 2006
Abstract
We discuss contention among streams of equal priority in the digital home. Next, we propose the use of random AIFSN as a solution, with draft text. Simulation results indicating the performance improvement are also included. The proposed scheme performs especially well under heavy channel loads.
Todor Cooklev
Todor Cooklev

3. Outline Contention in 802.11 scenarios for digital home
May 2006
doc.: IEEE /0657r0
May 2006
Outline
Contention in scenarios for digital home
Current e channel access mechanism
Proposed channel access mechanism
Proposed text
Simulation results
Implementation considerations
Conclusions
Todor Cooklev
Todor Cooklev

4. Contention in 802.11 scenarios for digital home
May 2006
doc.: IEEE /0657r0
May 2006
Contention in scenarios for digital home
Many traffic streams anticipated in CE dominated home…
HD Video
Support for 4-5 HD video streams Mbps
Content: Premium, time-shifted, DVD, personal, Gaming
Voice
Up to 4 streams with tight delay constraints – low throughput
Best-effort
Web, file transfer, printing - 10Mbps
Contention among equal-priority traffic is an important issue
Todor Cooklev
Todor Cooklev

5. Digital home of the future
May 2006
doc.: IEEE /0657r0
May 2006
Digital home of the future
HD content
HDTV
HDTV V4
HDTV V3 V2 V1
Live or time-shifted
Premium HD content
Voice 3-4
Data 2-3
Cable, Satellite
STB / PVR
Media PC
over WLAN
Internet, IPTV
Personal
content V5
Todor Cooklev
Todor Cooklev

6. 802.11e channel access mechanism
May 2006
doc.: IEEE /0657r0
May 2006
802.11e channel access mechanism
Fig.1 EDCA channel access mechanism
Arbitration inter-frame space (AIFS) is one of the EDCA parameters used to ensure traffic differentiation
Each AC has fixed AIFS duration associated with it. The smaller the AIFS the higher the medium access priority
But multiple streams of the same AC use same AIFSN leading to increased collision!
Fig. 2 From Table 20.2 of 11e
Todor Cooklev
Todor Cooklev

7. Proposed channel access mechanism
May 2006
doc.: IEEE /0657r0
May 2006
Proposed channel access mechanism
Fig. 3 Proposed channel access mechanism
Proposed Mechanism
AIFSN is random with a certain probability density function
For example the distribution can be uniform
Fig. 4 Proposed medium access control logic
Advantages
Possible to assign the ACs a non-integer AIFSN value on an average
Less collisions within an AC; higher throughput
Todor Cooklev
Todor Cooklev

8. Proposed Text : Section 9.2.3.4
May 2006
doc.: IEEE /0657r0
May 2006
Proposed Text : Section
Original…
A Non-AP QSTA computes the time periods for each AIFS[AC] from the dot11EDCATableAIFSN attributes in the MIB. QSTAs update their dot11EDCATableAIFSN values using information in the most recent EDCA Parameter Set element of Beacons received from the QAP of the QBSS (see ). A QAP computes the time periods for each AIFS[AC] from the dot11QAPEDCATableAIFSN attributes in its MIB.
NEW…
Non-AP QSTA compute AIFSN[AC] using some probability density function over a finite interval. This interval is bounded on one side by the default values in Table The other limit of this interval is from the dot11EDCATableAIFSN attributes in the MIB. QSTAs update their dot11EDCATableAIFSN values using information in the most recent EDCA Parameter Set element of Beacons received from the QAP of the QBSS (see ).
Todor Cooklev
Todor Cooklev

9. Proposed Text : Section 9.9.1.3
May 2006
doc.: IEEE /0657r0
May 2006
Proposed Text : Section
Original …
The value of AIFSN[AC] shall be greater than or equal to 2 for non-AP QSTAs and is advertised by the QAP in the EDCA Parameter Set Information Element in Beacons and Probe Response frames transmitted by the QAP. The value of AIFSN[AC] shall be greater than or equal to 1 for QAPs.
NEW…
QSTAs select the value of AIFSN according to some probability density function (PDF) over a finite interval. One of these limits (either the lower or the upper limit) is the default value specified in Table The other limit is in the dot11EDCATableAIFSN attribute in the MIB. Any PDF is acceptable, including a PDF where QSTA always select one number that belongs to this interval. The limit value AIFSN[AC] is advertised by the QAP in the EDCA Parameter Set Information Element in Beacons and Probe Response frames transmitted by the QAP. If this limit value is smaller than the default values in Table 20.2, then it becomes the lower bound of the interval. If this limit value is higher than the default values in Table 20.2, then it becomes the higher bound of the interval.
Todor Cooklev
Todor Cooklev

10. AIFSN intervals per AC May 2006
The limit value specified by the AP could be higher or lower
Todor Cooklev

11. Simulation results with random AIFSN
May 2006
doc.: IEEE /0657r0
May 2006
Simulation results with random AIFSN
Simulation Parameters
54 Mbps PHY, 12 nodes – 4 BE, 4 Video and 4 Voice
AIFSN[AC1] – fixed default values as in Table 20.2, except for video; AIFSN for video is random with uniform distribution over [2 4].
Global Throughput per AC
Global Delay per AC
Todor Cooklev
Todor Cooklev

12. Implementation Issues
May 2006
doc.: IEEE /0657r0
May 2006
Implementation Issues
The suggested operation is mandatory, so that it will not get advertised in beacons and probe responses.
Since any probability density function is acceptable, all current implementations using fixed AIFSN comply with this proposal, i.e. no change is necessary.
Those that do want to change and implement, for example, uniform probability density function, will not only see an improvement in their throughput, but will improve overall network performance.
Fairness? The improvement according to the proposed scheme does not come at the expense of degradation somewhere else. The performance of the entire network is improved.
Todor Cooklev
Todor Cooklev

13. May 2006
doc.: IEEE /0657r0
May 2006
Conclusions
Contention in wireless home scenarios is significant for equal-priority streams
The proposed random-AIFSN has benefit to all traffic classes – throughput increases and delay decreases
While the proposal is to make the suggested scheme mandatory, in reality it is optional because all current implementations comply with it.
Todor Cooklev
Todor Cooklev