May 2006
doc.: IEEE 802.11-06/0657r1
MAC improvement using random AIFSN
Date: 2006-05-13
Authors:
Name
Company
Todor Cooklev
Sudhanshu Gaur
Hitachi
America, Ltd.
Address
Phone
email
121 Miramonte Dr.,
Moraga, CA 94556
2000 Sierra Point
Pkwy., Brisbane,
CA 94005
925-377-6700 [email protected]
650-244-7251 sudhanshu.gaur@h
al.hitachi.com
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Submission
Slide 1
Todor Cooklev
May 2006
doc.: IEEE 802.11-06/0657r1
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.
Submission
Slide 2
Todor Cooklev
May 2006
doc.: IEEE 802.11-06/0657r1
Outline
•
•
•
•
•
•
•
Contention in 802.11 scenarios for digital home
Current 802.11e channel access mechanism
Proposed channel access mechanism
Proposed text
Simulation results
Implementation considerations
Conclusions
Submission
Slide 3
Todor Cooklev
May 2006
doc.: IEEE 802.11-06/0657r1
Contention in 802.11 scenarios for digital home
• Many traffic streams anticipated in CE dominated
home…
– HD Video
• Support for 4-5 HD video streams 100-125Mbps
• 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
Submission
Slide 4
Todor Cooklev
May 2006
doc.: IEEE 802.11-06/0657r1
Digital home of the future
HD content
V4
HDTV
HDTV
HDTV
V1
V3
V2
Live or time-shifted
Premium HD content
Voice 3-4
Data 2-3
STB / PVR
Cable, Satellite
over WLAN
Internet, IPTV
V5
Personal
content
Media PC
Submission
Slide 5
Todor Cooklev
May 2006
doc.: IEEE 802.11-06/0657r1
802.11e channel access mechanism
Fig.1 EDCA channel access mechanism
AC
CWmin
CWmax
AIFSN
BK
CWmin
CWmax
7
BE
CWmin
CWmax
3
VI
(CWmin+1)/2-1
CWmin
2
VO
(CWmin+1)/4-1
(CWmin+1)/2-1
2
• 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
Submission
Slide 6
Todor Cooklev
May 2006
doc.: IEEE 802.11-06/0657r1
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
Submission
Slide 7
Todor Cooklev
May 2006
doc.: IEEE 802.11-06/0657r1
Proposed Text : Section 9.2.3.4
• 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 7.3.2.14). 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 20.2. 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 7.3.2.14).
Submission
Slide 8
Todor Cooklev
May 2006
doc.: IEEE 802.11-06/0657r1
Proposed Text : Section 9.9.1.3
• 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 20.2. 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.
Submission
Slide 9
Todor Cooklev
May 2006
doc.: IEEE 802.11-06/0657r1
AIFSN intervals per AC
limit value specified by AP
limit value specified by AP
limit value specified by AP 2
limit value specified by AP 2
3
7
limit value specified by AP
limit value specified by AP
limit value specified by AP
limit value specified by AP
The limit value specified by the AP could be higher or lower
Submission
Slide 10
Todor Cooklev
May 2006
doc.: IEEE 802.11-06/0657r1
Simulation results with random AIFSN
•
Simulation Parameters
–
–
54 Mbps PHY (802.11g), 4 pairs each for BE, Video and Voice traffic
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 Delay per AC
Global Throughput per AC
Submission
Best Effort
Best Effort
Video
Video
Voice
Voice
Slide 11
Todor Cooklev
May 2006
doc.: IEEE 802.11-06/0657r1
Simulation results with random AIFSN [2]
•
Simulation Parameters
–
–
2x2 MIMO, 20MHz Channel, 5 pairs each of BE, Video and Voice traffic
AIFSN[AC1] – fixed default values as in Table 20.2, except for video; AIFSN for video is random with
uniform distribution over [1 3].
Delay per stream
Throughput per stream
Submission
Best Effort
Best Effort
Video
Video
Voice
Voice
Slide 12
Todor Cooklev
May 2006
doc.: IEEE 802.11-06/0657r1
Simulation results with random AIFSN [3]
Delay per stream
•
Simulation Parameters
– 2x2 MIMO, 20MHz Channel
– 5 Video direct links
– AIFSN for video is random
with uniform distribution over
[2 3].
Throughput per stream
Submission
Slide 13
Todor Cooklev
May 2006
doc.: IEEE 802.11-06/0657r1
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.
Submission
Slide 14
Todor Cooklev
May 2006
doc.: IEEE 802.11-06/0657r1
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.
Submission
Slide 15
Todor Cooklev