Aug 2007
doc.: IEEE 802.11-07/2334r1
Discussion on CCA Sensing on 20/40 MHz
Secondary Channel with PIFS and DIFS
Date: 2007-08-22
Authors:
Name
Company
Douglas Chan
Cisco Systems
Luke Qian
Cisco Systems
Address
Phone
170 W. Tasman Dr,
San Jose, CA 95134
4125 Highlander
Parkway, Richfield,
OH 44286
+1 408 5279344
+1 330 523
2051
email
[email protected]
[email protected]
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Submission
Slide 1
Chan, et al.
Aug 2007
doc.: IEEE 802.11-07/2334r1
Abstract
The 20/40 11n STA can transmit a 40 MHz PPDU if the
secondary channel is idle for at least PIFS. We suspect
this can give them an unfair advantage in accessing the
channel over 20 MHz BSSs, which uses at least DIFS.
We attempt to illustrate this with a simple calculation and
contrast its results with a previous submitted
simulations so we can open a discussion on this issue.
There’s a related comment to this issue: CID 70.
Submission
Slide 2
Chan, et al.
Aug 2007
doc.: IEEE 802.11-07/2334r1
The related comment
CID
Page
70
163.49
Line
Clause
Comment
Proposed Change
49
9.20.2
A 40 Mhz TXOP requires the
secondary channel to be idle for
at least PIFS before the end of
the backoff timer. If the
secondary channel has a nonHT BSS that is in a PCF or HCF
contention free period, PIFS is
used as the timeout for unacknowledged frames and PFPoll frames. Existing (non-HT)
channel access rules mean that
the HC / PC maintains access to
the medium. Clause 9.20.2 in
11n breaks this model. I also
have concerns that this clause
makes channel access unfair
because it gives the HT BSS
priority over a non-HT BSS
operating on the secondary
channel.
Use something longer than
PIFS (e.g. DIFS) as the period
during which the secondary
channel needs to be idle.
Proposed resolution: Counter to this, but accept in principle. That is, at least
wait for DIFS on the secondary channel before declaring it to be idle.
Submission
Slide 3
Chan, et al.
Aug 2007
doc.: IEEE 802.11-07/2334r1
Unfair better access for 20/40 BSSs using PIFS?
• Since D2.0, CCA sensing for 20/40 MHz is:
– STA permitted to transmit a 40 MHz PPDU if the secondary channel has
been idle for a duration of at least PIFS immediately preceding the backoff
counter expires
– The backoff counter is on the primary channel
– (This is Subclause 9.20.2, see next slide on D.2.05 version)
• But 20 MHz STAs (11n or a/b/g) are permitted to transmit only
after their channels are idle for a duration of at least DIFS
immediately preceding the backoff counter expires
• By inspection of these protocols, one may question if the 20 MHz
STAs have an unfairly smaller channel access than the 20/40 MHz
STAs that are sharing the secondary channel
Submission
Slide 4
Chan, et al.
Aug 2007
doc.: IEEE 802.11-07/2334r1
Current text in D2.05
Submission
Slide 5
Chan, et al.
Aug 2007
doc.: IEEE 802.11-07/2334r1
Illustrative calculation of better channel access
•
•
Let’s look at the worst case with an 11a BSS on the secondary channel of an 11n
20/40 BSS
– CW = 3 slots for both 11a and 11n 20/40 BSSs
– Assume also that there’s a 50% chance an HT transmission is successfully received
in the event of a collision
Since PIFS is shorter than DIFS by one slot:
– Contention window for the 11n STA is [1,4] from when the channel is cleared
– Contention window for the 11a STA is [2,5] from when the channel is cleared
Probability an 11n 20/40 STA gains access to channel
= P(11n STA’s backoff = 1) +
P(11n STA’s backoff = 2) * P(11a STA’s backoff = 2)* 0.5 +
P(11n STA’s backoff = 2) * P(11a STA’s backoff = [3,5]) +
P(11n STA’s backoff = 3) * P(11a STA’s backoff = 3)*0.5 +
P(11n STA’s backoff = 3) * P(11a STA’s backoff = [4,5]) +
P(11n STA’s backoff = 4) * P(11a STA’s backoff = 4)*0.5 +
P(11n STA’s backoff = 4) * P(11a STA’s backoff = 5)
= 1/4 + 3*(1/4 * 1/4 * 0.5) + 1/4 * (3/4 + 1/2 + 1/4)
= 72%
Submission
Slide 6
Chan, et al.
Aug 2007
doc.: IEEE 802.11-07/2334r1
Discussion
• Although the 20/40 11n STA’s backoff counter is on the primary
channel, giving it an additional constraint during the CCA, but
72% success probability is quite high and may still provide it with
an unfair channel access advantage
• However, previous simulations submitted [06/0608r2] indicate
small decrease in secondary channel BSS’s throughput
performance, implying using PIFS is still fair for the secondary
devices
– What could be the assumptions or factors absorbing this huge success
probability?
• Especially consider a scenario where the primary channel is
always clear, then during channel contention, should the 40 MHz
STA transmits right after PIFS of idling, or should it wait at least
DIFS of time like the other STAs in the secondary channel?
• Should this unfairness to these 20 MHz STAs prompt us to use
DIFS in CCA sensing on the secondary channel?
Submission
Slide 7
Chan, et al.