1. July 2007
doc.: IEEE /2090r0
Aug 2007
Discussion on CCA Sensing on 20/40 MHz Secondary Channel with PIFS and DIFS
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Chan, et al.
Eldad Perahia (Intel)

2. July 2007
doc.: IEEE /2090r0
Aug 2007
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.
Chan, et al.
Eldad Perahia (Intel)

3. The related comment Aug 2007
July 2007
doc.: IEEE /2090r0
Aug 2007
The related comment
CID
Page
Line
Clause
Comment
Proposed Change 70
163.49 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 non-HT BSS that is in a PCF or HCF contention free period, PIFS is used as the timeout for un-acknowledged frames and PF-Poll frames. Existing (non-HT) channel access rules mean that the HC / PC maintains access to the medium. Clause 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.
Chan, et al.
Eldad Perahia (Intel)

4. Unfair better access for 20/40 BSSs using PIFS?
July 2007
doc.: IEEE /2090r0
Aug 2007
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 , 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
Chan, et al.
Eldad Perahia (Intel)

5. Current text in D2.05 Aug 2007 July 2007 doc.: IEEE 802.11-07/2090r0
Chan, et al.
Eldad Perahia (Intel)

6. Illustrative calculation of better channel access
July 2007
doc.: IEEE /2090r0
Aug 2007
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%
Chan, et al.
Eldad Perahia (Intel)

7. July 2007
doc.: IEEE /2090r0
Aug 2007
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?
Chan, et al.
Eldad Perahia (Intel)