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Ticket #1928 (new defect)

Opened 14 years ago

Last modified 13 years ago

Problem with carrier sense in 802.11a?

Reported by: sanjitkaul@gmail.com Assigned to:
Priority: major Milestone:
Component: madwifi: other Version: v0.9.4
Keywords: Cc:
Patch is attached: 0 Pending:

Description

Problem with carrier sense in 802.11a?

Experiment Setup:


Madwifi Release 0.9.4:

Two radios are transmitting simultaneously, saturating the channel @ 6Mbps PHY rate, 802.11a, channel 44.

Both the radios use wlan0, with two virtual interfaces ath0 and ath1.

ath0 is in Master mode and ath1 is in monitor mode.

Acks are switched off using the setnoack ioctl.

Observation:


The radios don't share the channel for RSSI(s) as large as 28 and lesser, i.e., The received throughput at the nodes from the other is not close to typically expected (~ 2 Mbps). In fact it is very low, hardly any packets are received! However, the transmit rate at both the transmitters is consistently >5Mbps. It seems most packet transmissions end up in collisions. Note that the throughputs obtained when the radios are not transmitting simultaneously are as expected (>5Mbps).

The radios do share the channel for RSSI(s) 44 and larger.

It seems that Carrier sense is triggered only for large values of RSSI, as if only the energy based threshold for carrier sense was in place.

The same setup works fine in 802.11g mode giving expected throughputs.

Change History

(follow-ups: ↓ 2 ↓ 3 ↓ 4 ) 05/06/08 05:53:53 changed by mrenzmann

Blind guess: could this be an effect caused by the infamous ANI bug? Did you already try to disable ANI (via the intmit module parameter)?

(in reply to: ↑ 1 ) 05/06/08 06:14:38 changed by sanjitkaul

Replying to mrenzmann:

Blind guess: could this be an effect caused by the infamous ANI bug? Did you already try to disable ANI (via the intmit module parameter)?

That's a good point. I thought about it but didn't try disabling ANI as that problem seemed to be about low rssi links ending up in error and for links with rssi(s) lower than the ones I am playing with. Anyway, will give it a try.

(in reply to: ↑ 1 ) 09/02/08 20:28:08 changed by anonymous

Replying to mrenzmann:

Blind guess: could this be an effect caused by the infamous ANI bug? Did you already try to disable ANI (via the intmit module parameter)?

I disabled ANI and the results are the same, the problem remains. The rest of parameters are the same as mentioned earlier.

(in reply to: ↑ 1 ) 09/02/08 20:29:26 changed by sanjitkaul

Replying to mrenzmann:

Blind guess: could this be an effect caused by the infamous ANI bug? Did you already try to disable ANI (via the intmit module parameter)?

Re-posting using my id. (Sorry)

I disabled ANI and the results are the same, the problem remains. The rest of parameters are the same as mentioned earlier.

09/02/08 21:49:10 changed by anonymous

Well, sounds like "lots of crosstalk"

The problem is that your radios & antennas don't have enough "distance" in your PC... The 802.11a standard needs about 2m which means that you need some shielding and a long antenna cables if you want some speed...

09/23/08 20:57:53 changed by DarkMan

related to ticket #2109 ???