Subject: Re: ath0 carrier coming and going
To: David Young <firstname.lastname@example.org>
From: Steven M. Bellovin <email@example.com>
Date: 12/26/2004 00:01:28
In message <20041226023617.42DFC3BFEDB@berkshire.machshav.com>, "Steven M. Bell
>In message <20041224052255.GJ5287@che.ojctech.com>, David Young writes:
>>On Thu, Dec 23, 2004 at 09:39:37PM -0500, Steven M. Bellovin wrote:
>>> I'm running a very fresh -current on a Thinkpad T42 with an 802.11a/b/g
>>> card. I also run ifwatchd, and I see that carrier on the ath interface
>>> is dropped/restored about once per minute. Why is that? At the
>>> moment, it's the active interface I'm using to get out to the world.
>>If the interface idle, then maybe the AP is disconnecting you. Will you
>>see what's happening "on the air" at that time with 'tcpdump -ne -y
>>ieee802_11 -i ath0' ?
>>There is a radio calibration that Atheros does periodically. Maybe it
>21:30:45.737792 Beacon () [1.0* 2.0* 5.5 11.0 Mbit] ESS CH: 1, PRIVACY
>21:30:46.045003 Beacon () [1.0* 2.0* 5.5 11.0 Mbit] ESS CH: 1, PRIVACY
>21:30:46.147399 Beacon () [1.0* 2.0* 5.5 11.0 Mbit] ESS CH: 1, PRIVACY
>21:30:46.827858 ReAssoc Request (machshav) AP : 00:60:1d:22:5a:86
>21:30:46.829417 ReAssoc Response AID(5) : PRIVACY : Succesful
>21:30:47.069047 Beacon () [1.0* 2.0* 5.5 11.0 Mbit] ESS CH: 1, PRIVACY
>21:30:47.478655 Beacon () [1.0* 2.0* 5.5 11.0 Mbit] ESS CH: 1, PRIVACY
>There was a transition at 21:30:46, associated (I assume) with the
>ReAssoc messages. The question is why this should generate carrier
>down/up transitions to ifwatchd.
I should add -- for reasons that I don't pretend to understand, I see
this phenomenon when the laptop is about 1 meter from the access point.
I don't see it when I'm at the other end of the house. It's possible
that it's associating with the other access point in the house (though
I thought I checked for that); I'll have to bring the machine down to the
basement where there will be no doubt about which one it's using. Of
course, that AP handles 802.11g (as does the laptop), so that may
complicate matters. The upstairs AP is 802.11b-only.
--Prof. Steven M. Bellovin, http://www.cs.columbia.edu/~smb