Dan Williams wrote: > On Mon, 2008-03-10 at 23:40 -0400, Robert Moskowitz wrote: > >> Ver 0.6.4 in Centos 5.1 >> >> I am seeing 17 SSIDs in the current list. But I am not seeing one that >> I expected to see. And some of the listed SSIDs are 'stale'; that is >> they were visible in the part of the hotel I was in a couple minutes >> ago, but not in this part. So I guess a second question is how do you >> force a scan to produce a current SSID list? >> > > You don't force a scan. NetworkManager will periodically scan with a > backoff algorithm; it will start at 20 seconds and back off to 2 > minutes. APs are kept in the scan list for a maximum of 6 minutes > before being culled. > This is a problem when you are moving around a lot. Well maybe not so much a problem if you are always wanting to connect to SSID ietf-a, regardless of which AP. But a problem if you are moving around in an area with a lot of open networks and you are looking for something to ride on... > The problem is that wireless is hard, Tell me about it. I work on the standards. Will be in Orlando next week for the 802 plenary meeting. > and sometimes cards/drivers miss beacons. Of course. Until we change 'everything' with 802.11s, scanning requires the radio to listen to each channel, one at a time, and hope to catch the BEACON for that channel. And not just a BEACON, but all the APs using a given channel. The standard does not allow for a radio to listen on all channels. 802.11n does change this a bit. 11s basically requires it (well for the mesh nodes at least). > Often they will not report all the APs that are known to be > around at a given time. Because they frequently have table limit sizes and can only record so many. > So NetworkManager takes a composite of the last few scans as the scan list. > Ouch. Not good for an actively moving device. A person walking can easily encounter a few APs for a given SSID on the same channel. Which one is really current? So when you do an ASSOCIATE on a given channel, which AP do you put in as the destination BSSID? > 0.6.x also combines APs with the same SSID in the UI. As it should. People don't understand lots of APs in an SSID unless they install them! > 0.7 splits them out at the NetworkManager layer, AH, so NetworkManager controls the ASSOCIATE, not the device driver? > while the applet combines APs that are > similar based on more than just SSID (SSID, security settings, band, > channel). > Channel/band? well other than b/g vs a vs n. And within an SSID you cannot have different security settings, per the spec. >> Perhaps the question may be how many APs can be handled and then those >> are turned into the SSID list (when more than one AP per SSID is found >> as in the case of some of these SSIDs). >> > > Are any of the APs hidden? > If they are 'hidden' (which is a myth, read my paper on this), they are not of interest. Hidding an SSID is a waste of effort. And it seriously breaks AP roaming.
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