How (well) does autochannel select work?

I have used two different wireless routers that offer to choose the

80211.g wireless channel automatically: a Sonicwall TZ-150 and a Buffalo Tech WHR-HP-G54.

I am troubleshooting problems with other devices, so I would like to know how this feature works. It might be giving me trouble.

In particular, suppose that the router decides to move to a different channel. Is there an industry standard protocol for "telling" all the client devices to move to the new channel? Or does the router just begin using the new channel, and wait for the clients to error out and reassociate/reauthorize on the new channel? How disruptive is the channel changing process? I am streaming video over my wireless network, so I care about disruptions.

My particular network is going to move to an "all Buffalo Tech" configuration, so if this vendor has a proprietary protocol, I'd be interested in learning about it. I would still like to understand the general case though (self-education and all that).

Thanks in advance for any pointers.

Reply to
David Arnstein
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The access point sniffs for interference. Anything that doesn't sound like 802.11b or 802.11g is classified as interference. That includes MIMO and various proprietary enhancements. If the interference is sufficiently strong, or mangles desired data above some threshold, the access point changes channel in the vain hope of avoiding the interference.

Sounds good so far. The problem is that the access point has no way to inform the clients that it has just changed channel. So, it has to fake it. The access point issues multiple disconnect packets to all the clients, refuses any new connections, and *THEN* changes channels. The clients are suppose to recognize this as a clue to go scanning again for the access point, but on a different channel.

It seldom works correctly. Some clients don't hear the disconnect packets and hang onto the old channel for an inordinate amount of time. Some clients are stupidly set to connect to any available access point and frequently do exactly that. Some will not start scanning until the IP stack times out which could take a few minutes. Some will consider the disconnect a good time to reset the IP stack and therefore will fail to continue the interrupted session.

Obviously, switching channels too often is a bad idea. The network literally stops dead every time it switches until the clients catch up. The network also creates intentional disconnects, which can crash downloads and break VPN sessions. So, most access points don't switch unless the interference is so bad, that no communications is possible. In other words, it's almost disabled by default. I spent an afternoon trying to force a DI-624 to change channels. I'm not sure exactly what I did, but in 4 hours, I only saw it switch twice.

There are ways to do it right, but that requires proprietary extensions. The work on 802.11r (roaming) has made considerable progress in this area. CCX (Cisco Compatible Extensions) also address the problem.

However, neither Sonicwall or Buffalo offer CCX features.

There are also vendors that offer similar features to OEM's. For example, Netgear offers Autocell.

No. However, 802.11r (roaming) and 802.11k (management) will have this feature. Cisco CCK also has it.

Your worst nightmare is true. See previous explanation.

It depends on the client. It ranges from obnoxious to catastrophic. No way is it seamless at this time.

Fine. Then lock in the channel and also the speed. You won't go as fast and you might have problems if the neighbors land on your channel, but you won't have disruptions caused by the access point trying to optimize the connection.

Well, you could ask their support department if they have such a protocol. I couldn't find anything in their data sheets or web site.

Reply to
Jeff Liebermann

The 802.11 terms are Dynamic Channel Selection (DCS) and Dynamic Frequency Selection (DFS).

Reply to
Axel Hammerschmidt

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