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Old 01-03-2007, 11:41 PM
David Arnstein
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Default 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.
--
David Arnstein (00)
arnstein+usenet@pobox.com {{ }}
^^

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  #2 (permalink)  
Old 01-04-2007, 04:16 AM
Jeff Liebermann
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Default Re: How (well) does autochannel select work?

On Wed, 3 Jan 2007 23:41:09 +0000 (UTC), arnstein@panix.com (David
Arnstein) wrote:

>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.


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.
<http://www.cisco.com/web/partners/pr46/pr147/partners_pgm_concept_home.html>
However, neither Sonicwall or Buffalo offer CCX features.

There are also vendors that offer similar features to OEM's. For
example, Netgear offers Autocell.
<http://www.autocell.com>

>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?


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

>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?


Your worst nightmare is true. See previous explanation.

>How disruptive is the channel
>changing process?


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

>I am streaming video over my wireless network, so I
>care about disruptions.


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.

>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).


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.


--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558 jeffl@comix.santa-cruz.ca.us
# http://802.11junk.com jeffl@cruzio.com
# http://www.LearnByDestroying.com AE6KS

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  #3 (permalink)  
Old 01-04-2007, 08:30 AM
Axel Hammerschmidt
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Posts: n/a
Default Re: How (well) does autochannel select work?

David Arnstein <arnstein@panix.com> wrote:

> 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.


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

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