little otter <little.otter.2xmzag@no-mx.wirelessforums.org> hath
wroth:
>I've had a search for appropriate posts for this topic but had no luck,
>not in a way that I understand anyway. SO here's the thing...
>
>I've put together many wireless networks for friends etc so I'm not
>100% newby. I'm now looking at a slightly more demanding task that I'm
>not sure about.
>
>I wish to link 2 buildings together using a wireless bridge. I've done
>this before, but this time I want to use an EXTRA pair of wireless
>bridge units to create a fault tolerant link, that is, the link will
>still work if one of the wireless pairs dies (or is blocked by a
>nesting pterodactyl).
No flying dinosaurs yet, but lots of pigeons and pigeon droppings.
Incidentally, that's the way I test people for wireless experience. If
they don't know what solvent cleans up pigeon droppings, they've never
worked on an urban radio system.
>I have an initial scheme idea that I've drawn up in a little PDF to
>explain: http://www.peakelec.co.uk/RedundantWirelessScheme.pdf
Instant bridging loop. As drawn, there's no way to keep a packet from
going round and round and round the loop formed between switches. You
can simulate the problem by replacing the two wireless links with just
some CAT5 between the two ethernet switches. It won't take long to
see the problem by watching the flashing lights (or using SNMP
monitoring if the boxes are managed switches).
However, it will work if your switches can do STP (spanning tree
protocol), preferably Fast STP also known as 802.1w or MSTP (multiple
STP) also known as 802.1S or preferably 802.1Q. See:
<http://www.cisco.com/univercd/cc/td/doc/product/rtrmgmt/sw_ntman/cwsimain/cwsi2/cwsiug2/vlan2/stpapp.htm>
Also:
<http://en.wikipedia.org/wiki/Spanning_tree_protocol>
>But I'm sure that it won't be as simple as that, how would you solve
>this problem without going to very specialised hardware and/or
>complicated software?
Gee thanks. You give me a specialized problem and don't let me
suggest complicated and specialized hardware and protocols. That's no
fun, so I'll just ignore that restriction.
You can do load balancing between two bridged paths using VLAN's and
STP. See:
<http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a00800ae96a.shtml>
I've never tried doing this, but it looks like it will work.
(Translation: I don't really understand how it works until I've tried
it). The benefit is that you get to use both wireless links
simultaneously for double the thruput. The price is complexity.
>What types of products would you use and how would
>they be configured to provide a transparent switch-over in the event of
>a problem?
Any pair of ethernet switches that will do one of the numerous flavors
of STP.
You can also do fail over using routers, but that will complicate your
IP layout. RIP2 (router information protocol) can also be used to
switch paths. Details on request.
>Many thanks.
One is usually sufficient.
Incidentally, make sure you have some physical isolation between the
pairs of antennas. That means that you should not visually see the
"redundant" antenna across the 60 yard path. Cheapo wi-fi radios do
not have very good overload characteristics and are quite susceptible
to overload and interference from nearby off channel xmitters.
--
Jeff Liebermann
jeffl@cruzio.com
150 Felker St #D
http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
Re: Simple Redundant Wireless Link? 
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