[Beowulf] Wireless clusters (Bridging 802.11a)
James.P.Lux at jpl.nasa.gov
Wed Jan 14 20:36:16 EST 2004
I'm building a cluster using wireless interconnects, and it is getting to
be a royal pain to figure out what sort of adapters/interfaces to use. I'm
casting some bread upon the waters here to see if anyone has fooled with
this sort of thing.
I want to go 802.11a because of spectrum usage, not because of data
rate. If I go to 5GHz, I don't have to worry about accidentally connecting
to JPL's 802.11b (2.4GHz) network infrstructure, for instance, which will
keep netops off my back.
The processors in the cluster are Mini-ITX widgets with Compact Flash (CF)
drives, and, while booting off the net might be nice, I'm going to boot off
CF for now.
Here are some issues that have come up:
1) there's two ways to get the node to talk to the net:
via the ethernet connector and an external bridge
via a PCI card with a 802.11a adapter (most likely a 802.11a/b/g, since
that's what's available) (D=Link, Netgear, and Linksys all have them)
In all cases, I'd have an "access point" of some sort to talk to my head
Ideally, I'd like to set up the network in "ad-hoc" mode, where any node
can talk to any other directly, without having to be routed through an
access point. In "infrastructure" mode, many clients can talk to the access
point, but clients cannot talk to clients, except by going through the
access point, creating a single failure point (probably not important for
my initial work, but philosophically "bad").
2) It's unclear whether there are Linux drivers for any of the PCI based
802.11a cards. The mfrs don't seem to want to fool with that market, and,
chipset mfrs are quite reticent about releasing the intellectual property
needed to do a good job writing the drivers.
3) I could go with external bridging adapters (perhaps with integrated
routers, in case I add another ethernet device to the node, or, to hook up
a sniffer). Here the problem is that not all mfrs appear to support
bridging, at least with more than 2 (i.e. they can set up a point to point
bridge, but not a many to many bridge)
From some reading of the various manuals, it appears that some "access
points" can be set up to appear to be a "client" in infrastructure mode,
however that's a problem philosophically (and in terms of latency).
So, does anyone know which "access points" (i.e. a 802.11x to ethernet box)
can look like a client in an ad-hoc network.
(possible candidates: Netgear FWAG114, D-link DWL-774, DWL-7000AP, Linksys
WAP54A*, WAP51AB, WRT51AB. *Linksys says that the WAP54A doesn't do bridging)
Part 2 of the quest.................
I'm also looking for suggestions on performance and timing tests to run on
this cluster once it's assembled. Aside from the usual network throughput
(benchmark program recommendations requested), I'm interested in techniques
to look at latency, latency distribution, and dropped packets/retries,
since I suspect that wireless networks will have very "unusual" statistics
compared to the usual cluster interconnects.
And, bearing in mind our recent lengthy thread on timing and clocks, you
can be sure that I will do those sorts of tests too.
James Lux, P.E.
Spacecraft Telecommunications Section
Jet Propulsion Laboratory, Mail Stop 161-213
4800 Oak Grove Drive
Pasadena CA 91109
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