[Beowulf] Wireless clusters (Bridging 802.11a)

Jim Lux 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 
node/NFS, etc.

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
tel: (818)354-2075
fax: (818)393-6875

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