The arlan card is an ISA card, so nearly any old PC will do. 16MB of RAM is a good idea. Since, at least in this howto, you will be using the LRP code, you don't even need a hard disk. In general I prefer PCI hardware and Decchip tulip based cards, but we went with PCI based ne2000 clones for the network cards because they only cost 6 bux each.
So get a pair of P75 or better PCs, with 16MB of ram, a network card or two, and you're set.
Install the arlan cards (you might need to change their interrupts - these cards are not plug and play) and run their DOS based setup drivers. You'll need to pick a suitable network code, and spreadingCode. The networkcode is some random 6 digit number. We generated ours by dropping a cat on the numeric keypad, your methods may vary. It needs to be the same on both cards. The spreading Code is for 384Kbaud, 1Mbit, and 2Mbit operation. As a rule of thumb I'd suggest:
Be conservative. Your goal in life at this point is to make the thing work, not make it fast. You can always go back and make it go fast later.
If you picked out a different network card than the NE2000, you're going to have to download the linux router project's code, an appropriate kernel, and install the proper modules on the lrp disk. This can be a complex procedure, so here's Greg's notes on the topic.
The last issue is the registrationMode option. The current wireless card driver supports both point to point and Accesspoint mode as a client only. If you want mobile machines to connect to multiple antennas, you still need to use the Access point routers from an aironet reseller for the center locations. You can (in theory) continue to use the Linux software for your clients. Access Point mode is a proprietary mode not supported by this driver. It supports mobile IP (802.11) & connecting multiple machines via a single antenna with fairly strong authentication. Unfortunately the registration packet frame types are currently und ocumented. A registrationMode of 1 puts the card into Access Point mode.
Point to point (which is what you'll be using) has a registration mode of 0. It is possible to connect multiple nodes to a single antenna if you use host routing.
When you insert the driver module into the linux kernel, you should have a command line of:
insmod arlan registrationMode=0 spreadingCode=3Or whateveryou've chosen as your spreadingCode earlier in this document.
OK. Your wireless boxes are built. Time to start testing! Start off with the "rubber Duckie" antennas that come with the card. You need to configure LRP, which is not something we can easily help you with just yet. (note, although this document deals with the LRP code, it is much easier to setup conventional Linux boxes to do this job. Once the arlan module is inserted with the proper parameters, it looks just like an ethernet card to the rest of linux and you manage it with the same tools you are familiar with - ifconfig, route, etc. )
Once you can ping and telnet through each router at your local site, it's time to be ambitious with directional antennas, cabling, and the rest of the hardware hacks.
No matter how bad your cable or connectors are, even a bad connection acts as a short range whip antenna. Pick some site that is easy to see, at some distance more than a 1/2 mile, and setup your gear to talk to it. A diskless box eats very little power, so what we did was pack up the downstream router in the car, attached to a small UPS, and set up shop about 3 miles away at the local watering hole. You should be able to aim your antennas by eye at this distance. Get everything set up, and if you can ping with low latency and 0 packet loss, your gear is working perfectly.
If you can weather proof your router box, and run it, power, and ethernet outside, close to the antenna, more power to you. Short antenna cables are a good thing. However, we've noticed that the arlan radios appear to be tempurature sensitive - if the ambient temp gets above 38c or below 10c performance will start to drop.
Putting the router outside was not an option, so we built an 11' cable, mounted the wireless lan antenna on the TV arial, and drilled a hole in the side of the house to run the cable through. We ran power and a UPS up into the attic, and put the box up there.
We figure that it will stay warm enough through the winter, and we'll worry about the summer heat when the summer comes. Tenatively we're going to add a fan and switch to booting off a flashdisk in the springtime. We recognise that this is not a very scientific approach to the heat problem.
These numbers were achieved using an older version of the driver. Elmer Joandi claims that even a 386sx should now be able to run at full throttle with his latest driver.
As far as serial consoles go, I've allways liked having a single basic plug on a network device, and its alot easier to lug a laptop or connect a null modem than to plug up a monitor and keyboard. some situations require this (like if the system just refuses to boot), however most of the time a serial console set up on the LRP box will get you in (regardless of how badly you hosed the network). Our downstream box doesn't even have a video card.
It's hard to fit Linux on a single floppy. You get around this by installing tftp on the routers, and grabbing things like snmp from tftp servers and installing from there.
Over the network backups are easy once you have tftp installed.
tftp> put /dev/fd0 router.img