Jeff Duntemanns Drive-By Wi-Fi Guide

A Bridging Strategy

Cooking down my own experience and that of several people I know to a general logistical approach, here is the way to setup your bridge:

• If you've never done a bridge before, or are using unproven Wi-Fi equipment, it's smart to create a 'lab lashup' inside somewhere to prove the configuration out first. Set up two minimal LANs with routers, switches, and laptops or spare desktop machines, even if it's in the same room, and work the bugs out of bridging at the AP level before you go to great effort on the final physical setup, power over Ethernet, etc. It's entirely possible that the two APs you've chosen aren't compatible for some reason, and it's better to find out before you spend hours wedging an AP into a weathertight box and running Ethernet cable up to the roof.

• Once you have the access point configuration nailed, set your antennas up on photo tripods and do a ground-level 'dry run' across a parking lot or other unobstructed open space before hoisting the hardware onto the rooftops (see Figure 16.3). Align the antennas, make the software work, and take good notes.

  Figure 16.3: An Outdoor Lashup for Bridge Testing.

• On the ground or on the roof, mount and align the antennas first. It's hard enough to make two APs talk in bridge mode without troubleshooting a gain shortfall or intermittent alignment at the same time.

• Once you have the software configuration figured out and are passing packets over the air during your dry run, get the APs into their weathertight boxes and do another ground-level dry run over the same path to make sure everything still works. You want your time up on the roof (or, if worst comes to worst, up on a ladder) as short as possible.

• Get your hardware mounted up in the air, and do what you did during your dry run. The hard part here is pointing the antennas down one another's throats, but once you get that done, you should be able to place the APs in bridge mode and be finished.

There is a separate bridging mode called point-to-multipoint, in which one AP can bridge to multiple APs (say, from several different buildings to a central point) but no one I know has ever implemented this, and thus I can provide no useful advice. If you've pulled it off, I'd appreciate a note letting me know how it's gone for you!

Switches, Routers, and DHCP

The wireless bridge will link two physically separated Ethernet networks. However, if the two networks are intended to support the TCP/IP protocols (as they must if you intend to use the Internet and most modern networking software) they really must be configured as a single routable network, under the control of one router, with one DHCP server applying private IP addresses to all connected devices (including the access points) and one Network Address Translation (NAT) server handling intermediation between the private IP addresses used and the routable outside world of the Internet.

I once untangled a bridging scheme for a person I know who couldn't make it work, and it didn't work because he had two routers, one on each end of the link, and both routers were running DHCP servers and handing out private IP addresses in the same range. (If you don't understand IP addressing, routing, and DHCP servers, bail out here and read Chapters 2 and 3 now.)

Both legs of the bridged network must be separately switched or hubbed, but one router can and should handle Internet routing for both. A router/switch combo box like the popular Linksys BEFSR41 can act as a switch, but you have to disable its DHCP server. To minimize hassle, I recommend using a dedicated switch without router features, like the Linksys EZXS55W. These are very inexpensive and require no configuration at all.

All devices requiring IP addresses should be configured to request their IPs from a local DHCP server. This includes both computers and access points, and any other odd network hardware (like dedicated print servers) accessible via TCP/IP. Nearly all modern access points respond out of the box to a specific, preset non-routable IP address (usually in the 192.168.X.X range) but most can be reconfigured to replace the preset address with one from a DHCP server. If you use identical access points and leave both as they are shipped, responding to the same preset IP address, you will have problems!

One final reminder on IP addresses: You may have problems if the preset configuration screen addresses of your router and your APs are not on the same subnet. This can be a problem when you mix devices from different manufacturers. The preset configuration IP address for the Linksys BEFSR41 router is 192.168.1.1, and the preset configuration IP address for the D-Link DWL-900AP+ is 192.168.0.50. If like most people these days your subnet mask is 255.255.255.0, these are not on the same subnet!

What I've done is change the configuration address of my Linksys BEFSR41 router to 192.168.0.1. This is on the same subnet with the D-Link APs and allowed me to log in to the D-Links and change them from a preset configuration address to a dynamic address requested from the Linksys router's DHCP server.