Bridging 802.11 Networks with Linksys

by Glenn Fleishman

My downstairs office neighbor, a running coach named Tony, has an inquiring mind. He has heard me and my officemates wax sci-fi about wireless IEEE 802.11b networking. We somehow got him jonesing for it for when he expanded his coaching facilities to a second office in a building about 30 feet away, to which a wired extension to our shared network wasn't possible.

The new office is embedded bunker-like into a concrete foundation below ground, as well as having thick walls in its above-ground portion. Otherwise, we could have hung his new computers as wireless devices on our existing 802.11b network. He also has some legacy equipment that needs an Ethernet hub, so couldn't go entirely wire free.

We needed a solution that would allow us to extend our high-speed Internet service as well as the rest of the intranet in our existing building to Tony's new office 30 feet away. We wanted to bridge the gap wirelessly, making one seamless combination of wired and wireless networks. I'd been following discussions on the Bay Area Wireless User Group's (BAWUG) mailing list about the Linksys WAP11 access point (AP), and thought a pair of them might do the trick.

At a street price (with manufacturer's rebate) of about $185, this AP supports a firmware upgrade that turns it into the bargain wireless bridge of the century. Comparable devices from vendors such as Cisco can run $800 or more.

The Wireless Access Point

Typically, an access point acts as a central hub, router, bridge to Ethernet, and server for dynamic host configuration protocol (DHCP) and network address translation (NAT), as well as other functions. The standard home gateways often have a wide-area network (WAN) Ethernet port (to connect to a DSL or cable modem) and one or more local area network (LAN) ports for the local network.

The access point negotiates with wireless computers and other devices, hands out non-Internet-reachable NAT addresses via DHCP, and bridges traffic from the wireless LAN to the wired LAN and, through it, out to the Internet. (Home gateways with a WAN port bridge the traffic internally and send it all the broadband router.)

The Linksys WAP11 with its single Ethernet port handles all this perfectly fine. It has minimal but sophisticated options for set up, and removable antennas with standard connectors so that you could hang a higher-gain antenna off the back - important for connecting networks over longer distances.

Upgrading to build a bridge

But the bridging firmware makes the WAP11 more than just cheap and functional. In bridging mode, you can connect two or more WAP11s together to pass wired Ethernet traffic among one another, creating a super-network. If you have multiple facilities nearby or with line of sight between them, you can avoid telephone company digital line charges, as well as recurring fees for separate Internet connections in each facility.

Linksys's firmware upgrade 1.4f5, which supports bridging, was released, pulled and then re-released; the version number stayed the same. The two WAP11's I purchased for our multi-office installation came with release 1.3i installed; reports indicate that most units are still being shipped with the older firmware. Initially, the 1.4f5 upgrade utility failed on 1.3i installations, according to BAWUG list contributors, so apparently that's been fixed.

The WAP11 comes with a USB connection for configuration using a Windows-only application. It's critical to be able to use this, as once you've configured the unit, unless you give it a real IP number, you're not going to be able to reach it for reconfiguration without disconnecting it from the wired network.

Linksys provides an SNMP-based package as well, so an enterprising wireless advocate could release a pure SNMP configurator, too, for Linux or other platforms. I'm insane enough that, although I own a PC for testing, I ran Connectix Virtual PC with Windows 98 on a Mac Cube and had no problems accessing the unit over USB.

The firmware updater first installed 1.3k, not separately available, and then 1.4f5. It reported a failure on the latter, but on rebooting the units and reconnecting -- requiring a few USB plugs and unplugs -- the firmware version was reported correctly by the software and the units configured fine. (Oddly, the SNMP software shows the full firmware versioning information, while the USB shows just a fragment.)

You can link two WAP11's together by using each device's unique Ethernet or MAC (Media Access Layer) address. (It has nothing to do with Macintoshes; it's an acronym.) The USB and SNMP software displays the MAC address, a set of six two-character hexadecimal digits. You can also set up the WAP11 to talk to one or more identical devices using multipoint transmission; this also works if you don't want to enter the MAC numbers.

The two or more WAP11's need to be on the same 802.11b channel. The 802.11b protocol divides the available spectrum -- which can vary by country -- into overlapping channels of 22 MHz each. In the U.S., we have 83.5 MHz overall to play with, and 11 channels, numbered 1 through 11. Of those 11 channels, 1, 6 and 11 don't overlap at all, allowing as many as three access points to operate in the vicinity of one another.

Because the two WAP11s share the same channel, this contributes -- along with Ethernet overhead -- to reducing net bandwidth to about 4Mbps, according to Sam Habash, a BAWUG list contributor. The speed is close to the same with or without using 802.11b's built-in WEP (Wireless Equivalent Privacy) encryption system.

The bridging configuration allows WEP encryption keys at either 40/56/64 (technically all the same) or 128 bits. Even though WEP is weak and generically compromised, I'd recommend turning it on - especially since you're purposely broadcasting between multiple locations. This can reduce throughput, too.

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