Bridges: Taking wireless to the wide area

Wireless 802.11b bridges can stretch the meaning of the word “local” and give business users unexpected flexibility in extending corporate LANs.

An 802.11b bridge essentially is a media access control-level wireless access point that’s been configured slightly differently. Bridges, as the name implies, simply pass incoming wireless packets to one or more corresponding bridges. With these wireless bridges, network executives quickly and inexpensively can extend a wireless LAN by several hundred feet to a mile or more. With specialized antennae, the distance could be up to 5 or 6 miles.

Almost all wireless LAN vendors offer these products. Cisco Systems Inc., Agere Systems Inc., Enterasys Networks Inc. and Symbol Technologies Inc. are among the leading suppliers, but others that offer very inexpensive bridges include LinkSys Group Inc. and Breezecom. Prices range from US$400 to almost US$2,000. Some come with general-purpose antennae, others without, so businesses can add specialized, high-gain antennae for longer distances or tightly focused transmission beams. These antennae can add US$300 to US$1,000 to the bridge price.

These wireless connections traditionally are an alternative to leased, wired links, such as T-1 lines, eliminating the monthly telephone company charges and wait for activation, which some users report can take weeks or even months. Bridges, as with 802.11b access points, typically have an actual throughput of 3Mbps to 5.5Mbps, well above the 1.54Mbps of a T-1.

In Enfield, Conn., Charlene Bond, the town’s director of IT, estimates the municipality saves about US$3,000 per year in recurring telco charges by using the bridges. The town has five Enterasys wireless bridges. These fit the traditional usage pattern: They form a wireless connection, using the unlicensed 2.4GHz band, between two or more wired LANs.

But the municipality now is experimenting with wireless LANs: one in a historical building where network cabling could damage or disfigure architectural features, and another that will let wireless laptop users connect to the network in various conference rooms. In both cases, the bridges can tie these users into the town’s applications.

“It’s been very reliable and very adequate [in bandwidth],” Bond says.

Out With the Old

In some cases, 802.11b bridges replace older, more costly and proprietary high-bandwidth microwave (often dubbed “fixed wireless”) bridges. Fixed wireless vendors, such as Wi-Lan and RadioLAN, claim to at least double the actual throughput over longer ranges, for more users, with higher security. RadioLAN has just introduced its Campus BridgeLink-Lite, which uses the same radio technology as the other BridgeLink products but is simpler to install. It’s aimed at enterprise applications. Devices can be separated by about a mile, and at less than US$2,000, it competes with the 802.11b bridges.

Distances vary greatly for 802.11b bridges. Some network integrators say that in mountain ranges, with high-gain antennae, they can forge connections nearly 40 miles long. Cisco recommends a maximum distance of about 18 miles. But in the majority of cases, installations are less than 3 miles. They even can be used within buildings or building complexes, such as auto assembly lines and shipping centres.

One version of in-building bridging is the “workgroup bridge,” which can group up to eight client devices and wirelessly connect the devices to an 802.11b access point.

“The devices are wired to the bridge, which acts like a wireless [network interface] card for printers, older computers and other devices,” says Kenny Blankenship, network consultant with InterNetwork Experts, an Addison, Tex., network integrator specializing in Cisco wireless LANs.

Wireless LANs in a large warehouse could hop via a bridge across a street to connect to a wired administrative network, without the need to dig up the street or a parking lot to lay cable.

This extensibility has some surprising payoffs. The wireless network can become portable: It can be set up in temporary locations at disaster sites, exhibition halls or a parking lot, and bridged to wired networks. In what are being dubbed “town-area nets,” bridges link so-called “hot spots” – wireless LANs at a town hall, a recreational centre, the local library, the highway department and the like.

GlobalNet is a wireless ISP in Boseman, Mont., that exploited this extensibility. The company worked closely with Symbol to modify one of Symbol’s bridges and fine-tune the 802.11b wireless protocol for outdoor use. The result is a ready-to-install wireless bridge that mounts on the building or roof of a business or residence. A Category 5 cable runs from the bridge inside to an Ethernet router, for a wired LAN, or an access point, for a wireless one.

In effect, GlobalNet uses 802.11b as the wireless “last-mile” connection for its network customers, says Scott Johnson, GlobalNet’s CEO. The customer-mounted bridges connect to another bridge, often on a grain elevator in the Midwest. Eventually, several of these connections terminate at a managed switch. Finally, they are brought to a terrestrial T-1 or T-3 link at a consolidation site owned by carriers such as AT&T or WorldCom, bypassing the need for local-loop fees paid to local exchange carriers.

But reaping these benefits can take some work. Deploying multiple bridges requires some care, especially in more complex networks, says Yangmin Shen, director of technical marketing for Symbol.

Complicated Planning

Bridges can complicate bandwidth planning, he says. Wireless bandwidth decreases as distance increases. If customers aren’t careful, Shen says, they could create a choke point, where connections to two access points (and hence with a theoretical total of 22Mbps incoming) could overwhelm the 11Mbps link outgoing to a third access point.

“Your throughput is limited by the weakest [slowest] link,” Shen says. “You have to be smart about how to relay your data.” Unless you’re careful, you could inadvertently create a “loop” configuration in which broadcast traffic circles the network endlessly.

Another deployment issue is that 802.11b wireless is half duplex: In bridging, you have to choose the wireless channels carefully because when Point Y is transmitting to Point Z, the latter can’t be transmitting to the former at the same time. “Switching directions during transmission will cause delays,” Shen says. “You have to factor that in.”

Delays also occur with every wireless “hop.” If a packet makes too many hops, the time (or latency) required to get from one end of the network to the other will cause the Ethernet packet to timeout, with transmissions being incomplete. This is especially true if you’re hopping wireless LANs as if they were beads on a string. One solution is to reconfigure the network so that a bridge can act like a hub, with wireless access points radiating out from it like spokes on a wheel. In such a configuration, each endpoint makes only one hop to any other endpoint.

Another solution is to use a bridge that is “compatible” instead of “compliant” with 802.11b. Like GlobalNet and Symbol, Cisco increased the time it takes for an 802.11b packet to timeout so that its Aironet 350 bridge can reach farther to wireless clients, says Ron Siede, product line manager with Cisco’s wireless network group.

A wireless bridge requires a line-of-sight path to its companion bridge. That line of sight can be cluttered by foliage and plant or tree growth in the summer. It also is more difficult for 2.5GHz signals to get through foliage that is wet or that has high water content.

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