InformationWeek

The devices in the Karlnet group were generally easier to set up. Nearly all the tested products support the Simple Network Management Protocol; most of the vendor-supplied software consists of a basic SNMP interface with a MIB extension.

While the software side of wireless bridging is fairly simple, the physical installation of these bridges certainly isn't. Choosing the right antenna and making sure it's installed correctly is crucial. With the wrong antenna, you'll end up with poor performance as well as inefficient use of available bandwidth.

Certain antennas are designed for straight point-to-point applications, while others are omnidirectional, allowing point-to-multipoint bridging. Antenna installation consists of mounting the antenna-usually on the roof of the building-along with a lightning arrester. Most sites probably will want to use a professional installer to ensure line of sight. To make the task easier, some vendors-Enterasys and RadioLAN among them-provide utilities to get the antenna aimed correctly.

A wireless bridge basically connects two LAN segments. But, similar to high-end wired bridges on the market, the wireless devices we tested offer plenty of additional features for your wireless link.

One of the most helpful features we found was the ability to perform both protocol and broadcast packet filtering. The configuration software that comes with many of these products lets you choose which protocols you want filtered and whether to allow broadcast packets to be bridged. By restricting certain types of Ethernet packets, you can save considerable bandwidth.

Many of the devices we tested also offer an option that lets the bridge act as a router. And one of the most powerful options available on some of the bridges is the ability to do point-to-multipoint bridging. Cisco, Lucent, and Pinnacle offer products that can accommodate more than one radio.

By assigning each radio a different frequency, the capacity of the bridge and even the coverage area can be extended or multiple buildings can be connected to form a metropolitan area network. Point-to-multipoint configurations can be difficult to engineer because radio interference, bandwidth considerations, and other issues must be taken into account. Uniquely, Western Multiplex's Tsunami bridges have the ability to pass T1 traffic over the air.

We found that one of the primary differences among the bridges we tested was whether each device used a PC Card radio or an internal radio. The advantage of the PC Card approach is that it affords the ability to upgrade the radio, with the possibility of increasing the bandwidth or encryption options as budget and technology allow.

In addition, use of a PC Card-enabled device lets you begin with just one radio and eventually expand to take advantage of the point-to-multipoint option.

With the PC Card radio products, however, we noticed generally lower throughput compared with those units that used an internal radio. Also, the prices of the PC Card bridges usually don't include the PC Cards themselves, which must be purchased separately. Prices vary depending on encryption options. The majority of the PC Card-based devices we tested used PC Cards manufactured by Lucent, which explains the similar results we saw in testing their performance.

The IEEE 802.11 standard was developed to ensure interoperability among indoor wireless LAN devices; before the standard's introduction in 1997, you couldn't buy wireless equipment from multiple vendors. Today, issues concerning interoperability haven't been completely resolved, but the situation has improved significantly. IEEE 802.11 has had an impact on the wireless-bridge arena as well.

When implementing a point-to-point wireless bridge between two buildings, there is no specific need for interoperability, though many of the products we examined here adhere to the IEEE 802.11 standard. And compliance with the standard could become an issue if Internet service providers begin to offer wireless connectivity and you wish to own your own wireless bridge or router.

Ironically, IEEE 802.11 can hold back a wireless bridge's performance, in that it introduces a rather large gap between data pack-ets. Because of that gap, the actual data throughput can't realistically be close to the speed advertised. On average, we found that an 11-Mbps device was able to flush out data at around 4 Mbps (half-duplex).