Bridging Products

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Vendors offer specifically designed wireless bridging products, often referred to as point-to-point Ethernet bridges, which are capable of spanning distances of 30 miles (48 kilometers) or more. The sweet spot is a cost-effective 11 Mbps link that spans only a mile or so. Such products can not only provide better performance than using a T-1 line, they can also provide system reliability that approaches 99.999 percent.

It's easy to find a point-to-point bridging system for less than $1000, although most in this price range operate at 2.4 GHz and leverage commodity WLAN chipsets to deliver a maximum data rate of 11 Mbps. While these systems can be appealing, they often lack features important for enterprise implementations, including Power-over-Ethernet and flexible management and monitoring capabilities. In addition, because they're based on LAN protocols, they tend to require higher overhead, resulting in throughput that is as low as half the stated rate. Finally, the duty cycle of these products often fails to meet enterprise standards.

That's why many enterprise-grade bridging products are migrating toward the 5 GHz bands. These systems are designed typically from the ground up for fixed wireless applications. As such, their feature sets, performance, and overall reliability are better than lower-cost alternatives. The downside is that many of these systems will cost more than $10,000, though lower-cost options are emerging.

When considering total system cost, factor in not only the cost of the wireless bridges and antennae, but also the costs of components, installation charges, and maintenance required for a production system. In the past, system integrators have charged a healthy premium for installation and maintenance, though the trend these days is toward self-installation, at least in enterprise environments where adequate expertise exists.

Although all 2.4 GHz and 5 GHz bridging products discussed in this section operate in unlicensed spectrum, they still must adhere to rules defined by the FCC and other international regulatory bodies. The chart in Figure 9.4 shows some of the key regulations in those bands, but also see Appendix III: Regulatory Specifics re Wi-Fi.

Figure 9.4: FCC Regulations for 2.4 and 5 GHZ unlicensed radio bands. (In Europe, the ETSI is the regulatory body that regulates the Antenna/Radio power output and system range).

Finally, it's worth noting that wireless point-to-point systems aren't always based on radios. Second-generation FSO (free space optical) systems based on lasers and LEDs are an option for some applications. These systems provide significantly greater bandwidth than radio systems, but some may be vulnerable to adverse weather conditions. Vendors are hard at work trying to overcome these vulnerabilities.

5 GHz Products

Why 5 GHz? First, these devices may be less vulnerable to interference than those in the 2.4 GHz band, where wireless LANs and devices such as cordless phones and microwave ovens spew RF signals in all directions. Next, 5 GHz offers more than three times as much bandwidth as 2.4 GHz. That extra bandwidth not only lets applications run at higher data rates, it offers added flexibility to deploy multiple systems at a single location and to move to alternate channels should interference occur. The downside? You may have to sacrifice a little range, and 5 GHz products tend to be more expensive than 2.4 GHz products.

Network Computing magazine tested six different 5 GHz bridging products in late 2002 (see Fig. 9.5). All were worked as wireless Layer 2 bridges. In an age where Layer 3 switches and routers dominate the market, you may wonder why these products still run at Layer 2. The positive spin is that with a point-to-point system, you don't really need the sophisticated traffic-management capabilities of Layer 3 devices; also, many point-to-point systems link to backbone routers.

Figure 9.5: In late 2002, Network Computing magazine tested a group of 5 GHz Ethernet bridges. This chart lays out the results of those tests. The underlying radios and modulation systems varied considerably from product to product. In most cases, it's a trade-off between price and performance. Packing more bits into each clock cycle requires more sophisticated radio technology-and you'll pay for that luxury. Because regulations vary by sub-band at 5 GHz, you'll get longer range from products that operate in the 5.8-GHz UNII-3 sub-band.

So just how much geography can you cover with the 5 GHz systems? The vendors whose products Network Computing tested claimed ranges from 1 to 15 miles (1.6 to 24.1 kilometers), and while more expensive offerings are available from some vendors that could extend the range even farther, the products tested hit the sweet spot for most organizations. The maximum distance supported by the various bridging products vary depending on radio output power, receive sensitivity, and antenna size used. (With amplification some can send quality signals for 30 miles / 48 kilometers).

An interesting trend, visible in some of the products referenced in Fig. 9.5, involves integrating the antenna and modem into a single weatherproof enclosure. This simplifies installation and increases the effective system range by eliminating RF cabling, which is a source of significant signal loss. An alternative that yields similar benefits is to co-locate the modem and antenna on a single antenna mast; this strategy allows flexibility of antenna selection-and even more range-but it is more complex.

What about speed? All of the 5 GHz products provided enough speed to handle mainstream business applications. In the Network Computing list, among the six products tested, throughput ranged from a speedy 7 Mbps to a whopping 78 Mbps.

Finally, you should understand that most of today's 5 GHz bridging systems are based on custom radio implementations. However, it is likely that future products will leverage the cost-economies of commodity 802.11a chipsets. Standards are critical for WLAN products, but less important for point-to-point systems because interoperability is not a critical requirement. In fact, a lack of interoperability can be a benefit to the extent that proprietary radio designs and modulation schemes add to system security.

2.4 GHz Products

If you don't need the higher bandwidth, quality 2.4 GHz wireless bridge products are offered by vendors such as BreezeCom, Cisco Systems, Enterasys Networks, Lucent Technologies, Pinnacle Communications, and RadioLAN. These wireless bridges are best suited to joining small to midsize networks that don't have high bandwidth requirements. That is because the throughput delivered by such products is not sufficient for networks that need to send large amounts of data over the bridged link.

Note 

A word about wireless bridges and the 802.11 standards. These standards don't directly address wireless bridging, although they have had an impact on the wireless-bridge arena. Ironically, IEEE 802.11 can hold back a wireless bridge's performance, in that it introduces a rather large gap between data packets. Because of that gap, the actual data throughput realistically can't be close to the speed advertised. On average, an 11 Mbps device is able to flush out data at around 4 Mbps (half-duplex). Thus, not all bridging products strictly adhere to the IEEE 802.11 standards. So while, for example, Lucent and Pinnacle's devices, which use nearly identical management software, stay true to the 802.11 standards, other bridging products use varying methods and technologies to tackle the issue of transferring packets-many make some sort of effort to stick to the 802.11 standards, but others employ proprietary techniques.

In general, configuration involves setting each device with an IP address via software that came with the device, or a serial connection. Nearly all of the devices offered by the above-mentioned vendors support the Simple Network Management Protocol (SNMP), and most of the software consists of a basic SNMP interface with a management information base (MIB) extension.

One of the primary differences among the above-referenced bridges is in regard to whether the device uses a PC Card radio or an internal radio. The advantage of the PC Card approach is that it affords the ability of a radio upgrade, along 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 PC card radio products, however, you might notice generally lower throughput compared with those units that use an internal radio. Also, the price of the PC-card bridge usually doesn't include the PC card itself, which must be purchased separately. Prices also vary depending upon encryption options.

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