Examples of Wireless Bridging

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The following examples may give the reader some idea of how to take advantage of the benefits of wireless bridging. For additional ideas on how to use bridging technology, read the case studies at the end of this chapter.

To bridge buildings: Enterprises often use bridging to connect networks in different buildings within a corporate campus. Bridging access points are typically placed on top of buildings to achieve greater antenna reception. The typical distance over which the average AP with the typical antenna set-up can be connected wirelessly to another by means of bridging is approximately two miles (3.2 kilometers), although with the help of specialized equipment the distance can be extended to around 30 miles (48 kilometers). But even the two mile range will vary, depending on several factors including the specific receiver or transceiver being used.

3Com, one of the premier providers of Wi-Fi products including wireless bridging gear, offers a good hypothetical wireless bridging scenario. The principal of a local high school is pleased about a new addition to the school, which when completed, will encompass three computer labs, 15 classrooms, and more. But the projected completion date for the new addition is two years down the road. Classrooms are bursting at the seams now and more students are on the way. That means that the principal must find some way to network 50 new computers now, and not break the budget. The principal determines that by using wireless building-to-building bridging techniques, it is possible to turn one of the school's three new portable classrooms into a computer lab, and give the other two classrooms network access without running cables. One bridge is connected to the school network and another to a 3COM access point that was located in the portable computer lab. Now all of the new computers, including those in portable classrooms, have access to all necessary network resources, including the Internet.

Redundancy: An often-overlooked WLAN application is the use of wireless bridge links as a redundant, alternative path to the wired (fiber optic and T-1) network backbone. If the organization experiences disruption in its wired network services because of, say, a cut fiber optic cable or a failed building-to-building data T-1 transport line, a redundant wireless bridge link can take up the slack. Just remember that the redundancy is only for when the backbone service is interrupted in building-to-building connections, not when the service fails at the source or beyond the physical confines of the organization's facilities.

The benefit of wired cabling, such as fiber optics, is that it can provide data connectivity for approximately 1.25 miles (2 kilometers), whereas conventional copper cabling should not exceed 328 feet (100 meters) and three hops through repeater hubs. Thus fiber optics is often the backbone of choice for building-to-building connections. However, since fiber optic cabling often runs extensively through campus and industrial environments, it can be the victim of repeated service interruption due to construction or digging. Also, as luck would have it, redundant fiber optic link will often run in the same cable bundle as the primary link; thus, when the primary link is interrupted, it is highly likely that the redundant will also be lost. A wireless building-to-building bridge would effectively solve such a situation. Similarly, a wireless link can serve as a backup for an on-campus T-1 connection.

There are a couple of caveats to redundant wireless bridging:

• Networks using redundant bridge links must support Spanning Tree Protocol (STP) because STP constantly monitors the network for service interruptions and turns ports, switches, and bridges on and off to block and send traffic.

• Wireless bridge links require clear line of sight between the buildings connected by the redundant wireless pathway-obstructions, such as trees (which can be bare during winter months but overflowing with signal-blocking foliage other times of the year), can render such a pathway useless.

However, once the wireless bridge link is installed, some organizations may get rid of their wired building-to-building lines and install a second redundant wireless bridge. By eliminating recurring costs of, say, a T-1 line, an organization could save as much a $1500 per month. Additionally, any 802.11 (a, b or g) wireless bridge link is many times faster than a T-1's 1.5 Mbps connection.

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