Building-to-building Design
Installation considerations

When planning a wireless bridge deployment, care must be taken to select the best products. Things to consider include the following:

  • Bridge features, such as spanning-tree protocol or VLAN support
  • Distance and data rates needed
  • Optional antennas, to increase distance
  • Outdoor considerations, such as lightning arrestors
  • Sealing the coax connections

Typical considerations for bridges include the distance it will cover, the speed it will operate at, and the number of users it can support. One item that is very deceiving is the data rate. As with the LAN systems, data rate indicates how fast the RF passes data. This RF data includes the radio system overhead and the network data. The real item that should be discussed is throughput. This is the actual amount of network data that gets passed from one LAN to another. Remember that a higher data rate does not mean higher throughput.

The data rate of wireless bridges can be set to speeds of 1, 2, 5.5, and 11 Mbps. Reducing the speed increases the maximum distance that can be obtained, while increasing the speed lowers the maximum distance . Filtering can increase actual performance over the RF by eliminating unnecessary traffic. This has the same effect as increasing throughput. The number of users the bridge can support depends of the type of traffic that is being handled. Throughput is the real limiting factor.

Another consideration that can affect the distance and the data rates is the antenna choice. Cisco offers several directional, long-range antennas. The Yagi is a small, 46 x 8 cm (18 x 3 in.), and lightweight, 0.68 kg (1.5 lb), antenna that can be used for ranges up to 11.7 km (7.62 miles) at 2 Mbps, and up to 5.8 km (3.63 miles) at 11 Mbps. The solid dish is the best structural dish antenna on the market. It will withstand icing and winds over 117 kilometers per hour (110 miles per hour). It will allow 2 Mbps operation for up to 40 km (25 miles) and 11 Mbps operation for up to 33 km (20.52 miles). ,

When an antenna is installed outside of the building, there is a chance that it could be struck by lightning. Because of the extreme voltage associated with a lightning strike, the current could travel into the network, using the antenna, extension cable, and the Category 5 cable as a path. Once the current is on the Category 5 cable, it could travel throughout the entire network and damage any equipment connected to the cable. The best protection against a direct strike is fiber optic cabling. The Cisco Aironet lightning arrestor will not stop a direct strike. Because the conductor in fiber optic cabling is glass, the current cannot travel over the fiber. The energy is dissipated as heat and melts the fiber optic cabling .

Lightning does not need a direct hit to cause problems. An indirect hit can induce enough energy into the cable and antennas to cause damage to the bridge and other network devices. This is where a lightning arrestor can help. A lightning arrestor has two main purposes. The first is to remove any high static charges that collect on the antenna. This will prevent the antenna from attracting a lightning hit. The second purpose is to dissipate any energy that gets induced into the antenna or coax from a near lightning strike .

It is also necessary to seal the coax connectors. This is to prevent water from entering into the connectors. If water gets into the connectors, it will work its way up the coax, contaminating it and rendering the coax unusable.