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Understanding Bridge System Characteristics

Ranges for bridge systems vary as a result of frequency, transmitter power, and available antennas. Ranges of a couple miles or less cover the vast majority of bridge installations; however, longer ranges can be activated with appropriate selection of antennas, clear line of sight, and proper Fresnel zone clearance. (Refer to Chapter 2, "Understanding RF Fundamentals," for an introduction to Fresnel zones.) Note that typically only bridges, not access points (APs), have this extended range capability. The 802.11 specification was based on a presumption that a WLAN communication link (keeping in mind this is defining a local-area network) would be not more than 1000 feet. Therefore, distances for AP-to-client communication are limited to shorter distances for quality performance, irrespective of transmit power, cable, and antenna combinations. This is because timing restrictions in the 802.11 protocol, which synchronize the timing of the communications to support delays induced by the distance. Although most bridges might follow the 802.11 protocols, they do not strictly adhere to the timing parameters, or they have the ability to alter the timing (or distance) capability.

The data rate capabilities of bridges also vary. If a bridge follows one of the 802.11 specifications, the available data rates will be defined by the specifications, and throughput will be similar to a WLAN using the same technology. However, some bridges use proprietary modulation and, although they might be in the same frequency bands as 802.11 systems, they use proprietary channelization, which can enable higher throughput and data rates.

As discussed in Chapter 3, "Regulating the Use of 802.11 WLANs," you must adhere to various regulations when installing WLANs. The same is true for bridging. There are different Effective Isotropic Radiated Power (EIRP) limitations for different countries, as well as differences in available channels and even modulation schemes permitted. Review Chapter 3 for these regulations.