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.
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