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Wireless Local Area NetworksBog'liq garg200721.4.1 Infrared
Technology
Infrared is an invisible band of radiation that exists at the lower end of the visible
electromagnetic spectrum. This type of transmission is most effective when a clear
line-of-sight exists between the transmitter and the receiver.
Two types of infrared WLAN solutions are available: diffused-beam and
direct-beam (or line-of-sight). Currently, direct-beam WLANs offer a faster data
rate than the diffused-beam networks. Direct-beam is more directional since
diffused-beam technology uses refl ected rays to transmit/receive a data signal.
It achieves lower data rates in the 1–2 Mbps range.
Infrared is a short-range technology. When used indoors, it can be limited
by solid objects such as doors, walls, merchandise, or racking. In addition, the
lighting environment can affect signal quality. For example, loss of communica-
tion may occur because of the large amount of sunlight or background light in an
environment. Fluorescent lights also may contain large amounts of infrared. This
problem may be solved by using high signal power and an optimal bandwidth
fi lter, which reduces the infrared signals coming from an outside source. In an
outdoor environment, snow, ice, and fog may affect the operation of an infrared-
based system. Table 21.3 gives considerations for choosing infrared technology.
21.4.2 UHF Narrowband Technology
UHF wireless data communication systems have been available since the early
1980s. These systems normally transmit in the 430 to 470 MHz frequency range,
with rare systems using segments of the 800 MHz range. The lower portion
of this band — 430–450 MHz — is referred to as the unprotected (unlicensed), and
450–470 MHz is referred to as the protected (licensed) band. In the unprotected
band, RF licenses are not granted for specifi c frequencies and anyone is allowed to
use any frequencies, giving customers some assurance that they will have complete
use of that frequency.
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