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21 Wireless Local Area Networks
Because independent narrowband RF systems cannot coexist on the same
frequency, government agencies allocate specifi c RFs to users through RF site
licenses. A limited amount of unlicensed spectrum is also available in some coun-
tries. In order to have many frequencies that can be allocated to users,
the band-
width given to a specifi c user is very small.
The term
narrowband
is used to describe this technology because the RF
signal is sent in a very narrow bandwidth, typically 12.5 kHz or 25 kHz. Power
levels range from 1 to 2 watts for narrowband RF data systems. This narrow
bandwidth combined with high power results in larger
transmission distances
than are available from 900 MHz or 2.4 GHz spread spectrum systems, which
have lower power levels and wider bandwidths. Table 21.4 lists the advantages
and disadvantages of UHF technology.
Many modern UHF systems are synthesized radio technology. This refers
to the way channel frequencies are generated in the radio.
The crystal-controlled
products in legacy UHF products require factory installation of unique crystals
for each possible channel frequency. Synthesized radio technology uses a single,
standard crystal frequency and drives the required channel frequency by dividing
the crystal frequency down to a small value, then multiplying
it up to the desired
channel frequency. The division and multiplication factors are unique for each
desired channel frequency, and are programmed into digital memory in the radio
at the time of manufacturing. Synthesized UHF-based solutions provide the abil-
ity to install equipment without the complexity of hardware crystals.
Common
equipment can be purchased and specifi c UHF frequency used for each device
can be tuned based upon specifi c location requirements. Additionally, synthesized
UHF radios do not exhibit the frequency drift problem experienced in crystal
controlled UHF radios.
Modern UHF systems allow APs to be individually confi gured
for operation
on one of the several preprogrammed frequencies. Terminals are programmed
with a list of all frequencies used in the installed APs, allowing them to change
frequencies when roaming. To increase throughput,
APs may be installed with
overlapping coverage but use different frequencies.
