The technologies available for use in a WLAN include IR, UHF (narrowband), and SS implementation. Each implementation comes with its own set of advantages and limitations.
IR 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 IR 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 reflected rays to transmit/receive a data signal. It achieves lower data rates in the 1 – 2 Mbps range.
IR 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 communication may occur because of the large amount of sunlight or background light in an environment. Fluorescent lights also may contain large amounts of IR. This problem may be solved by using high signal power and an optimal bandwidth filter, which reduces the IR signals coming from an outside source. In an outdoor environment, snow, ice, and fog may affect the operation of an IR-based system.
UHF Narrowband Technology
UHF wireless data communication systems have been available since the early 1980s.
These systems normally transmit in the 430 – 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 specific frequencies and anyone is allowed to use any frequencies, giving customers some assurance that they will have complete use of that frequency.
Because independent narrowband RF systems cannot coexist on the same frequency, government agencies allocate specific RFs to users through RF site licenses. A limited amount of unlicensed spectrum is also available in some countries. In order to have many
frequencies that can be allocated to users, the bandwidth given to a specific 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 or 25 kHz. Power levels range from 1 to 2 W 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 SS systems, which have lower power levels and wider bandwidths. Table 5.4 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 ability to install equipment without the complexity of hardware crystals. Common equipment can be purchased and specific UHF frequency used for each device can be tuned based upon specific 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 configured 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.
Spread Spectrum Technology
Most WLANs use SS technology, a wideband RF technique that uses the entire allotted
spectrum in a shared fashion as opposed to dividing it into discrete private pieces (as with
narrowband). The SS system spreads the transmission power over the entire usable spectrum. This is obviously a less efficient use of the bandwidth than the narrowband approach. However, SS is designed to trade off bandwidth efficiency for reliability, integrity, and security. The bandwidth trade-off produces a signal that is easier to detect, provided that the receiver knows the parameters of the SS signal being broadcast. If the receiver is not tuned to the right frequency, a SS signal looks like background noise.
By operating across a broad range of radio frequencies, a SS device could communicate clearly despite interference from other devices using the same spectrum in the same physical location. In addition to its relative immunity to interference, SS makes eavesdropping and jamming inherently difficult.
In commercial applications, SS techniques currently offer data rates up to 2 Mbps. Because the FCC does not require site licensing for the bands used by SS systems, this technology has become the standard for high-speed RF data transmission. Two modulation schemes are commonly used to encode SS signals: direct sequence SS (DSSS) and frequency-hopping SS (FHSS).
FHSS uses a narrowband carrier that changes frequency in a pattern known to both transmitter and receiver. Properly synchronized, the net effect is to maintain a single logical channel. To an unintended receiver, FHSS appears to be a short-duration impulse noise.
DSSS generates a redundant bit pattern for each bit to be transmitted. This bit pattern is called a spreading code . The longer the code, the greater the probability that the original data can be recovered (and, of course, the more bandwidth will be required). To an unintended receiver DSSS appears as low-power wideband noise and is rejected by most narrowband receivers.
Source of Information : Elsevier Wireless Networking Complete
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