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As discussed in some detail in Chapter 9, Wireless LAN technology has enjoyed modest success in the LAN world during the last few years. Offering the obvious advantage of no wiring costs, Wireless LANs can be deployed to great benefit in a dynamic environment where there is frequent reconfiguration of the workplace. They also offer clear advantages in providing LAN connectivity in temporary quarters, where cabling soon would have to be abandoned. Wireless LANs are largely based on spread spectrum technology refined in World War II for use in radio-controlled torpedoes. This approach offers significantly increased security and throughput [12-3].
Standards are not yet entirely formalized, although the IEEE is examining the development of standards through the efforts of the 802.11 Working Group, which began its efforts in 1989. The emerging standard initially will aim at throughput of 2 Mbps [12-28]. Wireless LANs operate in three distinct radio frequency ranges, as well as on an infrared basis.
902 Mhz to 928 MHz are unlicensed ISM (Industrial, Scientific, Medical) frequencies. This approach avoids expensive and lengthy licensing by the regulatory authorities, but carries the potential for interference from other such systems in close proximity. As this frequency range was set aside by the FCC for unlicensed communications within buildings, such systems are susceptible to interference from other systems including cordless telephones and barcode scanning systems. Spread spectrum technology is generally used at these frequencies to mitigate issues of interference. Because power levels are low, distances generally are limited to 500 to 800 feet. Manufacturers include California Microwave, NCR and Proxim.
2.4 GHz-2.5 GHz and 5.8 GHz-5.9 GHz microwave systems using spread spectrum technology are also permitted to operate without licensing at low power levels and over limited distances. Manufacturers include Western Multiplex Corp. and Xircom. Many of the systems that use licensed frequencies in this range avoid the potential for interference, but do require that the manufacturer carefully police the deployment of such systems under the terms of an omnilicense. Alternatively, these frequencies can be used without licensing, if they are low-power and use spread-spectrum coding.
18 Ghz to 19 GHz are sometimes employed in a wireless LAN environment at low power levels. The same frequencies are used in commercial microwave systems offering the potential for interference unless spread-spectrum coding is employed. Manufacturers include Microwave Radio Corp. and Motorola.
Infrared light systems require no licensing. The potential for interference between systems is very limited and line-of-sight is required. Transmission rates of 4 Mbps are common, with some systems providing transmission of 10 Mbps and 16 Mbps. Typical applications include interbuilding connectivity. Manufacturers include A.T. Schindler Communications, InfraLAN Technologies, Laser Communications, Radiance Communications, and Spectrix Corp.
While Wireless LANs have gained a foothold in United States, their future is less certain elsewhere. Traditionally, DECT has been endorsed by ETSI as the standard, providing up to 1.14 Mbps. However, the 2.4 GHz (U.K.) and 18 GHz (Germany) bands are also being promoted. The concern of ETSI and the EC seems to be that the use of these frequencies will favor U.S. manufacturers.
An interesting application for Wireless LANs is that of certain grocery stores in northern California. Some shopping carts are equipped with wireless terminals which communicate with servers through hub antennae using unlicensed frequencies and spread-spectrum technology. The shopper can key into the terminal the general categories of items on the shopping list and be guided through the store with the aid of a map displayed on the terminal screen. Scattered throughout the store are special in-store coupon offers that can be accepted through wireless acknowledgement. Once the shopper reaches the checkstand, the coupon acknowledgement is communicated to the intelligent point-of-sale device (cash register) and is credited against the purchase, without any paper changing hands.
The worldwide local loop market is estimated at $100 billion, most of which is still twisted-pair. The low cost of installing and maintaining Wireless Local Loops is likely to position it as a strong competitive threat into the future. Nortel (nee Northern Telecom) estimates that the cost of copper local loops in North America ranges from $1,200 to $1,300 per residence, and can run as high as $5,000 where terrain is especially difficult or where population density is low. Nortel also estimates that WLL can be provided for no more than $900 per subscriber, with the added advantages of much more rapid deployment and much lower operating costs (25% less than copper), which include repair, reconfiguration, and loop testing.
As discussed briefly in Chapter 10 and illustrated in Figure 12.9, a WLL configuration involves either wired or microwave links to the Central Office from the low-power, omnidirectional antennae. Each antennae covers a relatively small geographic area, such as a neighborhood. Multiple channels are provided through frequency separation and time division multiplexing. The channels can be narrowband in support of voice-grade services, or can be as large as T1 in support of PBX trunking.
Figure 12.9 Wireless Local Loop (WLL) configuration.
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