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LANs are used almost exclusively for data communications over relatively short distances, such as within an office, office building or campus environment. LANs allow multiple workstations to share access to multiple host computers, other workstations, printers and other peripherals, and WAN connections. LANs increasingly are used for imaging applications, as well. They are also being used for video and voice communications, although currently on a very limited and generally unsatisfactory basis.
LAN applications include communications between the workstation and host computers, other workstations, and servers. The servers may serve to provide the workstation with access to text files, image files, applications, printers, or communications software for access to the WAN.
LAN benefits include the fact that a high-speed transmission system can be shared among multiple devices in support of large numbers of active terminals and large numbers of active applicationssort of a multi-user, multitasking computer network. LAN-connected workstations realize the benefit of decentralized access to very substantial centralized processors, perhaps in the form of a mainframe host computer and storage capabilities (information repositories). Additionally, current technology allows multiple LANS to be internetworked through the use of LAN switches, routers, and the like.
The shared medium for LANs include most of the transmission media discussed in Chapter 3. Although coaxial cable was the original medium and still is used widely in various configurations, twisted-pair has recently become the medium of choice in many environments. Fiber optic cable is used widely as a backbone technology, although it seldom is deployed to the desktop. Wireless LANs generally are limited to special radio technologies, although infrared technology is used in certain applications. Microwave and infrared systems are used to connect LANs and LAN segments in a campus environment. Satellite rarely is used in any way, as propagation delay renders it generally unsatisfactory for interactive communications
Coaxial Cable
Coaxial cable was the transmission medium first employed in LANs. Although it is expensive to acquire and costly to deploy and reconfigure, its performance characteristics are excellent. Additionally, DP/MIS managers always have been comfortable with coax, as it traditionally was specified in the mainframe and mid-range computer world. In fact, the technology initially didnt exist to make effective use of other options such as twisted-pair, fiber optics and radio systems.
In retrospect, perhaps the use of coaxial cable lessened the resistance of DP/MIS managers to the concept of LANs. Those who lived in the mainframe world (most did), regarded (some still do) PCs with disdain. They also sneered (some still do) at twisted-pair, which they refer to as telephone wire.
The advantages of coaxial cable include high bandwidth (500+ MHz), exceptional error performance, and lack of severe distance limitation. Additionally, security is high and durability is excellent. On the other hand, the costs of acquisition, deployment, and reconfiguration are high. The disadvantages of coaxial cable have been mitigated to a large extent through the development of new coax designs. Those designs also affect the performance of the system, however. Three variations of coax design are ThickNet, ThinNet, and Twinax.
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