Local Area Networks Lans

The IEEE (Ref. 1) defines a local area network as "a communication network to interconnect a variety of devices (e.g., personal computers, workstations, printers, file storage devices) that can transmit data over a limited area, typically within a facility."

The geographical extension or "local area" may extend from less than 100 ft (< 30 m) to over 6 miles (> 10 km). More commonly we can expect a LAN to extend over a floor in a building, and in some cases over a portion of a floor. Other LANs may cover multiple floors, groups of building in the same general area, or a college or industrial campus.

The transmission media encompass wire pair, coaxial cable, fiber-optic cable, and radio. The implementation of coaxial cable systems is slowing in favor of high-quality twisted wire pair. Wireless LANs, particular in the IEEE 802.11 series standards, are reaching maturity. Data rates vary from 1 Mbps to 1000 Mbps. LAN data rates, the number of devices connected to a LAN, the spacing of those devices, and the network extension depend on:

Fundamentals of Telecommunications, Second Edition, by Roger L. Freeman ISBN 0-471-71045-8 Copyright © 2005 by Roger L. Freeman

• The transmission medium employed

• The transmission technique (i.e., baseband or broadband)

• Network access protocol

• The incorporation of such devices as repeaters, bridges, routers, and switching hubs

• The access standard used, where there may be some upper limit on the number of accesses per segment

The preponderance of LANs operate without error correction with BERs specified in the range of 1 x 10-8 to 1 x 10-12 or better.

The most common application of a LAN is to interconnect data terminals and other processing resources, where all the devices reside in a single building or complex of buildings, and usually these resources have a common owner. A LAN permits effective cost sharing of high-value data processing equipment, such as mass storage media, mainframe computers or minicomputers, and high-speed printers. Resource sharing is probably equally as important where a LAN serves as the access vehicle for an intranet. Resource sharing in this context means that each LAN user has access to all other users' file and other data resources.

The interconnection of LANs in the local area (e.g., the floor of a building) with a high-speed backbone (e.g., between building floors) is very prevalent. LANs may connect through wide area networks (WANs) to other distant LANs. This is frame relay's principal application. The interface to the WAN may be via a smart bridge or a router.

There are two generic transmission techniques utilized by LANs: baseband and broadband. Baseband transmission can be defined as the direct application of the baseband signal to the transmission medium. Broadband transmission, in this context, is where the baseband signal from the data device is translated in frequency to a particular frequency slot in the RF spectrum. Broadband transmission requires a modem to carry out the translation. Baseband transmission may require some sort of signal-conditioning device. With broadband LAN transmission we usually think of simultaneous multiple RF carriers that are separated in the frequency domain. Present broadband technology comes from the cable television (CATV) industry.

The discussion in this section will essentially cover baseband LANs. An important aspect is that only one user at a time may access a LAN segment at a time. We expect segments to be isolated one from another by smart bridges or switched hubs.

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