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The Considerations That You Must Evaluate in Choosing the 'Best' Local-Area Network.

The growing interest in local-area data communication networks by business and industry raises the question of which kind of local-area network will best servce each organization's specific needs. Typically, such communications systems may include monitoring and control of machine operations, data processing, inventory control, closed-circuit television, security/alarm systems, energy management and employee-time accounting. Further, such a system may be spread over several floors of a large building or among several buildings, possibly covering an area a few miles wide.

Selection of the correct local-area network (LAN) is highly dependent on how many of these diverse elements and conditions must be tied together into one cohesive, integrated communications system.

Each of the four possible communications network types--twisted-wire, baseband, broadband and fiber-optic--must be evaluated with these considerations in mind.

Specific considerations in evaluating these four network types include:

* Is network intelligence needed?

* How large a capacity is required?

* Are there long distances between the various stations in the proposed network?

* How important is data speed?

* Will be proposed network interface with other systems?

* What degree of data error rate is acceptable?

* What are the projected requirements for network expansion, component relocation and maintenance?

Of the four communications network types, the oldest is twisted-wire. Consisting of two or more wires connecting each remote device to a central switching unit, this design is commonly used in a business telephone system, or private branch exchange (PBX). Twisted-wire networks offer network intelligence through a PBX, limited (not full-motion) videoconferencing unlimited distance with proper modems via the public switched network, interface capability and full transparency to user devices and other communication systems.

However, twisted-wire networks also have several disadvantages. Because of their narrow bandwidth, their inherent capacity is limited, and their relatively low top speed of approximately 64 kb/s (or 9,600 baud) makes them inadequate for television, high-speed graphics and modern high-speed data links. Also, their lack of shielding at critical points makes twisted-wire systems highly susceptible to noise interference from such sources as AC power lines, industrial and medical equipment, and color-TV oscillators; this noise interference will cause errors and lost data at a high rate of 10.sup.-5.

In addition, twisted-wire networks need large, expensive ducts to carry the necessary multitude of dedicated wires. And troubleshooting and fault-isolation in a paired wiring systen can be tough because of network complexity and the difficulty of identifying the correct fault source. Baseband Obviates Dedicated Wire

A baseband LAN makes dedicated wires unnecessary by putting all communications on a single channel in a coaxial cable. In some applications, baseband networks can function without a central controller, although one is generally used in larger systems for efficient network management. Also, baseband is perfectly adequate for most smaller installations, such as interconnecting computers, word processors and similar devices on a single floor or in a limited area of an office building.

Baseband is fast--up to 10 Mb/S--and has an improved data error rate of 10.sup.-7. Furthermore, baseband systems offer plug-in ease of installation, as long as the number of devices on each cable is limited and real-time video or high-speed graphics aren't required. They are relatively trouble-free and easy to repair.

But there are drawbacks to baseband too. It is still just a single channel, on which users must take turns sending or receiving voice or data messages; an automatic controller assigns places in the "queue," holding up traffic as necessary to prevent "crashes= between users' data.

Baseband systems are limited to 1,500 meters in length, unless expensive repeaters are used. Security against data loss is just moderate; so-called "vampire taps" (used to connect devices to the network) are prone to noise interference. Interfacing baseband with other networks requires special equipment. Further, at which twisted-wire, baseband networks require ducting, conduit or, at a minimum, a fireproof coating to meet building codes. Taps must be carefully plugged in, and only at specified points along the cable.

A broadband communications network has all the advantages of a baseband network, and goes beyond baseband by eliminating many of its drawbacks. Broadband, true to its name, can carry information on up to several hundred channels over a single cable TV-type coaxial cable this capability eliminates the competition factor that limits baseband. The wide 440-MHz bandwidth of broadband allows simultaneous accommodation of hundreds of voice, video and data channels (including full-motion video for industrial monitoring, security, educational TV or teleconferencing), along with widely dispersed data processing terminals and devices. And broadband networks are fully transparent to user devices and other communication systems because they use dedicated channels.

A broadband system can easily be extended well beyond 40 miles, by placing standard, low-cost CATV amplifiers every 2,000 feet. What's more, it can handle transmission speeds up to 10 Mb/s on each channel, making total capacity virtually limitless. Moreover, broadband allows data transmission in two directions at the same time.

Data-loss protection with broadband networks is excellent, an average data error rate of 10.sup.-9. assures high data accuracy. Data security is also enhanced with broadband, because a potential "tapper" must know which channel he wants as well as the encoding scheme being used. Equally important, the fully shielded cable used in a broadband system and the taps used to connect user devices to the network are simple to install and virtually immune to noise from outside sources.

Further, broadband coaxial cable does not require conduit or protective ductwork; its ready adaptability allows it to be fastened directly to walls or ceilings, or to be buried for connecting system elements in outlying buildings. Maintenance is also simplified; splicing is easy and repair can be accomplished in just a few minutes. Fiber-Optic Has Its Advantages

The fourth network type, fiber-optic, has several distinct advantages. Employing light beams traveling through transparent glass threads, each fiber can carry many channels, and multiple fibers can be packed into a relatively slender cable. Hence, the capacity of a fiber-optic network is very large. The bandwidth is virtually unlimited, and the capacity for accommodating network system terminals is extremely high. Also, fiber-optic multiplexer-modems allow dedicated transmission over virtually unlimited distances, using repeater devices every 4,000 feet.

Fiber-optic systems are extremely fast, with a capacity of 200 Mb/s in a typical installation. These systems can be interfaced with other networks or links using a dedicated optical-fiber circuit. Fiber-optic networks also have very low data error rates, provide very high security and are immune to outside noise.

With all its advantages, however, fiber-optic networks currently have severe limitations as well. The greatest drawback is that fiber optics is limited to point-to-point, dedicated-circuit applications. Fiber-optic point-to-point links can be used to interconnect distributed, computerized PBX elements in large, multi-building facilities. But fiber optics cannot be employed in an "information utility" in which data processing devices and terminals are frequently relocated. Maintain-ability is also a major problem; while fiber-optic systems require little routine maintenance, it is extremely difficult to splice a damaged cable with its minuscule, identical-looking optical fibers. Twisted-Wire Has Been Around

Twisted-wire networks have been around for a long time; telephone companies in particular have a great deal of experience in installing and servicing these type systems. Fiber-optic systems, on the other hand, are still experimental, no one really knows as yet how well a fiber-optic network will perform in a typical business, industrial or institutional environment. Baseband networks vary; one well-publicized system has only a limited number of working installations, while another baseband system has some 1,800 working installations within its computer terminal base.

There are approximately 500 broadband networks installed current, Of these, 300 are Videodata LAN/1 high-speed networks developed by Interactive Systems/3M. As a general rule, broadband has proven to be very cost effective in networks that have large terminal populations or that encompass multiple floors or buildings. They have also appeared to be preferable in smaller installations with terminals communicating at very high data rates, because broadband avoids the delays that can develop on baseband networks due to heavy or conflicting traffic demands.

The system you choose must depend on your specific application. Careful evaluation of how and where your network will e used, compared with the advantages and disadvantages of each network type, will determine your choice.
COPYRIGHT 1984 Nelson Publishing
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Copyright 1984 Gale, Cengage Learning. All rights reserved.

Article Details
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Author:Geisler, C.
Publication:Communications News
Article Type:buyers guide
Date:May 1, 1984
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