Local Area Networks (LANs)

TRANSMISSION METHODS USED BY LANs

LANs function because their transmission capacity is greater than any single terminal on the system. As a result, each station terminal can be offered a certain amount of time on the LAN, like a time-sharing arrangement. To economize on this small window of opportunity, stations organize their messages into compact packets that can be quickly distributed. When two messages are sent simultaneously, they could collide on the LAN causing the system to be temporarily disrupted. Busier LANs usually utilize special software that virtually eliminates the problem of collisions by providing orderly, non-contention access.

The transmission methods used on LANs are either baseband or broadband. The baseband medium uses a high-speed digital signal consisting of square wave DC voltage. While it is fast, it can accommodate only one message at a time. As a result, it is suitable for smaller networks where contention is low. It also is very simple to use, requiring no tuning or frequency discretion circuits. This transmission medium may be connected directly to the network access unit and is suitable for use over twisted wire pair facilities.

By contrast, the broadband medium tunes signals to special frequencies, much like cable television. Stations are instructed by signaling information to tune to a specific channel to receive information. The information within each channel on a broadband medium may also be digital, but they are separated from other messages by frequency. As a result, the medium generally requires higher capacity facilities, such as coaxial cable. Suited for busier LANs, broadband systems require the use of tuning devices in the network access unit that can filter out all but the single channel it needs.

THE FILE SERVER

The administrative software of the LAN resides either in a dedicated file server; in a smaller, less busy LAN; or in a personal computer that acts as a file server. In addition to performing as a kind of traffic controller, the file server holds files for shared use in its hard drives, administers applications such as the operating system, and allocates functions.

When a single computer is used as both a workstation and a file server, response times may lag because its processors are forced to perform several duties at once. This system will store certain files on different computers on the LAN. As a result, if one machine is down, the entire system may be crippled. If the system were to crash due to undercapacity, some data may be lost or corrupted.

The addition of a dedicated file server may be costly, but it provides several advantages over a distributed system. In addition to ensuring access even when some machines are down, its only duties are to hold files and provide access.

OTHER LAN EQUIPMENT

LANs are generally limited in size because of the physical properties of the network including distance, impedance, and load. Some equipment, such as repeaters, can extend the range of a LAN. Repeaters have no processing ability, but simply regenerate signals that are weakened by impedance. Other types of LAN equipment with processing ability include gateways, which enable LANs operating dissimilar protocols to pass information by translating it into a simpler code, such as ASCII. A bridge works like a gateway, but instead of using an intermediate code, it translates one protocol directly into another. A router performs essentially the same function as a bridge, except that it administers communications over alternate paths. Gateways, bridges, and routers can act as repeaters, boosting signals over greater distances. They also enable separate LANs located in different buildings to communicate with each other.

The connection of two or more LANs over any distance is referred to as a wide area network (WAN). WANs require the use of special software programs in the operating system to enable dial-up connections that may be performed by telephone lines or radio waves. In some cases, separate LANs located in different cities—and even separate countries—may be linked over the public network.

LAN DIFFICULTIES

LANs are susceptible to many kinds of transmission errors. Electromagnetic interference from motors, power lines, and sources of static, as well as shorts from corrosion, can corrupt data. Software bugs and hardware failures can also introduce errors, as can irregularities in wiring and connections. LANs generally compensate for these errors by working off an uninterruptable power source, such as batteries, and using backup software to recall most recent activity and hold unsaved material. Some systems may be designed for redundancy, such as keeping two file servers and alternate wiring to route around failures.

Security problems can also be an issue with LANs. They can be difficult to manage and access because the data they use is often distributed between many different networked sources. In addition, many times these data are stored on several different workstations and servers. Most companies have specific LAN administrators who deal with these issues and are responsible for the use of LAN software. They also work to backup files and recover lost files.

PURCHASING A LAN

When considering if a LAN is suitable for a business, several things must be considered. The costs involved and the administrative support needed often far exceed reasonable predictions. A complete accounting of potential costs should include such factors as purchase price of equipment, spare parts, and taxes, installation costs, labor and building modifications, and permits. Operating costs include forecasted public network traffic, diagnostics, and routine maintenance. In addition, the buyer should seek a schedule of potential costs associated with upgrades and expansion and engineering studies.

The vendor should agree to a contract expressly detailing the degree of support that will be provided in installing and tuning up the system. In addition, the vendor should provide a maintenance contract that binds the company to make immediate, free repairs when performance of the system exceeds prescribed standards. All of these factors should be addressed in the buyer's request for proposal that is distributed to potential vendors.

LANs can also be purchased for home use. Initially, these kits were expensive and slow and transmitted data via the phone lines in the home. New products have emerged that are faster, more affordable and use wireless technology to allow multiple computers to share printers and perform other LAN functions. This technology allows phone lines, cable connections, and LANs to be used simultaneously and is perfect for a small business owner who works out of his or her home.

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