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Micro House PC Hardware Library Volume II: Network Interface Cards And Modems Micro House PC Hardware Library Volume II: Network Interface Cards And Modems
by Micro House International, Inc. and Scott Mueller
Que, Macmillan Computer Publishing
ISBN: 078971664x   Pub Date: 06/17/98
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Network Topologies

Each workstation on the network is connected with cable (or some other medium) to the other workstations and to one or more servers. Sometimes a single piece of cable winds from station to station, visiting all the servers and workstations along the way. This cabling arrangement is called a bus topology, as shown in Figure 1-9. (A topology is simply a description of the way the workstations and servers are physically connected.) The potential disadvantage to this type of wiring is that if a workstation has a problem, it can cause all of the stations beyond it on the bus to lose their network connections.


FIG. 1-9  The linear bus topology, attaching all network devices to a common cable.

Sometimes separate cables run from a central wiring nexus, often called a hub or a concentrator, to each workstation. Figure 1-10 shows this arrangement, called a star topology. Sometimes the cables branch out repeatedly from a root location, forming the star-wired tree shown in Figure 1-11. Bus cabling schemes use the least amount of cable but are the hardest to diagnose or bypass when problems occur.


FIG. 1-10  The star topology, connecting the LAN’s computers and devices with cables that radiate outward, usually from a file server.


FIG. 1-11  The star-wired tree topology, linking the LAN’s computers and devices to one or more central hubs, or access units.

The other topology often listed in discussions of this type is a ring, in which each workstation is connected to the next, and the last workstation is connected to the first again (essentially a bus topology with the two ends connected). Data travels around a Token-Ring network in this fashion, for example. However, the ring is not physically evident in the cabling layout for the network. In fact, the ring exists only within the hub (called a multistation access unit or MSAU on a Token-Ring network). Signals generated from one workstation travel back to the hub, are sent out to the next workstation, and then back to the hub again. The data is then passed to each workstation in turn until it arrives back at the computer that originated it, where it is removed from the network. Therefore, although the wiring topology is a star, the data path is theoretically a ring. This is called a logical ring.

If you have to run cables (of any type) through walls and ceilings, the cable installation can be the most expensive part of setting up a LAN. At every branching point, special fittings connect the intersecting wires. Sometimes you also need various additional components along the way, such as hubs, repeaters, or access units.

A few companies, such as Motorola, are working on LANs that do not require cables at all. Wireless LAN uses infrared or radio waves to carry network signals from computer to computer, but have not yet achieved the speed and reliability needed for today’s applications.

Planning the cabling layout, cutting the cable, attaching connectors, and installing the cables and fittings are jobs usually best left to experienced workers. If the fittings are not perfect, you may get electronic echoes on the network, which cause transmission errors. There are also a great many physical specifications for each network type that must be observed if the network is to function properly. Coaxial cable costs about 15 cents per foot, whereas STP costs about 25 cents per foot. This may sound like a moderate expense, even for a large LAN, but the cost of installing cable, at about $45 per hour, overshadows the cost of the cable itself. The moral of this story is to have the installer run much more cable than you initially need so that you won’t have to have them come back and install more. The only time that you might consider installing LAN cable yourself is when you have a group of computers located on adjacent desks and you do not have to pull cable through the walls or ceiling.

Building codes almost always require you to use fireproof plenum cables. Plenum cables are more fire-resistant than some other cables. A professional cable installer should be familiar with the building codes in your area. You would be very upset if you installed ordinary cable yourself and were later told by the building inspector to rip out the cable and start over again with the proper kind.


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