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Of course, the execution of all of these steps takes only a fraction of a second. While you were reading about these steps, thousands of frames could have been sent across the LAN. Network adapter cards and the support software recognize and handle errors, which occur when electrical interference, collisions (in CSMA/CD networks), or malfunctioning equipment cause some portion of a frame to be corrupted. Errors generally are detected through the use of a cyclic redundancy check (CRC) data item in the frame. The CRC is checked by the receiver; if its own calculated CRC doesnt match the value of the CRC in the frame, the receiver tells the sender about the error and requests retransmission of the frame in error. Several products exist that perform network diagnostic and analysis functions on the different types of LANs, should you find yourself in need of such troubleshooting. The different types of network adapters vary not only in access method and protocol, but also in the following elements:
LAN CablesGenerally speaking, the cabling systems described in the next few sections use one of three distinct cable types. These are twisted pair, shielded and unshielded (also known as STP and UTP or 10BaseT), coaxial cable, thin and thick (also known as 10Base2 and 10Base5, respectively), and fiber-optic cable. The kind of cable you use depends mostly on the kind of network layout you select, the conditions at the network site, and of course your budget. Using Twisted Pair CableTwisted pair cable is just what its name implies: insulated wires within a protective casing, with a specified number of twists per foot. Twisting the wires reduces the effect of electromagnetic interference on the signals being transmitted. Shielded twisted pair (STP) refers to the amount of insulation around the cluster of wires and therefore its noise immunity. You are familiar with unshielded twisted pair (UTP); it is often used for telephone wiring. Figure 1-4 shows unshielded twisted pair cable; Figure 1-5 illustrates shielded twisted pair cable.
Using Coaxial CableCoaxial cable is fairly prevalent in your everyday life; you often find it connected to the backs of television sets and audio equipment. Thin and thick, of course, refer to the diameter of the coaxial cable. Standard Ethernet cable (Thick Ethernet) is as thick as your thumb. Thin Ethernet (sometimes called Thinnet or CheaperNet) cable is slightly narrower than your little finger. The thick cable has a greater degree of noise immunity, is more difficult to damage, and requires a vampire tap (a connector with teeth that pierce the tough outer insulation) and a drop cable to connect to a workstation. Although thin cable carries the signal over shorter distances than the thick cable, Thinnet uses a simple BNC (Bayonet-Neill-Concelman) connector (a bayonet-locking connector for thin coaxial cables), is lower in cost, and was at one time the standard in office coaxial cable. Thinnet is wired directly to the back of each computer on the network, and generally installs much more easily than Thicknet, but it is more prone to signal interference and physical connection problems. Figure 1-6 shows an Ethernet BNC coaxial T-connector, and Figure 1-7 illustrates the design of coaxial cable.
Using Fiber-Optic CableFiber-optic cable uses pulses of light rather than electricity to carry information. It is therefore completely resistant to the electromagnetic interference that limits the length of copper cables. Attenuation (the weakening of a signal as it traverses the cable) is also less of a problem, allowing fiber to send data over huge distances at high speeds. It is, however, very expensive and difficult to work with. Splicing the cable, installing connectors, and using the few available diagnostic tools for finding cable faults are skills that very few people have.
Fiber-optic cable is simply designed, but unforgiving of bad connections. Fiber cable usually consists of a core of glass thread, with a diameter measured in microns, surrounded by a solid glass cladding. This, in turn, is covered by a protective sheath. The first fiber-optic cables were made of glass, but plastic fibers also have been developed. The light source for fiber optic cable is a light-emitting diode (LED); information usually is encoded by varying the intensity of the light. A detector at the other end of the cable converts the received signal back into electrical impulses. Two types of fiber cable exist: single mode and multimode. Single mode has a smaller diameter, is more expensive, and can carry signals over a greater distance. Figure 1-8 illustrates fiber-optic cables and their connectors.
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