Transcript
The cables that bind the Network These days, it seems as if the world is tied together with network cable. Although we may take a wired world for granted, the consequence is that we rely on wires to connect us to anything and everything we do. There are three types of cabling systems in use: twisted pair, coaxial and optical fiber.
Twisted Pair Twisted-pair cable is the traditional wiring used by telephone companies: Two insulated copper wires wrapped around each other. Each pair carries information via differential signaling (also known as balanced input), wherein the desired signal generates equal but opposite currents in the two wires. The twisting keeps the two wires as close together as possible so both wires experience the same total amount of interference. If the wires simply ran parallel, they would be significantly more susceptible to noise and interference. Twisted-pair cables come in different categories. Lower end categories are used primarily in homes, and higher end categories are used as a cheaper alternative to coaxial cable in LANs. Twisted-pair cables can be bundled inside a larger cable. This allows for phones, modems, Ethernet and the like to be wired using only one cable. The smaller twisted pairs are often color-coded to denote their use. The bundled cables can also be shielded using a rubberlike covering, often for business uses. This is known as a shielded twisted pair. Coaxial Invented in 1929, coaxial cable was used when AT&T Corp. built its first cross-conti nental transmission system in 1940. Coaxial cable (called "coax") is best known from the cable TV hookups in homes. Like
twisted-pair cable, coax involves two copper-based channels that carry signals. The difference is that both channels are contained in a single wire. The central copper wire is surrounded by a layer of insulation. Around the insulation is a braided copper mesh channel, and around that is the outside insulation for the wire. Coaxial is considered a sturdier design than twisted pair, but it also costs more. Telephone companies sometimes use coaxial cables for the wiring into their central offices, but twisted pair is used more often. Business uses include corporate Ethernet and LANs. Like twisted-pair cables, coaxial can also be bundled in larger cables. Optical Fiber With optical fiber, light pulses are sent along a glass or plastic fiber, avoiding electromagnetic interference and the need for retransmission that occurs with copper wire. While fiber can carry much more information than copper, light pulses dissipate over long distances. To solve this problem, a repeater is used to receive a signal, amplify it and retransmit it along the next leg of the cable. Analog signals hold together better than digital signals, so analog repeaters can be placed up to 18 kilometers apart (a little more than 11 miles). Digital repeaters must be closer together typically two to six kilometers (one to four miles), but they can purge unwanted static from the signal, whereas analog repeaters pass along any static or interference. In a fiber-optic network, a repeater consists of a photocell, an amplifier and an LED or infrared-emitting diode. The diode fires the signal along the next segment of its trip. For longer distances, something called single-mode fiber is used. This cable carries a single ray of light to ensure that a signal reaches its intended destination. Single-mode fiber is commonly used in long-distance phone lines. Multimode fiber sends multiple rays of light concurrently and is used for shorter distances. Multimode has a larger core than single mode, and rays are sent along at slightly differentiated reflection angles within that core. Glass fiber also requires a thicker coating for physical protection than a copper wire. Repeaters, thicker coating and labor-intensive installation all drive up the cost of optical fiber, which is one reason why it's not normally used to connect local customers to a telephone company branch office despite the fact that it can carry a greater amount of information. Captors We can't talk about cables without mentioning the plugs that allow us to connect them to devices. Each type of cable uses several different types of connectors, and standardization of these connectors is as important as the cable standards themselves. Twisted-pair cables tan have many different types of connectors. e two most, common are : registered Jack (RJ) 11 and RJ45. RJl1 connectors are commonly used in U.S. telephones with four or six contact points. RJ45 connectors are similar but wider, with eight contact points, and are generally used in heavy-duty computing environments. Coaxial network cables generally use Bayonet Neill-Concelman (BNC) twist-lock connectors. The most common fiber-optic cable connectors are ST (a twist-lock device), FC/PC (which screws on) and the snap-in SC.
