Typical LAN Cable

If you look closely at this figure, you can see each copper wire, as well as the plastic coating on the wire. The copper wire is thin, making it brittle. In fact, the wire could easily break in your hand. To help prevent the wire from breaking, a thin plastic coating is painted onto each wire. Conveniently, each wire uses a different color of plastic coating, so you can look at each end of the cable and figure out which wire is which. As you might guess, and as you will learn more about in the next few pages, it is important that you can identify a particular wire on each end of the cable.

Also note that the wires in Figure 4-5 are twisted together in pairs. Each pair of wires is cleverly called a twisted pair. The term refers to a pair of wires twisted around each other to reduce the amount of electrical interface on the wires. In layman's terms, electromagnetic interference (EMI) occurs when electrical signals that exist in the aircaused by other wires or other nearby electrically powered deviceschange the electrical currents on the wire. If outside EMI changes the signal on the wire, the receiving computer might misinterpret a 0 as a 1 or a 1 as a 0, or it might not have a clue what the sender really sent. Sending the electrical signals over a twisted pair rather than a single wire eliminates a lot of EMI effects. (Besides, the wires are pretty skinny anyway, and copper is cheap, so why not use two?)

Another thing that can be added to the cable to reduce EMI is shielding. Shielding, as the name implies, shields the wires inside the cable from the effects of EMI. However, shielded cabling has more stuff in it, making the cable less bendable and more expensive to produce. Shielded cables are called shielded twisted pair (STP), and you could probably guess that unshielded cables are called unshielded twisted pair (UTP). LAN technology has evolved to the point where less expensive UTP cabling can be used in most environments, with STP cabling being used in environments where significant EMI issues exist. Figure 4-5 showed UTP cabling; Figure 4-6 shows an example of an STP cable.