Twisted-Pair Cable

In its simplest form, twisted-pair cable consists of two insulated strands of copper wire twisted around each other. Figure 2.12 shows the two types of twisted-pair cable: unshielded twisted-pair (UTP) and shielded twisted-pair (STP)cable.

Figure 2.12 Unshielded twisted-pair and shielded twisted-pair cables

A number of twisted-pair wires are often grouped together and enclosed in a protective sheath to form a cable. The total number of pairs in a cable varies. The twisting cancels out electrical noise from adjacent pairs and from other sources such as motors, relays, and transformers.

Unshielded Twisted-Pair (UTP) Cable

UTP, using the 10BaseT specification, is the most popular type of twisted-pair cable and is fast becoming the most popular LAN cabling. The maximum cable length segment is 100 meters, about 328 feet.

Traditional UTP cable, as shown in Figure 2.13, consists of two insulated copper wires. UTP specifications govern how many twists are permitted per foot of cable; the number of twists allowed depends on the purpose to which the cable will be put. In North America, UTP cable is the most commonly used cable for existing telephone systems and is already installed in many office buildings.

Figure 2.13 UTP cable

The 568A Commercial Building Wiring Standard of the Electronic Industries Association and the Telecommunications Industries Association (EIA/TIA) specifies the type of UTP cable that is to be used in a variety of building and wiring situations. The objective is to ensure consistency of products for customers. These standards include five categories of UTP:

• Category 1 This refers to traditional UTP telephone cable that can carry voice but not data transmissions. Most telephone cable prior to 1983 was Category 1 cable.

• Category 2 This category certifies UTP cable for data transmissions up to 4 megabits per second (Mbps). It consists of four twisted pairs of copper wire.

• Category 3 This category certifies UTP cable for data transmissions up to 16 Mbps. It consists of four twisted pairs of copper wire with three twists per foot.

• Category 4 This category certifies UTP cable for data transmissions up to 20 Mbps. It consists of four twisted pairs of copper wire.

• Category 5 This category certifies UTP cable for data transmissions up to 100 Mbps. It consists of four twisted pairs of copper wire.

Most telephone systems use a type of UTP. In fact, one reason why UTP is so popular is because many buildings are prewired for twisted-pair telephone systems. As part of the prewiring process, extra UTP is often installed to meet future cabling needs. If preinstalled twisted-pair cable is of sufficient grade to support data transmission, it can be used in a computer network. Caution is required, however, because common telephone wire might not have the twisting and other electrical characteristics required for clean, secure, computer data transmission.

One potential problem with all types of cabling is crosstalk. Figure 2.14 shows crosstalk between two UTP cables. (As discussed earlier in this lesson, crosstalk is defined as signals from one line interfering with signals from another line.) UTP is particularly susceptible to crosstalk, but the greater the number of twists per foot of cable, the more effective the protection against crosstalk.

Figure 2.14 Crosstalk occurs when signals from one line bleed into another line

Shielded Twisted-Pair (STP) Cable

STP cable uses a woven copper-braid jacket that is more protective and of a higher quality than the jacket used by UTP. Figure 2.15 shows a two-twisted-pair STP cable. STP also uses a foil wrap around each of the wire pairs. This gives STP excellent shielding to protect the transmitted data from outside interference, which in turn allows it to support higher transmission rates over longer distances than UTP.