Shielded Twisted-Pair (STP) Cable

Shielded Twisted-Pair (STP) Cable

Shielded twisted-pair cabling consists of one or more twisted pairs of cables enclosed in a foil wrap and woven copper shielding. Figure 3.12 shows IBM Type 1 cabling, the first cable type used with IBM Token Ring. Early LAN designers used shielded twisted-pair cable because the shield further reduces the tendency of the cable to radiate EMI and thus reduces the cable’s sensitivity to outside interference.

Coaxial and STP cables use shields for the same purpose. The shield is connected to the ground portion of the electronic device to which the cable is connected. A ground is a portion of the device that serves as an electrical reference point, and usually, it literally is connected to a metal stake driven into the ground. A properly grounded shield prevents signals from getting into or out of the cable.

As shown in Figure 3.12, IBM Type 1 cable includes two twisted pairs of wire within a single shield. Various types of STP cable exist, some that shield each pair individually and others that shield several pairs. The engineers who design a network’s cabling system choose the exact configuration. IBM designates several twisted-pair cable types to use with their Token Ring network design, and each cable type is appropriate for a given kind of installation. A completely different type of STP is the standard cable for Apple’s AppleTalk network.

Because so many different types of STP cable exist, stating precise characteristics is difficult. The following sections, however, offer some general guidelines.

Further Information