Shielded Twisted-Pair (STP) Cable

Shielded twisted-pair cabling consists of one or more twisted pairs ofcables enclosed in a foil wrap and woven copper shielding. Figure shows IBM Type 1 cabling, the ?rst cable type used with IBMToken Ring. Early LAN designers used shielded twisted-pair cable because the shield performed double duty, reducing the tendency of the cable to radiate EMI and reducing 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 is literally connected to a metal stake driven into the ground. A properly grounded shield prevents signals from getting into or out of the cable. The picture in Figure is an example of IBM Type 1 cable, an STP cable, and 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, describing pre- cise characteristics is dificult. The following sections, however, offer

some general guidelines.