|  | 5.1 | Most Common LAN Media
| 5.1.1 | STP
Shielded twisted-pair cable (STP) combines the techniques of shielding, cancellation, and twisting of wires. As specified for use in Ethernet network installations, STP provides resistance to both electromagnetic interference and radio frequency interference without significantly increasing the weight or size of the cable. Shielded twisted-pair cable has all of the advantages and disadvantages of unshielded twisted-pair cable. STP affords greater protection from all types of external interference, but is more expensive than unshielded twisted-pair cable. Unlike coaxial cable, the shield in STP is not part of the data circuit, therefore the cable needs to be grounded at both ends. Usually, installers ground STP at both the wiring closet and the hub, even though it is not always easy to do, particularly if the installers attempt to use older patch panels that were not designed to accommodate STP cable. If improperly grounded, STP can become a major source of problems, because it allows the shield to act like an antenna, absorbing electrical signals from other wires in the cable and from electrical noise sources outside the cable. Finally, STP cable cannot be run as far as other networking media (such as coaxial cable) without the signal being repeated. Another type of STP is specified for Token Ring installations. In this type of cable, known as 150 ohm STP, not only is the entire cable shielded to reduce electromagnetic interference and radio frequency interference, but each pair of twisted wires is shielded from each other in order to reduce crosstalk. Although the shielding used on 150 ohm STP cable is not part of the circuit, as it is in coaxial cable, it still must be grounded at both ends. This type STP cable requires an increased amount of insulation and a larger amount of shielding. These factors combine to considerably increase the size, weight, and cost of the cable. It also requires the installation of large wiring closets and large wiring ducts, luxuries that many older buildings cannot provide.
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