 a single point of failure for the entire cabling system. Hardwired methods of terminating cables are more fault tolerant. Shielded Twisted Pair There is another type of twisted pair cable known as STP (Shielded Twisted Pair). The shielding is an extra layer of metal insulation surrounding the twisted pair. This is sometimes rec- ommended in cases where a lot of outside electromagnetic interference is present or anticipated. If it is not necessary, it is better to do without the shielding, since it can also have the ef- fect of keeping electromagnetic fields generated within the cable trapped inside the shield, causing distortion of the signal. If shield- ing is used it is important that it be properly grounded. Otherwise, it may attract the interference that it was designed to repel. The grounding should only take place at the MDF. If it is grounded at both ends, more interference in the signal may result. In general, shielding should only be used in low-level signal situations. Shielding works best when each pair of wires is shielded sepa- rately. Overall shielding of a cable containing multiple pairs will still keep out external interference but will not prevent interfer- ence between the pairs. This interference is known as crosstalk and results in conversations spilling over to other pairs of wires SO that one conversation is heard in the middle of another. Good quality shielded pair cable is expensive. The expense is justified since these cables must meet rigid manufacturing speci- fications which address the diameter and strength of each conductor, the properties of the insulation, the twisting of each pair, the shielding of each pair by a metallic foil, and the shield- ing of the entire cable by an outer layer of shielding material. 254 The Telecom Handbook CHAPTER 12 Screened Twisted Pair Cable Also knows as SCTP, this is a variation of shielded twisted pair cable developed by ITT. It can carry a signal 75% further than regular UTP. Grounding An important consideration relating to the installation of equ