The STP cable is a balanced cable with paired stranded and shielded cores. The standard design of STP cables are two- and four-pair. As a rule, a foil shield made of an aluminum-laminated polyester foil is used as shielding for the pairs of wires, while a braided shield is used as overall shielding.
The shielding has the advantage that electromagnetic fields, which are due to the current flow in the cores, do not propagate through other cable pairs; on the other hand, the shielding also provides protection against external interference fields. Compared to unshielded cables, the properties of shielded cables are much more stable and independent of the environment. There are hardly any impedance or reactance deviations; the transmission parameters are relatively constant even at high frequencies and are better than those of UTP cables.
The classic STP cable is a multicore cable whose stranded wire pairs are insulated. The wire pairs of the STP cable are individually shielded. There is no overall shield. If the individual shielding is foil shielding, then the cable is referred to as a cable with pairs of cores shielded in metal foil, or PiMF for short. There are also cables which, in addition to the individual shielding of the wire pairs, have an overall shielding of foil or braided shielding. These cables are called S/STP cables: Screened Shielded Twisted Pair. The first S for Screened means a braided shield. If the overall shielding is a metal foil, an F, F/STP occurs in place of the S. STP cables with double outer shielding, a combination of foil shielding and braided shielding, have the designation SF/STP, Screened Foiled Shielded Twited Pair.
In addition to cables that have individual shielding for the individual wire pairs, there is also a variant with additional overall shielding for all wire pairs. STP cables are available with impedances of 100, 120 and 150 ohms, S/STP cables only with 100 ohms impedance. To put a stop to the proliferation of cable diversity, various standardization bodies( EIA/ TIA, ISO, IEC, DIN) have drawn up standards in which cables are classified into categories. The cabling standard, for example, specifies an impedance of 100 ohms, which is also becoming increasingly common.