The single biggest difference between shielded (STP) and unshielded (UTP) twisted pair cables is the presence of the shielding layer. The whole idea of the shielding layer is to make the cable less susceptible to Electro-Magnetic Interference (EMI), noise/cross-talk and the signal degradation they cause. We have seen a number of different shielding strategies employed for twisted pair cabling and the ones used in computing networking scenarios are defined by the manner of their construction and include:
One Pair Shielding – The shielding layer covers both members of a pair and only one pair at a time.
Multiple Shielded Pairs – When a cable is composed of more than a single shielded pair the sets of shielded pairs of wires are contained in a sheath which may or may not be shielded as well.
Screening – When the shielding layer covers bunches of twisted pairs be they shielded or not it is referred to as screening. In the case of cables with small numbers of pairs manufacturers tend to implement the screening as a single shielding layer that covers the whole bunch of twisted pairs within the cable.
Screened Shielded Twisted Pair (S/STP) – Also known as Screened Fully Shielded Twisted Pair; the shielding layer covers both members of a twisted pair and an additional shielding layer is added to cover the whole bunch of shielded twisted pairs thereby producing a multi shielded and screened bunch of twisted pairs. The Screened Shielded Twisted Pair (S/STP) is the most resistant to the effects of external and internal interference of all the varieties of Shielded Twisted Pair (STP) and not unexpectedly it is also the most expensive.
It is important to note that the shield layer must be grounded in order for it to work and that since the shielding layer is usually metallic if installed correctly it can be used to serve as the ground for the cable. This does have its drawbacks however with shield functional failure being well-known to engineers.
Special Purpose Drain Wire – To avoid this shielded and/or screened twisted pair cable is usually manufactured with an additional special purpose grounding wire known as a drain wire built in which produces a far more effective and reliable grounding mechanism than does using the shield as the ground.
In fact; the manufacture of larger cables composed of many bundles of shielded twisted pair wires has seen the drain’s purpose change from a nicety to an essential component without which very few of the twisted pairs contained within it could be trusted to function reliably without suffering the negative effects of EMI and cross-talk from other pairs in the same large bundle in particular.
Shielded Twisted Pair (STP) Uses
Token Ring Networks – The IBM Cabling System specifications for Token Ring Networks specified the use of 150 ohm shielded twisted pair cables
Mission critical functionalities and services infrastructure, environments containing other electrical and electronic equipment sensitive to the negative effects of EMI, life support systems, medical facilities in general and special purpose uses in areas of high EMI and large cable bundles were scenarios in which STP was commonly implemented.
Today however; fiber optic cable is a far superior, less bulky and cheaper solution with the capacity to deliver higher bandwidth and data throughput reliability whilst being immune to the negative effects of EMI.