Twisted Pair - Cable Shielding

Cable Shielding

Twisted pair cables are often shielded in an attempt to prevent electromagnetic interference. Because the shielding is made of metal, it may also serve as a ground. However, usually a shielded or a screened twisted pair cable has a special grounding wire added called a drain wire.

This shielding can be applied to individual pairs, or to the collection of pairs. When shielding is applied to the collection of pairs, this is referred to as screening. Shielding provides an electric conductive barrier to attenuate electromagnetic waves external to the shield and provides conduction path by which induced currents can be circulated and returned to the source, via ground reference connection.

Shielded twisted pair (STP or STP-A)

150 ohm STP shielded twisted pair cable is defined by the IBM Cabling System specifications and is used with token ring or FDDI networks. This type of shielding protects cable from external EMI from entering or exiting the cable and also protects neighboring pairs from crosstalk.

Screened twisted pair (ScTP or F/TP)

ScTP cabling offers an overall sheath shield across all of the pairs within the 100 Ohm twisted pair cable. F/TP uses foil shielding instead of a braided screen. This type of shielding protects EMI from entering or exiting the cable.

Screened shielded twisted pair (S/STP or S/FTP)

S/STP (Screened Shielded Twisted Pair) or S/FTP (Screened Foiled Twisted Pair) cabling offer shielding between the pair sets and an overall sheath shield within the 100 Ohm twisted pair cable. This type of shielding protects EMI from entering or exiting the cable and also protects neighboring pairs from crosstalk.

S/STP cable is both individually shielded (like STP cabling) and also has an outer metal shielding covering the entire group of shielded copper pairs (like S/UTP). This type of cabling offers the best protection from interference from external sources, and also eliminates alien crosstalk.

Shielded foiled Twisted pair (SFTP)

SFTP is different with S/FTP, SFTP is a kind of cable which has Foil and shield together. Usually use for outdoor cables for better protection against electrical signals and physical effects.

Note that different vendors and authors use different terminology (i.e. STP has been used to denote both STP-A, S/STP, and S/UTP). See below for the ISO/IEC attempt to internationally standardise the various designations.

Comparison of some old and new abbreviations, according to ISO/IEC 11801:

The code before the slash designates the shielding for the cable itself, while the code after the slash determines the shielding for the individual pairs:

TP = twisted pair

U = unshielded

F = foil shielding

S = braided shielding