Twisted pair cables often incorporate shielding in an attempt to prevent electromagnetic interference. Shielding provides an electrically conductive barrier to attenuate electromagnetic waves external to the shield; and provides a conduction path by which induced currents can be circulated and returned to the source via ground reference connection.
Such shielding can be applied to individual pairs or quads; or to a collection of pairs. Individual pairs are foil shielded, while an overall cable (of multiple pairs) may use any of braided screen or foil or braiding with foil.
When shielding is applied to a collection of pairs, it is usually referred to as screening, but usage among vendors and authors in applying such words as screening, shielding, and STP (shielded twisted pair) can be subject to variability.
ISO/IEC 11801:2002 (Annex E) attempts to internationally standardize the various designations for shielded cables by using combinations of three letters – U for unshielded, S for braided shielding (in outer layer only), and F for foil shielding – to explicitly indicate the type of screen for overall cable protection and for protecting individual pairs or quads, using a two-part abbreviation in the form of x/xTP.
Shielded Cat 5e, Cat 6/6A, and Cat 8/8.1 cables typically have F/UTP construction, while shielded Cat 7/7A and Cat 8.2 cables use S/FTP construction.
Because the shielding is made of metal, it may also serve as a path to ground; usually a screen shielded, twisted pair cable has an integrally incorporated, grounding wire, called a drain wire, which makes electrical contact with the shield or screen. The purpose of the drain wire is for easy connection of the screen to terminals, which are usually designed for connection of round wires.
Common shield construction types include:
Individual Shield (U/FTP)
Individual shielding with aluminum foil for each twisted pair or quad. Common names: pair in metal foil, shielded twisted pair, screened twisted pair. This type of shielding protects cable from external EMI entering or exiting the cable and also protects neighboring pairs from crosstalk.
Overall Shield (F/UTP, S/UTP, and SF/UTP)
Overall foil, braided shield or braiding with foil across all of the pairs within the 100 ohm twisted pair cable. Common names: foiled twisted pair, shielded twisted pair, screened twisted pair. This type of shielding helps prevent EMI from entering or exiting the cable.
Individual and Overall Shield (F/FTP, S/FTP, and SF/FTP)
Individual shielding using foil between the twisted pair sets, and also an outer foil or braided shielding. Common names: fully shielded twisted pair, screened foiled twisted pair, shielded foiled twisted pair, screened shielded twisted pair, shielded screened twisted pair. This type of shielding helps prevent EMI from entering or exiting the cable and also protects neighboring pairs from crosstalk.
Industry Nomenclature for Cable Construction Types
|Industry abbreviations||ISO/IEC 11801 Designation||Cable shielding||Pair shielding|
|STP, ScTP, PiMF||U/FTP||None||Foil|
|FTP, STP, ScTP||F/UTP||Foil||None|
|SFTP, S-FTP, STP||SF/UTP||Braiding and Foil||None|
|SSTP, SFTP, STP, STP PiMF||S/FTP||Braiding||Foil|
|SSTP, SFTP, STP||SF/FTP||Braiding and Foil||Foil|