Shielded twisted-pair cable (STP) combines the techniques of shielding and twisting wires. Each pair of wires is twisted and then wrapped in metallic foil. All four pairs of wires are wrapped in an overall metallic braid or foil. STP is usually 150-ohm cable. As specified for use in Ethernet network installations, STP reduces electrical noise. This includes pair-to-pair coupling, or crosstalk, from within the cable, and electromagnetic interference (EMI) and radio frequency interference (RFI) from outside the cable. STP cable must follow precise specifications as to how many twists or braids exist for every 30 cm (1 ft) of cable. Shielded twisted-pair cable shares many of the advantages and disadvantages of unshielded twisted-pair cable (UTP). Properly installed STP offers greater protection from all types of external interference, but is more expensive and difficult to install than UTP. A hybrid of UTP and STP is screened UTP (ScTP), also known as foil twisted pair (FTP), or pairs in metal foil (PiMF). ScTP is essentially UTP wrapped in a metallic foil shield, or screen. It is usually 100-ohm cable.
If improperly grounded, or if any discontinuities exist in the entire length of the shielding material, (for example, due to poor installation) STP and ScTP become susceptible to major noise problems. This is because noise problems cause the shield to act like an antenna picking up unwanted signals. This effect works both ways. Not only does the foil prevent incoming electromagnetic waves from causing noise on the data wires, but it also minimizes the outgoing radiated electromagnetic waves, which could cause noise in other devices. STP and ScTP cables cannot be run as far as some networking media, such as coaxial cable or optical fiber, without the signal being repeated. Also, more insulation and shielding combine to considerably increase the size, weight, and cost of the cables. The shielding materials make terminations more difficult and susceptible to poor workmanship. Despite their shortcomings, STP and ScTP cables are still useful in high electrical or RF noise environments, such as near an airport radar installation. These cables are also popular in Europe.