Shielded twisted-pair cable (STP) combines the techniques of shielding and
twisting wires. STP cabling is shown in Figure
. 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).
This is shown in Figure
. 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.