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Category 5 UTP consists of 2 or more pairs of 22 or 24 AWG wire. Category 5 cable
is constructed and insulated such that the maximum attenuation of a 10 MHz
signal in a cable run at the control temperature of 20 C is 65 dB/km. A cable that
has a higher maximum attenuation than 65 dB/km does not meet the Category 5
requirements.
Shielded Twisted Pair (STP)
The TP-PMD specification is also able to utilize high-quality Shielded Twisted
Pair, or STP cable. Shielded Twisted Pair cabling is a multistranded cable most
often constructed of four 26 AWG conductive copper solid or stranded core wires.
Each wire is surrounded by a non-conductive insulating material such a
Polyvinyl Chloride (PVC). These wires are twisted around one another in a
specific arrangement to form pairs. The pairs are made up of associated wires -
transmit wires are paired with transmit wires, receive wires are paired with
receive wires.
Each pair in the STP cable is then surrounded by a metallic foil shield that runs
the length of the cable. Some types of STP incorporate an additional braided or
foil shield that surrounds each of the shielded pairs in the cable. The overall cable
is wrapped in an insulating jacket that covers the shields and holds the wires
together.
Twisting the pairs together throughout the cable helps to reduce the effects of
externally-induced electrical noise on the signals that pass through the cable. In
each pair, one wire carries the normal network signal, while its associated wire
carries a copy of the transmission that has been inverted.
The twisting of associated pairs helps to reduce the interference of the other
strands of wire throughout the cable. This is due to the method of transmission
used with twisted pair transmissions.
Cabling Types
Outer Jacket
Figure 11-4. STP Cable Pair Association
Overall Shield
Tx+
Tx-
Rx-
Rx+
Pair Shield
1845n21
FDDI Media
11-5
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