2 cissp ® Official Study Guide Eighth Edition

Twisted-Pair
Twisted-pair cabling
is extremely thin and flexible compared to coaxial cable. It con-
sists of four pairs of wires that are twisted around each other and then sheathed in a 
PVC insulator. If there is a metal foil wrapper around the wires underneath the exter-
nal sheath, the wire is known as 
shielded twisted-pair (STP)
. The foil provides addi-
tional protection from external EMI. Twisted-pair cabling without the foil is known as 
unshielded twisted-pair (UTP)
. UTP is most often used to refer to 10BaseT, 100BaseT, 
or 1000BaseT.
The wires that make up UTP and STP are small, thin copper wires that are twisted in 
pairs. The twisting of the wires provides protection from external radio frequencies and 
electric and magnetic interference and reduces crosstalk between pairs. Crosstalk occurs 
when data transmitted over one set of wires is picked up by another set of wires due to 
radiating electromagnetic fields produced by the electrical current. Each wire pair within 
the cable is twisted at a different rate (in other words, twists per inch); thus, the signals 
traveling over one pair of wires cannot cross over onto another pair of wires (at least within 
the same cable). The tighter the twist (the more twists per inch), the more resistant the cable 
is to internal and external interference and crosstalk, and thus the capacity for throughput 
(that is, higher bandwidth) is greater.
There are several classes of UTP cabling. The various categories are created through 
the use of tighter twists of the wire pairs, variations in the quality of the conductor, and 
variations in the quality of the external shielding. Table 11.9 shows the original UTP 
categories.

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Cabling, Wireless, Topology, Communications, and Transmission Media Technology