Twisted-Pair Cable

Twisted-Pair Cable

Twisted-pair cable has become the dominant cable type for all new network designs that employ copper cable. Among the several reasons for the popularity of twisted-pair cable, the most significant is its low cost. Twisted-pair cable is inexpensive to install and offers the lowest cost per foot of any cable type.

A basic twisted-pair cable consists of two strands of copper wire twisted together (see Figure 3.11). This twisting reduces the sensitivity of the cable to EMI and also reduces the tendency of the cable to radiate radio frequency noise that interferes with nearby cables and electronic components. This is because the radiated signals from the twisted wires tend to cancel each other out. (Antennas, which are purposely designed to radiate radio frequency signals, consist of parallel, not twisted, wires.)

Twisting also controls the tendency of the wires in the pair to cause EMI in each other. Whenever two wires are in close proximity, the signals in each wire tend to produce noise, called crosstalk, in the other. Twisting the wires in the pair reduces crosstalk in much the same way that twisting reduces the tendency of the wires to radiate EMI.

Two types of twisted-pair cable are used in LANs: shielded and unshielded.

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