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. Your telephone cable is an
example of a twisted-pair type cable. A basic twisted-pair cable consists of two strands of copper wire
twisted together (see Figure). The twisting reduces the sensitivi- ty 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, 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 of the wires also controls the tendency of the wires in thepair to cause EMI in each other. As noted previously, whenever twowires are in close proximity, the signals in each wire tend to producecrosstalk in the other. Twisting the wires in the pair reduces crosstalkin much the same way that twisting reduces the tendency of thewires to radiate EMI.A twisted-pair cable is used in most cases to connect a PC to either a HUB or a MAU. "Connectivity Devices and Transfer Mechanisms." Two types oftwisted-pair cable are used in LANs: shielded and unshielded,
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