Twisted-pair cable is widely used, inexpensive, and easy to install. Twisted-pair cable used for telecommunication purposes is generally unshielded (UTP). It can transmit data at an acceptable rate - up to 100 Mbps in some network architectures. The most common twisted-pair wiring is telephone cable, which is unshielded and is usually voice-grade, rather than the higher-quality category 5 cable used for data networks. In a twisted-pair cable, two conductor wires are wrapped around each other. The information signal is transmitted as a difference voltage between the two conductor wires. The current flows in opposite directions in each wire of the active circuit, as shown in Figure 3.4.
Since these currents are equal and opposite and in close proximity, their magnetic fields cancel each other, and also cancel any magnetic interference caused by outside noise sources. This type of cable is therefore self-shielding and is less prone to interference (Figure 3.5).
Shielding of twisted-pair cables
Twisting the pairs of wires minimizes crosstalk between pairs. The twists also help deal with electromagnetic interference (EMI) and radio frequency interference (RFI), as well as balancing the mutual capacitance of the cable pair. The performance of a twisted-pair cable can be influenced by changing the number of twists per meter in a wire pair. Each of the pairs in a 4-pair category 5 cable, for example has a different twist rate to reduce the crosstalk between them.
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