The Effects of Longitudinal Imbalance on Crosstalk

A multipair cable consists of many insulated but unshielded conductors within a protective conducting sheath. The individual conductors are used to form circuits. In one configuration, called a metallic circuit, two conductors are paired and form the circuit. Signals are applied between them. This is called metallic excitation of the circuit, and the signal is said to propagate in the metallic mode. In another configuration, called a longitudinal circuit, two conductors are paral1227 leled and these, plus the conducting cable sheath, form the circuit. Signals are applied between the paralleled conductors and the conducting cable sheath, which is grounded. This is called longitudinal excitation of the circuit, and the signal is said to propagate in the longitudinal mode. It is also possible for one wire pair to be used for both circuits simultaneously and, consequently, for one wire pair to be simultaneously excited in the metallic and longitudinal modes of propagation. This happens when the terminal equipment is longitudinally unbalanced, as will be explained. Because the conductors are not shielded and are in close proximity to each other, electromagnetic fields generated by current flowing through the conductors cause energy to be coupled from one circuit to another. This is called crosstalk and is undesirable, since it may cause noise in other circuits that can impair the performance of digital and analog systems, or even be intelligible speech that is overheard and leads to loss of privacy.