Electrical current flowing through any conductor (like the copper wires connecting his phone) will produce a surrounding electromagnetic field. If another conductor is within the surrounding field, an inductively coupled current will flow through the adjacent conductor.
Inductively Coupled Electromagnetic Flux
In the figure above current flowing through the conducting wire will produce an inductively coupled current in the adjacent wire. If the varying signal current represents a voice transmission the conversation can crossover from one line to another and voices can be heard on one line from another line conversation. Usually this is only an annoyance since crosstalk signal levels are typically low when compared with the signal levels of the conversation on the primary line. On the other hand digital data transmissions are extremely sensitive to crosstalk. Crosstalk can cause bit misinterpretation and will typically require a retransmission of the damaged data.
There are two types of crosstalk, near end and far end.
Near End Crosstalk (NEXT)
Near end crosstalk occurs between a transmitted signal and a received signal. Transmitted signals are typically stronger that a signal that is being received and interfere with the received signals.
Near End Crosstalk
Far End Crosstalk (FEXT)
Far end crosstalk occurs between two signals transmitted in the same direction. The adjacent conductors each produce a magnetic field and can interfer with each other.
Far End Crosstalk
The most common way to reduce crosstalk between adjacent wires is to twist the wires together in a way that cancels the crosstalk flux. That's why Unshielded Twisted Pair (UTP) cabling is used for high speed data cabling like Ethernet. In addition shielding, in the form of foil or metallic braid is also used in Shielded Twisted Pair (STP) cable.
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