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Now, bear with me, you need to understand some of this stuff...

The 10BASE-T and 100BASE-TX Ethernets consist of two transmission lines.  Each transmission line is a pair of  twisted wires.  One pair receives data signals and the other pair transmits data signals.  A balanced line driver or transmitter is at one end of one of these lines and a line receiver is at the other end.  A (much) simplified schematic for one of these lines and its transmitter and receiver follow:

Pulses of energy travel down the transmission line at about the speed of light (186,000 miles/second).  The principal components of one of these pulses of energy is the voltage potential between wires and current flowing near the surface of the wires.  This energy can also be considered as residing in the magnetic field which surrounds the wires and the electric field between the wires.  In other words, an electromagnetic wave which is guided by, and travels down the wires.

The main concern is the transient magnetic fields which surrounds the wires and the magnetic fields generated externally by the other transmission lines in the cable, other network cables, electric motors, fluorescent lights, telephone and electric lines, lightning, etc. This is known as noise.   Magnetic fields induce their own pulses in a transmission line which may literally bury the Ethernet pulses, the conveyor of the information being sent down the line.

The twisted-pair Ethernet employs two principle means for combating noise.  The first is the use of balanced transmitters and receivers.  A signal pulse actually consists of two simultaneous pulses relative to ground: a negative pulse on one line and a positive pulse on the other.  The receiver detects the total difference between these two pulses.  Since a pulse of noise (shown in red in the diagram) usually produces pulses of the same polarity on both lines one pulse is essentially canceled by out the other at the receiver.  Also, the magnetic field surrounding one wire from a signal pulse is a mirror of the one on the other wire. At a very short distance from the two wires the magnetic fields are opposite and have a tendency to cancel the effect of each other out.  This reduces the line's impact on the other pair of wires and the rest of the world.

The second and the primary means of reducing cross-talk--the term cross-talk came from the ability to (over) hear conversations on other lines on your phone--between the pairs in the cable, is the double helix configuration produced by twisting the wires together.  This configuration produces symmetrical (identical) noise signals in each wire.  Ideally, their difference, as detected at the receiver, is zero.  In actuality it is much reduced.

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