Why twistes in UTP and How it reduces Noice

Exp I.

Twisted pair cable consists of a pair of insulated wires twisted together. It is a cable type used in telecommunication for very long time. Cable twisting helps to reduce noise pickup from outside sources and crosstalk on multi-pair cables.

Twisted pair cable is good for transferring balanced differential signals. The practice of transmitting signals differentially dates back to the early days of telegraph and radio. The advantages of improved signal-to-noise ratio, crosstalk, and ground bounce that balanced signal transmission bring are particularly valuable in wide bandwidth and high fidelity systems. By transmitting signals along with a 180 degree out-of-phase complement, emissions and ground currents are theoretically canceled. This eases the requirements on the ground and shield compared to single ended transmission and results in improved EMI performance.

When cable twisted at constant twist rate over the lenght of the cable, a cable with wel dfined characteristic impedance is formed. Characteristic impedance of twisted pair is determined by the size and spacing of the conductors and the type of dielectric used between them. Balanced pair, or twin lines, have a Zo which depends on the ratio of the wire spacing to wire diameter and the foregoing remarks still apply. For practical lines, Zo at high frequencies is very nearly, but not exactly, a pure resistance. Because the impedance of a cable is actually a function of the spacing of the conductors, so separating the conductors significantly changes the cable impedance at that point.

When many twisted pairs are put together to form a multi-pair calbe, individual conductors are twisted into pairs with varying twists to minimize crosstalk. Specified color combinations to provide pair identification.

The most commonly used twisted pair cable impedance is 100 ohms. It is widely used for data communications and telecommunications applications in structured cabling systems. In most twisted pair cable applications the cable impedance is between 100 ohms and 150 ohms. When a cable has a long distance between the conductors, higher impedances are possible. Typical wire conductor sizes for cables used in telecommunications 26, 24, 22 or 19 AWG.

• 100 ohms: This impedance is the standardized impedance to be used in the twisted pair wiring used in structured wiring systems standardized EIA/TIA 568 standard. Both unshielded and shielded "CAT5 and better" cables used on this kind of applications have 100 ohms impedance (usually at +-15% or better accuracy).

Exp II

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.