Twisted Pair Cable

The twisted pair consists of two insulated copper wires arranged in a spiral pattern. The pair acts as a single communication link. Normally, a number of these twisted pair cables are wrapped together in a protective sheath. One of the wires is used to carry signals to the receiver and the other is only used to capture the amount of noise. The receiver will then calculate the difference between the two wires. Assuming the wires received the same amount of noise, once the receiver calculates the difference, the receiver would be able to recreate the original signal.

Wire A = original signal + noise

Wire B = noise

original signal = Wire A – Wire B

Twisted pair is normally used in the local loop of the telephone line to provide voice and data channels.

Types

There are two types of twisted pair cables – the Shielded Twisted Pair (STP) and the Unshielded Twisted Pair (UTP). As the name suggests, STP has a metal shield which protects it from noise. This metal shield is absent in the UTP. The STP was created by IBM. Although the shield provides additional protection from noise the cable is bulkier and more expensive. The STP is seldom used outside IBM.

Categories

The Electronic Industries Association (EIA) has developed standards to classify unshielded twisted pair cables into seven categories. Categories are determined by cable quality. Each category is best used with a particular technology.

Category	Bandwidth	Data Rate	Digital/Analog	Use
1	very low	<>	Analog	Telephone
2	<>	2 Mbps	Analog/digital	T-1 lines
3	16 MHz	10 Mbps	Digital	LANs
4	20 MHz	20 Mbps	Digital	LANs
5	100 MHz	100 Mbps	Digital	LANs
6 (draft)	200 MHz	200 Mbps	Digital	LANs
7 (draft)	600 MHz	600 Mbps	Digital	LANs

Why are the wires twisted?

The wires are twisted to reduce noise. Because the wires are twisted, they are affected by noise almost equally. Suppose in one twist one wire is close to the source of noise, in the next twist, the other wire would be closer to the source of noise and so forth. The noise in one wire will cancel out the noise in the other wire. This means that the receiver who calculates the difference between the two receives almost no noise.

Wire A = original signal + noise A

Wire B = noise B

original signal = Wire A – Wire B if an only if noise A = noise B

Connectors

The standard connector for twisted pairs are Registered Jacks (RJ). Some of the common RJs are RJ45 and RJ11.

Advantages

1. Low-cost for local networking
2. Wires are thin and flexible

Disadvantages

1. Limited distance due to strong attenuation
2. The twist loosens over time which makes a difference in the noise received by the wires.
3. Limited bandwidth and data rate (250Khz – 4Mbps)
4. A lot of repeaters or amplifiers are needed for long distance use.