Twisted Pair Cables

The oldest and still the most common transmission medium is twisted pair. A twisted pair consists of two insulated copper wires, typically about 1mm thick. The wires are twisted together in a helical form, just like a DNA molecule. The purpose of twisting the wires is to reduce the electrical interference from similar pairs close by. The following figure illustrate pairs of wires twisted.

Twisted – pair medium is inexpensive and easy to install and use. Twisted pair is used for audio telephone communication with speech signal bandwidth of 4 KHz. Nearly all telephones are connected to the telephone company office by a twisted pair. Twisted pairs can run several kilometers without amplification, but for longer distances, repeaters are needed. When many twisted-pairs run in parallel for a substantial distance, such as all the wires coming from an apartment building to the telephone office, they are bundled together and enclosed in a protective sheath.

Twisted pairs can be used for either analog or digital transmission. The bandwidth depends on the thickness of the wire and the distance traveled, but several megbit/sec, can be achieved for a few kilometers in many cases. Due to their adequate performance and low cost, twisted pairs are widely used and are likely to remain so for years to come.

Twisted-pairs used to connect terminals to a computer may be used upto 9-6 kbps if the length is less than 100 meters. Noise pick up by twisted pair wires limit their use.

Twisted pair cabling comes in several varieties, two of which are important for computer networks.

CAT-3 and Category 3 twisted pairs consist of two insulated wires gently twisted together. Four such pairs are typically grouped together in a plastic sheath for protection and to keep eight wires together. Prior to about 1988, most office buildings had one category 3 cable running from a central wiring closet on each floor into each office. This scheme allowed upto four regular telephones or two multiline telephones in each office to connect to the telephone company equipment in the wiring closet.

CAT-5 or category 5 twisted pairs were introduced after 1988. They are similar to category-3 pairs, but with more twists per centimeter and Teflon insulation, which results in less cross talk and a better quality signal over longer distance, making them more suitable for high speed computer communication.

Both of these wiring types are often referred to as UTP (Unshielded Twisted pair), to contrast them with the bulky, expensive, shielding twisted pair cables IBM introduced in the early 1980s but which have not proven popular outside of IBM installations.

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