The Unshielded Twisted Pair, or UTP as it is commonly known, is a copper cable common in Ethernet cabling that is used to reduce crosstalk. Crosstalk is a form of interference. Generally, many home and business computers are connected to the telephone company using the twisted pair. Twisted pairs contain more than one circuit and in the 4-pair Unshielded Twisted Pair, the adjacent pairs differ in twist rate in order to reduce the crosstalk. Twisted wires reduce interference between the wires because the loop between the wires (the determinant of the magnetic coupling into the signal) is diminished. In order to get some protection from interference, the twisted pair circuits use a kind of signal transmission known as balanced-pair transmission. Some of the attributes of balanced-pair circuits include;
* Both conductors are at signal potential, or are at above-ground potential because the impedance, if measured per leg to the ground, is equal. This is in contrast to the coaxial circuit, where the signal transmission is imbalanced.
* Conductors in balanced-pair transmissions are driven in “push-pull,” or differential mode. Thus, the signal at any point along the circuit when measured from one leg of the pair to the ground is at par in magnitude and opposite in polarity to a similar signal from another pair.
* If any electromagnetic field couples into the cable, it causes the same induction of voltage in both magnitude and polarity onto the cable. The interference voltage is thus in a common mode.
* The terminating equipment at the end of the cable is designed to react to differential-mode signals and reject common-mode signals. This means that if the twisted pair picks up unwanted signals through coupling, it picks them in a form that can be rejected by the terminating equipment.
Ideally, that’s how UTP work on paper. Reality, however, may not match theory. Apart from small imperfections in the cable structure, the installation process may add further minor deformations, all of which might in the end add up to geometric imperfections. This might cause the cables to be imbalanced. The result is be that a small part of the common-mode interference signal picked up through induction will be converted into a differential-mode signal by the cable imbalances. This unwanted signal adds noise to the circuit, helped by the fact that it is in the same mode as the wanted signal and the terminating equipment will not be able to tell the difference between it and the wanted signal.
The imbalance is described by a parameter known as the longitudinal conversion loss (LCL) and is measured in decibels. The best cables have the highest specified LCLs. If the cable is mishandled during installation, its LCL also suffers. UTP cables are generally cheap compared to other LAN cables. However, as already discussed, maintaining the integrity of the cable during the installation process is perhaps the most important step in ensuring that the cable performs as it is expected to. Thus, they have strict requirements for maximum puling tension as well as minimum bend radius.
For more information on UTP read this article.
