Unshielded Twisted Pair (UTP) has been designed for use in voice applications and local area networks. Because of the comparatively low cost, this cable is widely used and is available in several different performance categories (Categories 3, 4, 5 and 6). Over 99% of our copper installations are UTP versus shielded twisted pair (STP), which is rarely used.
Electromagnetic interference is present in all types of cabling to some degree. There are two basic types of electromagnetic interference (EMI) that cable engineers worry about-electromagnetic emissions and electromagnetic immunity. Emissions refer to energy that is radiated by the cable, and immunity is the ability of the cable to reject outside signals. In local area networks (LANs), failure to properly manage EMI can have an adverse effect on the integrity of the signal being transmitted.
In an unshielded cable, careful design of the cable and the associated electronic equipment results in a "balance" of the currents in the two conductors of a pair. That is, the currents in the two conductors are equal in magnitude but flowing in opposite directions. Proper installation and termination of the cable is also critical to maintaining this balance. In a balanced system, there is very little radiation of EMI since the external field from one conductor is effectively canceled by the external field from the other conductor of the pair.
Generally, the more twists per foot of cable, the better the cable is electrically balanced. Category 5 cable has more twists per foot than Category 3 or 4 cables and, therefore, offers better protection from EMI problems.