Wire Transmission
Wire was the first medium used in telecommunications. Today, wire transmission facilities are used in both the local loop and inter-office trunks. Wire transmission facilities include the signal being transmitted, the equipment located in central offices, and the equipment along the transmission route. Open, unshielded wires also called open wire lines was the first type of transmission media used for telephones. To reduce crosstalk and power-line noise, open wire pairs are transposed, or frogged, at regular intervals. Transposition enables each wire in the pair to be equally exposed to power lines and to other wire pairs. Noise is reduced through a cancellation effect.
Open wires have an advantage over twisted pair cables. There is less resistance in open wire lines because they are made of thicker wires. This helps to reduce signal loss. Twisted pair cables are used for local loops, and for analog and digital carrier systems that cover relatively short distances, that is 200 miles or less. A twisted pair cable gets its name from the arrangement of wires inside the cable. One twisted pair is made up of two individual wire conductors usually made up of copper. Many twisted pairs are packed into a cable. A plastic insulated cable can carry from 6 to 1800 pairs of cables. Cables have an outer protective sheath that may be made of aluminum, steel or polyethylene.