Telephone line

A telephone line or telephone circuit (or just line or circuit industrywide) is a single-user circuit on a telephone communication system. It is designed to reproduce speech of a quality that is understandable. It is the physical wire or other signaling medium connecting the user's telephone apparatus to the telecommunications network, and usually also implies a single telephone number for billing purposes reserved for that user. Telephone lines are used to deliver landline telephone service and digital subscriber line (DSL) phone cable service to the premises. Telephone overhead lines are connected to the public switched telephone network. The voltage at a subscriber's network interface is typically 48 V between the ring and tip wires, with tip near ground and ring at –48 V. In 1878, the Bell Telephone Company began to use two-wire circuits, called the local loop, from each user's telephone to end offices which performed any necessary electrical switching to allow voice signals to be transmitted to more distant telephones. These wires were typically copper, although aluminium has also been used, and were carried in balanced pairs of open wire, separated by about 25 cm (10′′) on poles above the ground, and later as twisted pair cables. Modern lines may run underground, and may carry analog or digital signals to the exchange, or may have a device that converts the analog signal to digital for transmission on a carrier system. Often the customer end of that wire pair is connected to a data access arrangement; the telephone company end of that wire pair is connected to a telephone hybrid. In most cases, two copper wires (tip and ring) for each telephone line run from a home or other small building to a local telephone exchange. There is a central junction box for the building where the wires that go to telephone jacks throughout the building and wires that go to the exchange meet and can be connected in different configurations depending upon the subscribed telephone service.