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 consistent 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 the United States

In 1878, the Bell Telephone Company began using 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. They may also 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,

and the telephone company end of that wire pair is connected to a telephone hybrid.