How Does Telephone Wiring Work?

Telephone Wiring is Everywhere

For telephone wiring to work correctly, it must be able to complete a circuit between the telephone handset and the switching equipment in the telephone company's central office. To accomplish this exceptionally large circuit, telephone wiring starts with a set of two wires -- known as a cable pair -- in the home or office; this wiring runs from the telephone jack to which a telephone is connected throughout the home or office to a box mounted outside. This box, known as a network interface device (NID for short), is the interface between the home or office wiring and the telephone company network. From the NID, the two wires continue on -- either via an aerial cable or underground conduits -- to the closest telephone company substation. The two wires are transferred to a large collection of circuits, known as a trunk, for transport the rest of the way to the central office.

At the other end of the trunk, the cable pair emerges from the trunk and physically connects to a large central office wiring structure known as a "frame." The frame keeps tens of thousands of cable pairs organized and routes them to an individual line card in the telephone company switching equipment. Depending on the type of switch used -- popular models include the Nortel DMS 100 and the Lucent 5ESS -- the line card may be configured somewhat differently, but individual line cards are programmed to provide the services to which a telephone customer subscribes (in addition to basic dial tone service, these services may include features like call waiting and caller ID or advanced features like call restrictions or hot line service).

In essence, the cable pair that serves a telephone subscriber serves to create a very large closed circuit between the desktop telephone and the switching equipment in the central office; this circuit is necessary because telephones work off of electrical impulses. When a telephone user lifts the handset, the line card on the switch recognizes a slight change in the electrical continuity on the line and plays a dial tone sound while waiting for the user to dial a number. During dialing, the telephone itself generates sounds -- known to technophiles as dual-tone multifrequency, or DTMF sounds -- to indicate the dialed number to the switch; these sounds are converted into electrical impulses and sent over the circuit to the switch. Likewise, when a user speaks into the telephone, a microphone converts her voice into electrical signals and sends them over the circuit; the electrical signals are passed through the telephone network and converted back into sound by a speaker in the other party's telephone handset. Through the use of electrical signals and telephone wiring, conversations of virtually limitless duration and complexity can be created.