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A telephone line or telephone circuit (or just line or circuit industrywide) is a single-user circuit on a telephone
communication Communication (from la, communicare, meaning "to share" or "to be in relation with") is usually defined as the transmission of information. The term may also refer to the message communicated through such transmissions or the field of inquir ...
system. It is designed to reproduce speech of a quality that is understandable. It is the physical
wire Overhead power cabling. The conductor consists of seven strands of steel (centre, high tensile strength), surrounded by four outer layers of aluminium (high conductivity). Sample diameter 40 mm A wire is a flexible strand of metal. Wire is c ...
or other signaling medium connecting the user's telephone apparatus to the
telecommunications Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of humans for communication over a distance greater than that fe ...
network, and usually also implies a single telephone number for billing purposes reserved for that user. Telephone lines are used to deliver
landline A landline (land line, land-line, main line, home phone, fixed-line, and wireline) is a telephone connection that uses metal wires or optical fiber telephone line for transmission, as distinguished from a mobile cellular network, which uses ...
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 Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to m ...
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 to use two-wire circuits, called the
local loop In telephony, the local loop (also referred to as the local tail, subscriber line, or in the aggregate as the last mile) is the physical link or circuit that connects from the demarcation point of the customer premises to the edge of the commo ...
, 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 Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...
, although
aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. I ...
has also been used, and were carried in
balanced pair In telecommunications and professional audio, a balanced line or balanced signal pair is a circuit consisting of two conductors of the same type, both of which have equal impedances along their lengths and equal impedances to ground and to other ci ...
s of open wire, separated by about 25 cm (10") on
poles Poles,, ; singular masculine: ''Polak'', singular feminine: ''Polka'' or Polish people, are a West Slavic nation and ethnic group, who share a common history, culture, the Polish language and are identified with the country of Poland in Ce ...
above the ground, and later as
twisted pair Twisted pair cabling is a type of wiring used for communications in which two conductors of a single circuit are twisted together for the purposes of improving electromagnetic compatibility. Compared to a single conductor or an untwisted ba ...
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 A carrier system is a telecommunications system that transmits information, such as the voice signals of a telephone call and the video signals of television, by modulation of one or multiple carrier signals above the principal voice frequency or ...
. Often the customer end of that wire pair is connected to a
data access arrangement The term data access arrangement (DAA) has the following meanings: #In public switched telephone networks, a single item or group of items at the customer side of the network interface device for data transmission purposes, including all equipmen ...
; the telephone company end of that wire pair is connected to a
telephone hybrid A telephone hybrid is the component at the ends of a subscriber line of the public switched telephone network (PSTN) that converts between two-wire and four-wire forms of bidirectional audio paths. When used in broadcast facilities to enable the a ...
. In most cases, two
copper wire Copper has been used in electrical wiring since the invention of the electromagnet and the telegraph in the 1820s. The invention of the telephone in 1876 created further demand for copper wire as an electrical conductor. Copper is the electri ...
s (
tip and ring Tip and ring are the two conductors or sides of a telephone line. Their names are derived from the telephone plugs used for connecting telephone calls in manual switchboards. One side of the line is connected to the metal ''tip'' of the plug, and ...
) for each telephone line run from a home or other small building to a local
telephone exchange A telephone exchange, telephone switch, or central office is a telecommunications system used in the public switched telephone network (PSTN) or in large enterprises. It interconnects telephone subscriber lines or virtual circuits of digital syst ...
. There is a central
junction box An electrical junction box (also known as a "jbox") is an enclosure housing electrical connections. Junction boxes protect the electrical connections from the weather, as well as protecting people from accidental electric shocks. Construction ...
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. The wires between the junction box and the exchange are known as the
local loop In telephony, the local loop (also referred to as the local tail, subscriber line, or in the aggregate as the last mile) is the physical link or circuit that connects from the demarcation point of the customer premises to the edge of the commo ...
, and the network of wires going to an exchange is known as the
access network An access network is a type of telecommunications network which connects subscribers to their immediate service provider. It is contrasted with the core network, which connects local providers to one another. The access network may be further ...
. The vast majority of houses in the U.S. are wired with 6-position modular jacks with four conductors (
6P4C A modular connector is a type of electrical connector for cords and cables of electronic devices and appliances, such as in computer networking, telecommunication equipment, and audio headsets. Modular connectors were originally developed fo ...
) wired to the house's junction box with copper wires. Those copper wires may be connected back to two telephone overhead lines at the local
telephone exchange A telephone exchange, telephone switch, or central office is a telecommunications system used in the public switched telephone network (PSTN) or in large enterprises. It interconnects telephone subscriber lines or virtual circuits of digital syst ...
, thus making those jacks
RJ14 A registered jack (RJ) is a standardized telecommunication network interface for connecting voice and data equipment to a service provided by a local exchange carrier or long distance carrier. Registration interfaces were first defined in th ...
jacks. More often, only two of the wires are connected to the exchange as one telephone line, and the others are unconnected. In that case, the jacks in the house are
RJ11 A registered jack (RJ) is a standardized telecommunication network interface for connecting voice and data equipment to a service provided by a local exchange carrier or long distance carrier. Registration interfaces were first defined in th ...
. Older houses often have 4-conductor telephone station cable in the walls color coded with Bell System colors: red, green, yellow, black as 2-pairs of 22 AWG (0.33 mm²) solid copper; "line 1" uses the red/green pair and "line 2" uses the yellow/black pair. Inside the walls of the house—between the house's outside junction box and the interior
wall jack A telephone jack and a telephone plug are electrical connectors for connecting a telephone set or other telecommunications apparatus to the telephone wiring inside a building, establishing a connection to a telephone network. The plug is inserte ...
s—the most common telephone cable in new houses is Category 5 cable—4 pairs of 24 AWG (0.205 mm²) solid copper. Inside large buildings, and in the outdoor cables that run to the telephone company POP, many telephone lines are bundled together in a single cable using the 25-pair color code.


References


External links

* {{DEFAULTSORT:Telephone Line Local loop Line Telecommunications infrastructure