A carrier system is a telecommunications system that transmits information, such as the voice signals of a

telephone call A telephone call is a connection over a telephone network between the called party and the calling party. First telephone call The first telephone call was made on March 10, 1876, by Alexander Graham Bell. Bell demonstrated his ability to " ...

and the video signals of

television Television, sometimes shortened to TV, is a telecommunication Media (communication), medium for transmitting moving images and sound. The term can refer to a television set, or the medium of Transmission (telecommunications), television tra ...

, by modulation of one or multiple

carrier signal In telecommunications, a carrier wave, carrier signal, or just carrier, is a waveform (usually sinusoidal) that is modulated (modified) with an information-bearing signal for the purpose of conveying information. This carrier wave usually has ...

s above the principal voice frequency or data rate.Western Electric (1969) ''Fundamentals of Telephone Communication Systems'', p.16.2 Carrier systems typically transmit multiple channels of communication simultaneously over the

shared medium In telecommunication, a shared medium is a medium or channel of information transfer that serves more than one user at the same time. In order for most channels to function correctly, no more than one user can be transmitting at a time, so a c ...

using various forms of

multiplexing In telecommunications and computer networking, multiplexing (sometimes contracted to muxing) is a method by which multiple analog or digital signals are combined into one signal over a shared medium. The aim is to share a scarce resource - a ...

. Prominent multiplexing methods of the

are

time-division multiplexing Time-division multiplexing (TDM) is a method of transmitting and receiving independent signals over a common signal path by means of synchronized switches at each end of the transmission line so that each signal appears on the line only a fracti ...

(TDM) and

frequency-division multiplexing In telecommunications, frequency-division multiplexing (FDM) is a technique by which the total bandwidth available in a communication medium is divided into a series of non-overlapping frequency bands, each of which is used to carry a separate ...

(FDM). A cable television system is an example of frequency-division multiplexing. Many television programs are carried simultaneously on the same

coaxial cable Coaxial cable, or coax (pronounced ) is a type of electrical cable consisting of an inner conductor surrounded by a concentric conducting shield, with the two separated by a dielectric ( insulating material); many coaxial cables also have a ...

by sending each at a different frequency. Multiple layers of multiplexing may ultimately be performed upon a given input

signal In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The '' IEEE Transactions on Signal Processing' ...

. For example, in the

public switched telephone network The public switched telephone network (PSTN) provides infrastructure and services for public telecommunication. The PSTN is the aggregate of the world's circuit-switched telephone networks that are operated by national, regional, or local telep ...

, many telephone calls are sent over shared trunk lines by time-division multiplexing. For long-distance calls several of these channels may be sent over a

communications satellite A communications satellite is an artificial satellite that relays and amplifies radio telecommunication signals via a transponder; it creates a communication channel between a source transmitter and a receiver at different locations on Ear ...

link by frequency-division multiplexing. At a given receiving

node In general, a node is a localized swelling (a "knot") or a point of intersection (a vertex). Node may refer to: In mathematics * Vertex (graph theory), a vertex in a mathematical graph * Vertex (geometry), a point where two or more curves, line ...

, specific channels may be demultiplexed individually.

History

The purpose of carrier systems is to save money by carrying more traffic on less infrastructure. 19th century telephone systems, operating at

baseband In telecommunications and signal processing, baseband is the range of frequencies occupied by a signal that has not been modulated to higher frequencies. Baseband signals typically originate from transducers, converting some other variable into ...

, could only carry one telephone call on each wire, hence routes with heavy traffic needed many wires. In the 1920s, frequency-division multiplexing could carry several circuits on the same balanced wires, and by the 1930s L-carrier and similar systems carried hundreds of calls at a time on

s. Capacity of these systems increased in the middle of the century, while in the 1950s researchers began to take seriously the possibility of saving money on the terminal equipment by using

. This work led to

T-carrier The T-carrier is a member of the series of carrier systems developed by AT&T Bell Laboratories for digital transmission of multiplexed telephone calls. The first version, the Transmission System 1 (T1), was introduced in 1962 in the Bell ...

and similar digital systems for local use. Due to the shorter

repeater In telecommunications, a repeater is an electronic device that receives a signal and retransmits it. Repeaters are used to extend transmissions so that the signal can cover longer distances or be received on the other side of an obstruction. Som ...

spacings required by digital systems, long-distance still used FDM until the late 1970s when

optical fiber An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparency and translucency, transparent fiber made by Drawing (manufacturing), drawing glass (silica) or plastic to a diameter slightly thicker than that of a Hair ...

was improved to the point that digital connections became the cheapest ones for all distances, short and long. By the end of the century, analog connections between and within

telephone exchange 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 syste ...

s became rare.

References

*{{FS1037C MS188

External links

J-carrier
Multiplexing

History

See also

References

External links