A carrier system is a
transmission system
In telecommunications, a transmission system is a communication system that transmits a signal from one place to another. The signal can be an signal (electrical engineering), electrical, fiber-optic communication, optical or radio wave, radio s ...
that
transmits information, such as the voice signals of a
telephone call
A telephone call, phone call, voice call, or simply a call, is the effective use of a connection over a telephone network between the calling party and the called party.
Telephone calls are the form of human communication that was first enabl ...
and the video signals of
television
Television (TV) is a telecommunication medium for transmitting moving images and sound. Additionally, the term can refer to a physical television set rather than the medium of transmission. Television is a mass medium for advertising, ...
, by modulation of one or multiple
carrier signal
In telecommunications, a carrier wave, carrier signal, or just carrier, is a periodic waveform (usually sinusoidal) that conveys information through a process called ''modulation''. One or more of the wave's properties, such as amplitude or frequ ...
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†...
. Prominent multiplexing methods of the
carrier signal
In telecommunications, a carrier wave, carrier signal, or just carrier, is a periodic waveform (usually sinusoidal) that conveys information through a process called ''modulation''. One or more of the wave's properties, such as amplitude or frequ ...
are
time-division multiplexing (TDM) and
frequency-division multiplexing
In telecommunications, frequency-division multiplexing (FDM) is a technique by which the total bandwidth (signal processing), bandwidth available in a communication channel, communication medium is divided into a series of non-overlapping freque ...
(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 Electrical conductor, conductor surrounded by a concentric conducting Electromagnetic shielding, shield, with the two separated by a dielectric (Insulat ...
by sending each at a different frequency. Multiple layers of multiplexing may ultimately be performed upon a given input
signal
A signal is both the process and the result of transmission of data over some media accomplished by embedding some variation. Signals are important in multiple subject fields including signal processing, information theory and biology.
In ...
. For example, in the
public switched telephone network
The public switched telephone network (PSTN) is the aggregate of the world's telephone networks that are operated by national, regional, or local telephony operators. It provides infrastructure and services for public telephony. The PSTN consists o ...
, 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 (satellite communications), transponder; it creates a communication channel between a source transmitter and a Rad ...
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, lines ...
, specific channels may be demultiplexed individually.
History
Carrier systems increase economic efficiency by carrying more traffic on comparable cost of
communication infrastructure. 19th century telephone systems, operating by direct
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 ...
transmission, could only carry one telephone call on each wire pair, hence routes with heavy traffic needed many wire pairs.
In the 1920s, frequency-division multiplexing could carry several circuits on the same
balanced wires, and by the 1930s
L-carrier The L-carrier system was one of a series of carrier systems developed by AT&T for high-capacity transmission for long-distance communications. It was the first designed to handle hundreds of Voice frequency, voice frequency telephone lines, compared ...
and similar systems carried hundreds of calls simultaneously on
coaxial cable
Coaxial cable, or coax (pronounced ), is a type of electrical cable consisting of an inner Electrical conductor, conductor surrounded by a concentric conducting Electromagnetic shielding, shield, with the two separated by a dielectric (Insulat ...
s. The capacity of these systems increased in the middle of the century.
In the 1950s, research began into further increasing the throughput of terminal equipment by using digital signals with
time-division multiplexing (TDM). 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 Syst ...
,
E-carrier
The E-carrier is a member of the series of carrier systems developed for digital transmission of many simultaneous telephone calls by time-division multiplexing. The European Conference of Postal and Telecommunications Administrations (CEPT) ori ...
and other similar digital systems.
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. Some ...
spacings required by digital systems, long-distance transmission still used
FDM until the late 1970s when
optical fiber
An optical fiber, or optical fibre, is a flexible glass or plastic fiber that can transmit light from one end to the other. Such fibers find wide usage in fiber-optic communications, where they permit transmission over longer distances and at ...
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
A telephone exchange, telephone switch, or central office is a central component of a telecommunications system in the public switched telephone network (PSTN) or in large enterprises. It facilitates the establishment of communication circuits ...
s became rare.
See also
*
Channel access method
In telecommunications and computer networks, a channel access method or multiple access method allows more than two terminals connected to the same transmission medium to transmit over it and to share its capacity. Examples of shared physical ...
References
*
{{refend
External links
J-carrier
Multiplexing