Carrier system
Encyclopedia
In telecommunication
, a carrier system (loosely, a synonym with carrier) is a multichannel
telecommunications system
in which a number of individual channels (e.g. data
, audio, video or combination thereof) are multiplexed for transmission
. The transmission occurs between nodes of a network
.
In carrier
systems, many different forms of multiplexing
may be used, such as time-division multiplexing
and frequency-division multiplexing
. A cable television system is an example of a carrier system that uses frequency-division multiplexing. Many different television programs are carried simultaneously on the same coaxial cable
by sending each at a different frequency. Multiple layers of multiplexing may ultimately be performed upon a given input signal; i.e., the output
resulting from one stage of modulation
may in turn be modulated. For example, in the public telephone network
, many telephone calls are sent over shared trunklines by time-division multiplexing; then for long distance calls several of these channels may be sent over a communications satellite
link by frequency-division multiplexing. At a given node
, specified channels, groups, supergroups, etc. may be demultiplexed by add-drop multiplexer
s without demultiplexing the others.
, 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 coaxial cable
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 time-division multiplexing
. This work led to T-carrier
and similar digital systems for local use. Due to the shorter repeater
spacings required by digital systems, long-distance still used FDM until the late 1970s when optical fiber
was improved to the point that digital connections became the cheapest ones for all distances, short and long.
Telecommunication
Telecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...
, a carrier system (loosely, a synonym with carrier) is a multichannel
Multichannel
Multichannel may refer to:*Multichannel audio, i.e.**Stereophonic sound, namely two channel audio**Surround sound, more than two channels though still technically stereo**Ambisonics, a studio or live way of recording with many channels...
telecommunications system
System
System is a set of interacting or interdependent components forming an integrated whole....
in which a number of individual channels (e.g. data
Data
The term data refers to qualitative or quantitative attributes of a variable or set of variables. Data are typically the results of measurements and can be the basis of graphs, images, or observations of a set of variables. Data are often viewed as the lowest level of abstraction from which...
, audio, video or combination thereof) are multiplexed for transmission
Transmission (telecommunications)
Transmission, in telecommunications, is the process of sending, propagating and receiving an analogue or digital information signal over a physical point-to-point or point-to-multipoint transmission medium, either wired, optical fiber or wireless...
. The transmission occurs between nodes of a network
Telecommunications network
A telecommunications network is a collection of terminals, links and nodes which connect together to enable telecommunication between users of the terminals. Networks may use circuit switching or message switching. Each terminal in the network must have a unique address so messages or connections...
.
In carrier
Carrier wave
In telecommunications, a carrier wave or carrier is a waveform that is modulated with an input signal for the purpose of conveying information. This carrier wave is usually a much higher frequency than the input signal...
systems, many different forms of multiplexing
Multiplexing
The multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred...
may be used, such as time-division multiplexing
Time-division multiplexing
Time-division multiplexing is a type of digital multiplexing in which two or more bit streams or signals are transferred apparently simultaneously as sub-channels in one communication channel, but are physically taking turns on the channel. The time domain is divided into several recurrent...
and frequency-division multiplexing
Frequency-division multiplexing
Frequency-division multiplexing is a form of signal multiplexing which involves assigning non-overlapping frequency ranges to different signals or to each "user" of a medium.- Telephone :...
. A cable television system is an example of a carrier system that uses frequency-division multiplexing. Many different television programs are carried simultaneously on the same coaxial cable
Coaxial cable
Coaxial cable, or coax, has an inner conductor surrounded by a flexible, tubular insulating layer, surrounded by a tubular conducting shield. The term coaxial comes from the inner conductor and the outer shield sharing the same geometric axis...
by sending each at a different frequency. Multiple layers of multiplexing may ultimately be performed upon a given input signal; i.e., the output
Output
Output is the term denoting either an exit or changes which exit a system and which activate/modify a process. It is an abstract concept, used in the modeling, system design and system exploitation.-In control theory:...
resulting from one stage of modulation
Modulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...
may in turn be modulated. For example, in the public telephone network
Public switched telephone network
The public switched telephone network is the network of the world's public circuit-switched telephone networks. It consists of telephone lines, fiber optic cables, microwave transmission links, cellular networks, communications satellites, and undersea telephone cables, all inter-connected by...
, many telephone calls are sent over shared trunklines by time-division multiplexing; then for long distance calls several of these channels may be sent over a communications satellite
Communications satellite
A communications satellite is an artificial satellite stationed in space for the purpose of telecommunications...
link by frequency-division multiplexing. At a given node
Node (networking)
In communication networks, a node is a connection point, either a redistribution point or a communication endpoint . The definition of a node depends on the network and protocol layer referred to...
, specified channels, groups, supergroups, etc. may be demultiplexed by add-drop multiplexer
Add-drop multiplexer
An add-drop multiplexer is an important element of an optical fiber network. A multiplexer combines, or multiplexes, several lower-bandwidth streams of data into a single beam of light...
s without demultiplexing the others.
History
The purpose of carrier systems is to save money by carrying more traffic on less infrastructure. 19th century telephone systems, operating at basebandBaseband
In telecommunications and signal processing, baseband is an adjective that describes signals and systems whose range of frequencies is measured from close to 0 hertz to a cut-off frequency, a maximum bandwidth or highest signal frequency; it is sometimes used as a noun for a band of frequencies...
, 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
L-carrier
SystemYearFrequencyCoax per cableDistance between repeatersVoice circuits per coax tubeL-119413 MHz48 miles600L-21942840 kHz416 miles360L-319508 MHz84 miles1,860L-4196717 MHz202 miles3,600L-5197257 MHz221 mile10,800L-5E197566 MHz221 mile13,200...
and similar systems carried hundreds of calls at a time on coaxial cable
Coaxial cable
Coaxial cable, or coax, has an inner conductor surrounded by a flexible, tubular insulating layer, surrounded by a tubular conducting shield. The term coaxial comes from the inner conductor and the outer shield sharing the same geometric axis...
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 time-division multiplexing
Time-division multiplexing
Time-division multiplexing is a type of digital multiplexing in which two or more bit streams or signals are transferred apparently simultaneously as sub-channels in one communication channel, but are physically taking turns on the channel. The time domain is divided into several recurrent...
. This work led to T-carrier
T-carrier
In telecommunications, T-carrier, sometimes abbreviated as T-CXR, is the generic designator for any of several digitally multiplexed telecommunications carrier systems originally developed by Bell Labs and used in North America, Japan, and South Korea....
and similar digital systems for local use. Due to the shorter repeater
Repeater
A repeater is an electronic device that receives asignal and retransmits it at a higher level and/or higher power, or onto the other side of an obstruction, so that the signal can cover longer distances.-Description:...
spacings required by digital systems, long-distance still used FDM until the late 1970s when optical fiber
Optical fiber
An optical fiber is a flexible, transparent fiber made of a pure glass not much wider than a human hair. It functions as a waveguide, or "light pipe", to transmit light between the two ends of the fiber. The field of applied science and engineering concerned with the design and application of...
was improved to the point that digital connections became the cheapest ones for all distances, short and long.