Return channel
Encyclopedia
In communications system
s that use star topologies
, the return channel (also reverse channel, back channel or return link) is the transmission link from a user terminal to the central hub.
Return links are often, but not always, slower than the corresponding forward links. Examples where this is true include ADSL
(where the "A" stands for "asymmetric"), cable modem
s, cellular Internet access facilities (e.g. 3G
) and satellite internet access (e.g. ASTRA2Connect
).
The return channel need not use the same medium as the main channel. For example, some "hybrid" Internet access services use a one-way cable television
system for the forward channel and a dial-up modem
and telephone line
for the return channel.
Even when the return and forward channels use the same medium, their differences
often dictate the use of very different data modulation
and coding
techniques. For example, in a star radio network,
only the central hub transmits on the forward link, so multiple access contention
is
a consideration only on the return link.
The "forward/return" terminology is also used for spacecraft
command and telemetry
links. Because the return link carries telemetry, it is often orders of magnitude faster than the forward link that transmits only
a few predefined spacecraft commands.
"Return" and "forward" links are distinct from, and should not be confused with, uplink
s and downlinks in satellite communication systems. For example, satellite Internet access with
conventional bent pipe spacecraft transponders requires a total of two uplinks and two downlinks. One uplink and downlink pair are used for the forward link from the central ground hub through the satellite to the user terminal, and another uplink/downlink pair are
used for the return link from the user terminal to the central hub.
Communications system
In telecommunication, a communications system is a collection of individual communications networks, transmission systems, relay stations, tributary stations, and data terminal equipment usually capable of interconnection and interoperation to form an integrated whole...
s that use star topologies
Star network
Star networks are one of the most common computer network topologies. In its simplest form, a star network consists of one central switch, hub or computer, which acts as a conduit to transmit messages...
, the return channel (also reverse channel, back channel or return link) is the transmission link from a user terminal to the central hub.
Return links are often, but not always, slower than the corresponding forward links. Examples where this is true include ADSL
Asymmetric Digital Subscriber Line
Asymmetric digital subscriber line is a type of digital subscriber line technology, a data communications technology that enables faster data transmission over copper telephone lines than a conventional voiceband modem can provide. It does this by utilizing frequencies that are not used by a voice...
(where the "A" stands for "asymmetric"), cable modem
Cable modem
A cable modem is a type of network bridge and modem that provides bi-directional data communication via radio frequency channels on a HFC and RFoG infrastructure. Cable modems are primarily used to deliver broadband Internet access in the form of cable Internet, taking advantage of the high...
s, cellular Internet access facilities (e.g. 3G
3G
3G or 3rd generation mobile telecommunications is a generation of standards for mobile phones and mobile telecommunication services fulfilling the International Mobile Telecommunications-2000 specifications by the International Telecommunication Union...
) and satellite internet access (e.g. ASTRA2Connect
ASTRA2Connect
ASTRA2Connect is a two-way satellite broadband Internet service available across Europe, which launched in March 2007, and uses the ASTRA series of geostationary satellites...
).
The return channel need not use the same medium as the main channel. For example, some "hybrid" Internet access services use a one-way cable television
Cable television
Cable television is a system of providing television programs to consumers via radio frequency signals transmitted to televisions through coaxial cables or digital light pulses through fixed optical fibers located on the subscriber's property, much like the over-the-air method used in traditional...
system for the forward channel and a dial-up modem
Modem
A modem is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data...
and telephone line
Telephone line
A telephone line or telephone circuit is a single-user circuit on a telephone communication system...
for the return channel.
Even when the return and forward channels use the same medium, their differences
often dictate the use of very different data 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...
and coding
Coding
Coding may refer to:* Channel coding in coding theory* Line coding* Computer programming, the process of designing, writing, testing, debugging / troubleshooting, and maintaining the source code of computer programs...
techniques. For example, in a star radio network,
only the central hub transmits on the forward link, so multiple access contention
Contention
Contention may refer to:* The main contention, in rhetoric, the main point being argued* Resource contention, a general concept in communications and computing, is competition by users of a system for the facility at the same time:...
is
a consideration only on the return link.
The "forward/return" terminology is also used for spacecraft
Spacecraft
A spacecraft or spaceship is a craft or machine designed for spaceflight. Spacecraft are used for a variety of purposes, including communications, earth observation, meteorology, navigation, planetary exploration and transportation of humans and cargo....
command and telemetry
Telemetry
Telemetry is a technology that allows measurements to be made at a distance, usually via radio wave transmission and reception of the information. The word is derived from Greek roots: tele = remote, and metron = measure...
links. Because the return link carries telemetry, it is often orders of magnitude faster than the forward link that transmits only
a few predefined spacecraft commands.
"Return" and "forward" links are distinct from, and should not be confused with, uplink
Uplink
A telecommunications link is generally one of several types of information transmission paths such as those provided by communication satellites to connect two points on earth.-Uplink:...
s and downlinks in satellite communication systems. For example, satellite Internet access with
conventional bent pipe spacecraft transponders requires a total of two uplinks and two downlinks. One uplink and downlink pair are used for the forward link from the central ground hub through the satellite to the user terminal, and another uplink/downlink pair are
used for the return link from the user terminal to the central hub.
See also
- Backhaul (telecommunications)Backhaul (telecommunications)In a hierarchical telecommunications network the backhaul portion of the network comprises the intermediate links between the core network, or backbone, of the network and the small subnetworks at the "edge" of the entire hierarchical network...
- BroadbandBroadbandThe term broadband refers to a telecommunications signal or device of greater bandwidth, in some sense, than another standard or usual signal or device . Different criteria for "broad" have been applied in different contexts and at different times...
- Satellite Internet