In a

telecommunications network A telecommunications network is a group of Node (networking), nodes interconnected by telecommunications links that are used to exchange messages between the nodes. The links may use a variety of technologies based on the methodologies of circuit ...

, a link is a

communication channel A communication channel refers either to a physical transmission medium such as a wire, or to a logical connection over a multiplexed medium such as a radio channel in telecommunications and computer networking. A channel is used for infor ...

that connects two or more devices for the purpose of

data transmission Data communication, including data transmission and data reception, is the transfer of data, signal transmission, transmitted and received over a Point-to-point (telecommunications), point-to-point or point-to-multipoint communication chann ...

. The link may be a dedicated physical link or a

virtual circuit A virtual circuit (VC) is a means of transporting data over a data network, based on packet switching and in which a connection is first established across the network between two endpoints. The network, rather than having a fixed data rate reser ...

that uses one or more physical links or shares a physical link with other telecommunications links. A telecommunications link is generally based on one of several types of information transmission paths such as those provided by

communication 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 Earth. ...

s, terrestrial

radio communications Radio is the technology of communicating using radio waves. Radio waves are electromagnetic waves of frequency between 3 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmitter connected t ...

infrastructure and

computer network A computer network is a collection of communicating computers and other devices, such as printers and smart phones. In order to communicate, the computers and devices must be connected by wired media like copper cables, optical fibers, or b ...

s to connect two or more points. The term ''link'' is widely used in computer networking to refer to the communications facilities that connect nodes of a network. Sometimes the communications facilities that provide the communication channel that constitutes a link are also included in the definition of ''link''.

Types

Point-to-point

A point-to-point link is a dedicated link that connects exactly two communication facilities (e.g., two nodes of a network, an intercom station at an entryway with a single internal intercom station, a radio path between two points, etc.).

Broadcast

Broadcast links connect two or more nodes and support ''broadcast transmission'', where one node can transmit so that all other nodes can receive the same transmission. Classic Ethernet is an example.

Multipoint

Also known as a ''multidrop'' link, a multipoint link is a link that connects ''two or more'' nodes. Also known as general topology networks, these include ATM and

Frame Relay Frame Relay (FR) is a standardized wide area network (WAN) technology that specifies the Physical layer, physical and data link layers of digital telecommunications channels using a packet switching methodology. Frame Relay was originally devel ...

links, as well as

X.25 X.25 is an ITU-T standard protocol suite for Packet switched network, packet-switched data communication in wide area network, wide area networks (WAN). It was originally defined by the CCITT, International Telegraph and Telephone Consultative Co ...

networks when used as links for a

network-layer protocol In the seven-layer OSI model of computer networking, the network layer is layer 3. The network layer is responsible for packet forwarding including routing through intermediate routers. Functions The network layer provides the means of transf ...

like IP. Unlike broadcast links, there is no mechanism to efficiently send a single message to all other nodes without copying and retransmitting the message.

Point-to-multipoint

A point-to-multipoint link (or simply a ''multipoint'') is a specific type of multipoint link which consists of a central connection endpoint (CE) that is connected to multiple peripheral CEs. All of the peripheral CEs receive any transmission of data that originates from the central CE while any transmission of data that originates from any of the peripheral CEs is only received by the central CE.

Private and public

Links are often referred to by terms that refer to the ownership or accessibility of the link. * A ''private link'' is a link that is either owned by a specific entity or a link that is only accessible by a particular entity. * A ''public link'' is a link that uses the public switched telephone network or other public utility or entity to provide the link and which may also be accessible by anyone. *

Direction

Uplink

*Pertaining to

radiocommunication service Radio is the technology of communicating using radio waves. Radio waves are electromagnetic waves of frequency between 3 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmitter connected to ...

, an uplink (UL or U/L) is the portion of a

feeder link A feeder link is – according to ''Article 1.115'' of the International Telecommunication Union´s (ITU) ITU Radio Regulations (RR)ITU Radio Regulations, Section IV. Radio Stations and Systems – Article 1.115, definition: ''feeder link'' – ...

used for the transmission of signals from an

earth station A ground station, Earth station, or Earth terminal is a terrestrial radio station designed for extraplanetary telecommunication with spacecraft (constituting part of the ground segment of the spacecraft system), or reception of radio waves fro ...

to a space radio station, space radio system or high altitude platform station. *Pertaining to

GSM The Global System for Mobile Communications (GSM) is a family of standards to describe the protocols for second-generation (2G) digital cellular networks, as used by mobile devices such as mobile phones and Mobile broadband modem, mobile broadba ...

and cellular networks, the radio uplink is the transmission path from the

mobile station A mobile station (MS) comprises all user equipment and software needed for communication with a mobile network. The term refers to the global system connected to the mobile network, i.e. a mobile phone or mobile computer connected using a mobi ...

(cell phone) to a

base station Base station (or base radio station, BS) is – according to the International Telecommunication Union's (ITU) Radio Regulations (RR) – a " land station in the land mobile service." A base station is called '' node B'' in 3G, '' eNB'' in L ...

(cell site). Traffic and signalling flowing within the BSS and NSS may also be identified as uplink and downlink. *Pertaining to computer networks, an uplink is a connection from data communications equipment toward the network core. This is also known as an upstream connection.

Downlink

*Pertaining to

, a downlink (DL or D/L) is the portion of a

used for the transmission of signals from a space radio station, space radio system or high altitude platform station to an

. *In the context of

satellite communications 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 Earth. ...

, a downlink (DL) is the link from a satellite to a

ground station A ground station, Earth station, or Earth terminal is a terrestrial radio station designed for extraplanetary telecommunication with spacecraft (constituting part of the ground segment of the spacecraft system), or reception of radio waves fr ...

. *Pertaining to

cellular network A cellular network or mobile network is a telecommunications network where the link to and from end nodes is wireless network, wireless and the network is distributed over land areas called ''cells'', each served by at least one fixed-locatio ...

s, the radio downlink is the transmission path from a

cell site A cell site, cell phone tower, cell base tower, or cellular base station is a cellular frequencies, cellular-enabled mobile device site where antenna (electronics), antennas and electronic communications equipment are placed (typically on a Rad ...

to the

cell phone A mobile phone or cell phone is a portable telephone that allows users to make and receive calls over a radio frequency link while moving within a designated telephone service area, unlike fixed-location phones ( landline phones). This radio ...

. Traffic and signalling flowing within the base station subsystem (BSS) and

network switching subsystem Network switching subsystem (NSS) (or GSM core network) is the component of a GSM system that carries out call out and mobility management functions for mobile phones roaming on the network of base stations. It is owned and deployed by mobi ...

(NSS) may also be identified as uplink and downlink. *Pertaining to

s, a downlink is a connection from data communications equipment toward

data terminal equipment Data terminal equipment (DTE) is an end instrument that converts user information into signals or reconverts received signals. It is also called data processing terminal equipment or tail circuit. A DTE device communicates with the data cir ...

. This is also known as a downstream connection.

Forward link

A forward link is the link from a fixed location (e.g., a

) to a mobile user. If the link includes a communications relay satellite, the forward link will consist of both an

uplink In a telecommunications network, a link is a communication channel that connects two or more devices for the purpose of data transmission. The link may be a dedicated physical link or a virtual circuit that uses one or more physical links or shar ...

(base station to satellite) and a downlink (satellite to mobile user).Basics of C & Ku Band
Scatmag.com

Reverse link

The reverse link (sometimes called a ''

return channel In communications systems, the return channel (also reverse 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 wher ...

'') is the link from a mobile user to a fixed base station. If the link includes a communications relay satellite, the reverse link will consist of both an

(mobile station to satellite) and a downlink (satellite to base station) which together constitute a half hop.

References

* * {{Telecommunications Telecommunications Telecommunications engineering Communication circuits Broadcast engineering Telecommunications infrastructure