In telecommunications a link is the communications channel that connects two or more communicating devices. This link may be an actual physical link or it may be a logical link that uses one or more actual physical links. When the link is a logical link the type of physical link should always be specified (e.g., data link, uplink, downlink, fiber optic link, point-to-point link, etc.) This term is widely used in computer networking (see Data link) to refer to the communications facilities that connect nodes of a network.[1][a]
A telecommunications link is generally one of several types of information transmission paths such as those provided by communication satellites, terrestrial radio communications infrastructure and computer networks to connect two or more points.
Types of links
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. 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 links, as well as X.25 networks when used as links for a network layer protocol 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 is a specific type of multipoint link which consists of a central connection endpoint (CE) that is connected to multiple peripheral CEs. Any transmission of data that originates from the central CE is received by all of the peripheral CEs while any transmission of data that originates from any of the peripheral CEs is only received by the central CE.
This term is also often used as a synonym for multipoint, as defined above.
Private and Public - Accessibility and Ownership
Links are often referred to by terms which refer to the ownership and / 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 specific 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 satellite communications, an uplink (UL or U/L) is the portion of a communications link used for the transmission of signals from an Earth terminal to a satellite or to an airborne platform. An uplink is the inverse of a downlink. An uplink or downlink is distinguished from reverse link or forward link.
- Pertaining to GSM and cellular networks, the radio uplink is the transmission path from the mobile station (cell phone) to a base station (cell site). Traffic and signalling flows 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
- In the context of satellite communications, a downlink (DL) is the link from a satellite to a ground station.
- Pertaining to cellular networks, the radio downlink is the transmission path from a cell site to the cell phone. Traffic and signalling flows within the base station subsystem (BSS) and network switching subsystem (NSS) may also be identified as uplink and downlink.
- Pertaining to a computer networks, a downlink is a connection from data communications equipment towards data terminal equipment. This is also known as a downstream connection.
Forward link
A forward link is the link from a fixed location (e.g., a base station) to a mobile user. If the link includes a communications relay satellite, the forward link will consist of both an uplink (base station to satellite) and a downlink (satellite to mobile user).[2]
Reverse link
The reverse link (sometimes called a return channel) 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 uplink (mobile station to satellite) and a downlink (satellite to base station) which together constitute a half hop.
Sources
See also
Notes
- ^ Sometimes the communications facilities that provide the communications channel that constitutes a link are also included in the definition of link.
References
This article incorporates