Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.

A concentrator is a device that provides a central connection point for the connection of terminal, computer, or communication devices. It can be a central point where cables converge. Technically, a concentrator merges a certain number of incoming lines with a number of outgoing lines, or provides a central communication link for a number of devices. Several types of concentrators are listed here:

• Concentrators    In the mainframe environment, a concentrator can merge the lines from a number of terminals and provides a link to another concentrator in a hierarchical arrangement, or links directly to the front-end processor of a host computer. Data from the low-speed terminal lines is transferred over a high-speed line using a multiplexing method or a contention method. In the multiplexing method, a terminal gets a fixed time slot in the multiplexed stream. In the contention method, each low-speed line gets full access to the high-speed line for a brief period. See "Mainframe" for more information.


• Front-end processors    A front-end processor is similar to the concentrator just described in function, but it is usually a dedicated computer in its own right that performs concentration functions at higher speeds and supports more attached devices. See "Mainframe" for more information.


• LAN concentrators    In the LAN (local area network) environment, concentrators have grown from simple wire management facilities to hub devices that provide "collapsed backbone," bridging, and routing functions. A collapsed backbone is equivalent to shrinking a bus cable system like Thinwire coaxial Ethernet down to the size of a small box. A separate wire of inexpensive twisted-pair wire then runs to each workstation. See "Network Connection Technologies." "Switch Fabrics and Bus Design," and "Ethernet."


• Port-sharing and selector units    Port-sharing units provide a way for multiple terminals at a remote site to share a modem connection to a computer or host system. The device fits in between the terminals and the modem.


• Multiplexers    The original design of a multiplexer was based on a need to reduce the cost of data transmissions for terminal devices that needed to communicate with a host device over a telecommunication link. A multiplexer is a device that merges the data from multiple terminals into one line and then ships the merged data over the link, where it is demultiplexed at the other end. Multiplexing cost-justifies the leasing of a high-speed digital line such as T1. See "Multiplexing and Multiplexers" for more details.

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