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General Background
Broadly, the three basic components of a telecommunications
system are station equipment, transmission facilities, and
switches. Station equipment is generally located on the
customer's premises and includes such items as the telephone at a
residential customer's house. Transmission facilities provide
the paths over which information is transmitted between customers
(whether the medium is used for local network transmission or
transmission over trunks, which are lines between different
networks) and consist of transmission media such as copper and
fiber optic cable, as well as the equipment used to amplify and
regenerate the transmitted signals. Switches connect
transmission facilities at key locations and route incoming and
outgoing calls.
The basic objective of the telephone switch is to connect
any calling outlet with any wanted inlet, a process that can be
visualized by picturing an operator sitting at an old manual
switchboard. As a call is made, the operator pulls a flexible
cord connected to the caller's line and physically plugs it into
a receptacle connected to another line in the same network or to
a trunk line if the recipient is in a different network.
The process of switching actually involves four sequential
phases, each consisting of certain activities or functions. The
first phase, preselection, encompasses activities related to
recognizing a new call request and determining how to route it.
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Last modified: May 25, 2011