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5.2
Basic Structure of a Telephone Network System
As the number of telephones to be connected to the exchange increases
and the conversation area expands, it becomes more economic to set up
two or more exchanges and connect between exchanges by telephone lines
rather than terminating all of the telephone lines in a single exchange.
The line connecting two exchanges is called a transit trunk.
The line connecting an exchange and a telephone is called a subscriber
line.
As the number of telephone centers increases, it becomes more economic
to set up an exchange for terminating only the transit trunks and connect-
ing this exchange in the center in shape of a star rather than connecting
telephone centers to others in the shape of an interlinked network by
directly connected transit trunks.
This kind of exchange is called a transit exchange. An exchange that con-
nects subscriber lines is called a subscriber exchange.
Exchange
Subscriber line
Fig. 1-21 Subscriber Line and Transit Trunk
Subscriber exchange
Without a transit exchange (Mesh)
Fig. 1-22 Transit Exchange System
BASIC OF TELEPHONE
Transit trunk
Transit exchange
With a transit exchange (Star)
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