Once a call is initiated from ET1 to ET2, a dedicated link (circuit) is
established across the entire network, connecting the two end terminals together.
An Introduction to Telephony and Asterisk
[ 10 ]
In the following diagram the currently allocated circuit from ET1 to ET2 is shown.
In the above diagram observe that the connection between ET1 and ET2 prevents
other terminals from establishing a circuit to ET1 or ET2. At the same time, other end
terminals are fully capable of establishing circuits among themselves, even while
traversing the same trunks as the circuit from ET1 to ET2.
The trunks connecting the Central Office Exchanges would usually be of type SS7,
connections from the Central Office Exchange to a Private Branch Exchange would
usually be of types PRI, BRI, or FXO, and the connection from the Central Office
Exchange to an analog end terminal would always be of type FXO. Each of these
connectivity methods is discussed in the following sections.
Signalling System # 7 (SS7)
While SS7 is a common trunk when interconnecting Central Office Exchanges, its
usage in the Asterisk environment is still highly experimental. The various aspects
of the SS7 signalling protocol are beyond the scope of this book; however, some
preliminary information is included.
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