Signalling System Number 7 (SS#7) is the protocol
used by the telephone companies for interoffice signalling.
In the past, in-band signalling techniques were used
on interoffice trunks. This method of signalling used
the same physical path for both the call-control signalling
and the actual connected call. This method of signalling
is inefficient and is rapidly being replaced by out-of-band
or common-channel signalling techniques.
A
network utilizing common-channel signalling is actually
two networks in one:
- First
there is the circuit-switched "user" network
which actually carries the user voice and data traffic.
It provides a physical path between the source and
destination.
- The
second is the signalling network which carries the
call control traffic. It is a packet-switched network
using a common channel switching protocol.
The
original common channel interoffice signalling protocols
were based on Signalling System Number 6 (SS#6). Today
SS#7 is being used in new installations worldwide. SS#7
is the defined interoffice signalling protocol for ISDN.
It is also in common use today outside of the ISDN environment.
The
primary function of SS#7 is to provide call control,
remote network management, and maintenance capabilities
for the inter- office telephone network. SS#7 performs
these functions by exchanging control messages between
SS#7 telephone exchanges (signalling points or SPs)
and SS#7 signalling transfer points (STPs).
The
switching offices (SPs) handle the SS#7 control network
as well as the user circuit-switched network. Basically,
the SS#7 control network tells the switching office
which paths to establish over the circuit-switched network.
The STPs route SS#7 control packets across the signalling
network. A switching office may or may not be an STP.
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