The Signalling System Number 7 is an
ITU standard for signalling in telephone networks.
Signalling System Number 7
is an international telecommunications standard developed
by the International Telecommunication Union (ITU) Telecommunication Sector (ITU-T). The
Signalling System Number 7 standard defines the procedures and protocols by
which elements in the public switched telephone network (PSTN) exchange
out-of-band signalling information over a digital signalling network. The protocols
and procedures define such things as call set-up and tear-down,
call management, call routing, call forwarding, display of calling party
number, wireless services, and so on. The Signalling System Number 7
standard refers to out-of-band signalling, that is signalling that is
not sent down the same channel as the voice call.
In
GSM, the Signalling System Number 7 is
used for all signalling on all links except for the
air interface (or radio interface).