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1991-12-31
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Recommendation Q.721
FUNCTIONAL DESCRIPTION OF THE SIGNALLING SYSTEM No. 7
TELEPHONE USER PART (TUP)
1 General
Use of Signalling System No. 7 for telephone call control signalling
requires:
- application of Telephone User Part (TUP) functions, in combination with
- application of an appropriate set of Message Transfer Part (MTP)
functions.
A general description of the signalling system and the division of
functions between the Message Transfer Part and the Telephone User Part are
presented in Recommendation Q.700 and the requirements of interaction between
those two parts are contained in Recommendation Q.701.
2 Telephone User Part
The Telephone User Part specified in these specifications defines the
necessary telephone signalling functions for use of Signalling System No. 7 for
international telephone call control signalling. It is specified with the aim of
providing the same features for telephone signalling as other CCITT telephone
signalling systems.
Signalling System No. 7 can be used to control the switching of all types
of international circuits to be used in a worldwide connection, including
circuits with speech interpolation and satellite circuits.
The system meets all requirements defined by the CCITT concerning the
service features for worldwide international semiautomatic and automatic
telephone traffic. It is designed for the bothway operation of speech circuits.
When used with homogeneous digital telephone circuits the continuity of
these circuits is ensured by the means for transmission quality supervision and
failure detection that are inherent in the digital systems providing these
circuits. However, the system includes means for link-by-link assurance of
continuity check of the speech path when used with analogue telephone circuits
and/or digital circuits including certain types of equipment, where fault
indications are lost, e.g., circuit multiplication equipment.
The signalling system is suitable for national telephone applications.
Most telephone signalling message types and signals specified for international
use are also required in typical national applications. In addition to these,
national applications typically require additional signalling message types and
signals; the system provides ample spare capacity for such additions.
The standard label structure specified for telephone signalling messages
requires that all exchanges using the signalling system are allocated codes from
code plans established for the purpose of unambiguous identification of
signalling points. The principles to apply to the international signalling
network are specified in Recommendation Q.708.
3 Message Transfer Part
The Message Transfer Part of Signalling System No. 7 is specified in
Recommendations Q.701 to Q.709. An overview description of the Message Transfer
Part is contained in Recommendation Q.701.
The Message Transfer Part defines a range of functions by which different
signalling modes and different signalling network configurations may be realized.
Any application of Signalling System No. 7 requires that an appropriate selection
of these functions is applied depending on the intended use of the system and the
characteristics of the telecommunications network concerned.
Fascicle VI.8 - Rec. Q.721 PAGE1