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