CCITT No. 7 user parts

Abstract

This chapter describes the user part of CCITT No. 7, which utilises the message-transfer part (MTP) to transfer signalling messages through the signalling network. Whereas the MTP provides a comprehensive transfer mechanism, including a dynamic routeing capability, the MTP cannot interpret the meaning of the Level-4 messages being transferred. It is the user part that defines the meaning of the messages that are being transferred and determines the sequence in which messages are sent. It is the user part that also interacts with the call control function within an exchange to establish an overall controlling mechanism for calls. Three user parts have been specified for CCITT No. 7: the telephone user part (TUP) ι, the ISDN user part (ISUP) and the data user part (DUP). All three are defined primarily to control the establishment and release of traffic circuits and they are, therefore, circuit-related in design. The data user part (DUP) was defined in the early development of CCITT No. 7 to control the establishment and release of circuit-switched data calls. The implementation of the DUP is not extensi e, with only a few network operators having implemented dedicated circuit-switched-data networks. Requirements for data networks will in future be handled by the ISUP, with the result that the DUP is unlikely to be used extensively in telecommunications networks. The DUP is therefore not described in this book.