SIGNALING CONNECTION CONTROL PART The message transfer part (MTP) of SS7 (Chapter 8) was designed to transfer TUP (and later on, ISUP) messages between exchanges at the ends of a trunk. The signaling connection control part (SCCP), in combination with MTP, provides the transfer of messages that are not related to individual trunks, for example, transaction messages [l-3]. The combination of MTP and SCCP is known as the network service part (NSP) of SS7, and is the equivalent of layers 1,2, and 3 of the OS1 (open systems interconnection) protocols in data communication systems () | Signaling in Telecommunication Networks. John G. van Bosse Copyright 1998 John Wiley Sons Inc. ISBNs 0-471-57377-9 Hardback 0-471-22415-4 Electronic 14 SIGNALING CONNECTION CONTROL PART The message transfer part MTP of SS7 Chapter 8 was designed to transfer TUP and later on ISUP messages between exchanges at the ends of a trunk. The signaling connection control part SCCP in combination with MTP provides the transfer of messages that are not related to individual trunks for example transaction messages 1-3 . The combination of MTP and SCCP is known as the network service part NSP of SS7 and is the equivalent of layers 1 2 and 3 of the OSI open systems interconnection protocols in data communication systems . SCCP has been documented by CCITT 4 and later by ITU in a series of Recommendations 5-8 . The . version has been specified by ANSI 9 . INTRODUCTION Figure shows SCCP and its relations with the other parts of SS7. TCAP and ISUP are SCCP users. In turn the ASEs at a signaling point are users of TCAP and can be considered as indirect SCCP users. Each SCCP user at a signaling point has a subsystem number SSN that ranges 1 through 127. When SCCP receives an incoming message from MTP it uses SSN to deliver the message to the appropriate SCCP user in the case of ASEs the message is passed to TCAP which then delivers it . In this chapter the term subsystem is used to denote an SCCP user. 385 386 SIGNALING CONNECTION CONTROL PART Figure Position of SCCP in signaling system . From Rec. . Courtesy of ITU-T. Message Transfer Enhancements SCCP enhances the message transfer capabilities of SS7 in two principal ways. In the first place MTP uses the service indicator SI in an incoming message to deliver it to the appropriate MTP user SCCP is one of them . SI has a range 0 through 15 limiting the number of MTP users to 16. SCCP expands the addressing capability of SS7 allowing up to 127 subsystems at a signaling point. In the .
