The ISDNUser Part (ISUP) is one of several possible user parts (UP) of the SS7 signalling. The ISDN User Part is the ISDN-specific part of the transmission of additional information via the signalling channel between exchanges
no. 1 (DSS1), on the subscriber side. ISUP is the prerequisite for a complete ISDN service offering in national and international traffic. Theavailability of all supplementary services depends on the national implementations
. The ISDN User Parts is entered in the data field for the service indicator, the Service Indicator (SI). The SI data field is a 4-bit long data field in the Message Transfer Part (MTP) of the Message Signal Unit (MSU).