ISDN user part (ISUP)
The ISDN User Part (ISUP) is one of several possible user parts( UP) of the SS7 signaling. The ISDN User Part is the ISDN-specific part of the transmission of additional information via the signaling channel between exchanges.
Functionally, it is comparable to the 1TR6 protocol or the DSS1 protocol, Digital Subscriber System no. 1 (DSS1), on the subscriber side. ISUP is the prerequisite for a complete ISDN service offering in national and international traffic. The availability of all additional 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).