The functional part of the Signaling System No. 7 (SS7) protocol, i.e., the part that specifies the interexchange signaling procedures for the set up and tear down of trunk calls between networks. The ISDN User Part is one of the specified users of the Message Transfer Part (MTP) transport capability of the SS7 interconnections. The user part identification is encoded in the service indicator (SI) portion of the service information octet (SIO. )