. There are national and international signalling points that operate with the corresponding national and international Signalling Point Codes (SPC). The marking of the Signalling Points into national and international is done in the Network Indicator (NI) data field of the Message Transfer Part
(MTP). Between the Signalling Points the messages and control information are exchanged with the Message Signal Unit (MSU).
The signalling point is described in ITU-T recommendation Q.704.