The Message Transfer Part (MTP) of the SS7 signalling system is used for the reliable and fast transmission of signalling messages. It is used to exchange the message signal units (MSU) between the signalling points (SP) by means of point codes (PC). The message transfer part operates with error protection procedures
and ensures error-free message transport and the correct chronological sequence of the messages. The message transfer part is subdivided into several levels, which have the designations Level 1, 2 and 3 and functionally cover the lower layers of
the SS7 architectureMessage Transfer Part Level 1 (MTP1) is the architecture level of SS7 in which the physical, electrical and functional properties of the digitallink
with 64 kbit/sMTP Level 2 (MTP2) ensures the exact end-to-end transmission of messages via a Signalling Link (SLK). Flow control, message validation, error control and retransmission are defined in this level. If an error
occurs on a Signalling Link (SLK), the message is sent back. MTP Level 2 corresponds functionally to thelink
layer of the OSI reference model
MTP Level 3 (MTP3) supports message routing between the Signalling Points (SP) and corresponds functionally to the network layer of the OSI reference model. In MTP3, messages are routed according to the routing labels in the Signalling Information Field (SIF), which includes the Destination Point Code (DPC), Originating Point Code (OPC) and Signalling Link Selection (SLS). Point Codes (PC) are numeric addresses used to uniquely identify the various signaling points in an SS7 network. MTP3 supports the ISDN User Part (ISUP) and the Telephony User Part
(TUP), which are located above the MTP3 layer.Unit (MSU) and consists of various data fields such as the Flags (F), the Check Bits (CK), the User Part (UP), Service Indic ator (SI), Length Indic ator (LI), Forward Indicator Bit (FIB), the Forward Sequence Number (FSN), the Backward Indicator Bit (BIB) and the Backward Sequence Number (BSN).