MESSAGE TRANSFER PART

The 'Message Transfer Part' (MTP) is part of the Signalling System 7 (SS7) used for communication in Public Switched Telephone Networks. MTP is responsible for reliable, unduplicated and in-sequence transport of SS7 messages between communication partners.
MTP is made up of three levels, corresponding to layers in the OSI model: MTP Level 1 corresponds to OSI Layer 1 (the physical layer), MTP Level 2 to OSI Layer 2 (the data link layer), and MTP Level 3 to OSI Layer 3 (the network layer). MTP Level 3 is usually abbreviated as MTP3. Likewise MTP Level 2 and MTP Level 1 are abbreviated as MTP2 and MTP1.
MTP1 normally uses a timeslot in an E-carrier or T-carrier.
MTP2 provides error detection and sequence checking, and retransmits unacknowledged messages. MTP2 uses packets called ''signal units'' to transmit SS7 messages. There are three types of signal units: Fill-in Signal Unit (FISU), Link Status Signal Unit (LSSU), Message Signal Unit (MSU).
MTP3 provides routing functionality to transport signaling messages through the SS7 network to the requested endpoint. Each network element in the SS7 network has a unique address, the Point Code (PC). Message routing is performed according to this address. A distinction is made between a Signaling Transfer Point (STP) which only performs MTP message routing functionalities and a Signaling End Point (SEP) which uses MTP to communicate with other SEPs (that is, telecom switches). MTP3 is also responsible for network management; when the availability of MTP2 data links changes, MTP3 establishes alternative links as required and propagates information about route availability through the network.
MTP is formally defined in ITU-T recommendations Q.701-Q.705. Tests for the MTP are specified in the ITU-T recommendations Q.781 for MTP2 and in Q.782 for MTP3. These tests are used to validate the correct implementation of the MTP protocol.
Different countries use different variants of the MTP protocols. In North America, the formal standard followed is the Bellcore document GR-246-CORE.