Message Transfer Part
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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 the correct and reliable end to end data transport of SS7 messages between communication partners. In the OSI model, MTP Level 2 corresponds to OSI Layer 2 (data link layer) and MTP Level 3 to the OSI Layer 3 (network layer). The physical layer for the MTP transport normally uses a time slot in an E-carrier or T-carrier.
MTP Level 2 provides error detection, sequence checking, and initiates retransmission in case of erroneous reception of messages. MTP Level 2 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).
MTP Level 3 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 Signaling Point Code (SPC). Message routing is performed according to this address.
MTP is formally defined in ITU-T recommendations Q.701-Q.705.
In the SS7 network 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).
MTP Level 3 is often abbreviated to MTP3. The same applies to the other two levels: MTP2 and MTP1.