Message Transfer Part
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The Message Transfer Part is part of the Signalling System 7 (SS7) which is used for communication in Public Switched Telephone Network.
The MTP is responsible for the correct and reliable end to end data transport of SS7 messages between communication partners. According to the OSI model the MTP Level two corresponds to OSI Layer 2 (data link layer) and the 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/T-carrier.
MTP Level two provides error detection, sequence checking and it initiates retransmission in case of erroneous reception of messages. The MTP Level2 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 three 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 the MTP to communicate with other SEPs e.g. telecom switches.