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message transfer part (MTP)

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

aredefined. As physical interfaces, MTP Level 1 defines E1 lines with 2,048 kbit/s out of 32 64-kbit/s lines, the DS transmission interface DS-0 with 64 kbit/s and DS-1 with 1,544 kbit/s, and V.35

with 64 kbit/s

Protocol stack of the SS7 protocol

Protocol stack of the SS7 protocol

MTP 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.

Structure of the Message Transfer Part (MTP)

Structure of the Message Transfer Part (MTP)

The Message Transfer Part is part of the Message Signal 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).

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Englisch: message transfer part - MTP
Updated at: 08.10.2011
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