MTP 3 - Message Transfer Protocol 3

Message Transfer Protocol 3 (MTP 3) is part of the protocol stack used within the Common Channel Signalling (CCS) network. The CCS network provides a means to transfer out of band signalling information between network nodes within the telephony network. MTP 3 is specified as part of the Signalling System 7 protocol and is also referred to as part of the B-ICI interface for ATM. MTP 3 sits between MTP 2 and the user parts (ISUP, TUP, SCCP and TCAP) of the SS7 protocol stack.

MTP 3 provides a connectionless message transfer system for passing information across a network. MTP 3 defines the functions and procedures of the signalling system for signalling message handling and signalling network management. Signalling message handling consists of the actual transfer of a signalling message and directing the message to the proper signalling link or user part. Signalling network management consists of controlling the signalling message routing and configuration of the signalling network facilities based on predetermined information and the status of the signalling network facilities.

The MTP 3 protocol is specified by the ITU Q.704 Recommendation, ANSI T1.111 Recommendation and other national recommendations. Each of these recommendations is different. The ITU and ANSI Recommendations can be purchased from Global Engineering at 1-800-624-3974 and other sources. There is a good tutorial on Signalling System 7 in the July 1990 issue of IEEE Communications. Thanks to Jeff Lawrence of Trillium Digital Systems, Inc. for the above definition.

MTP3 (Message Transfer _Part_ level 3) is the network layer of the SS7 signalling transport system. It routes SS7 signalling messages to public network nodes by means of Destination Point Codes, and to the appropriate signalling entity within a node by means of a Service Info Octet. B-ISUP is an Application Layer protocol run over MTP3.

MTP3 includes a number of link-protection features, to allow automatic rerouting of signalling messages around broken signalling transfer points. It includes certain management functions for congestion control on signalling links.

The protocol is defined in Q.704, available from ITU, Place de Nations, Geneva, Switzerland. Write or use gopher@info.itu.ch.

MTP3 is widely deployed for existing narrowband SS7 networks. It will be used for the transport of B-ISUP, but don't expect the document to mention ATM. MTP3 assumes it is running over MTP2 (data link protocol), and the SAAL is specifically designed to mimic this and leave MTP3 unchanged.

Thanks to Mark Jeffrey of GPT ltd for this definition.