SUBJECT E2)

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. MTP3 is specified as part of the Signalling System 7 protocol and is also referred to as part of the B-ICI interface for ATM. MTP3 sits between MTP2 and the user parts (ISUP, TUP, SCCP and TCAP) of the SS7 protocol stack. 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.

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 ATM SAAL is specifically designed to mimic this and leave MTP3 unchanged.

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Last Changed 25 August 1996