Message Transfer Part
The objectives of the Message Transfer Part (MTP) are to provide reliable transport and delivery of User Part signaling information across the signaling network and appropriate reactions to system and network failures. The users of the MTP are the Signaling Connection Control Part (SCCP) and ISDN User Part (ISUP).
The components of the MTP relate directly to the OSI Reference Model’s lower three layers. The Physical Layer is known as the Signaling Data Link; the details can be found in Recommendation Q.702. The Signaling Link is a Data Link Layer protocol found in Recommendation Q.703. And finally, the Network Layer is represented by the Signaling Network functions (Q.704) and Signaling Connection Control Part (Q.711-Q.716).
Signaling System 7 is a connectionless network. It was purposely designed that way to avoid the overhead necessary to establish signaling connections. That is, the designers wanted a network that would have good performance in the face of heavy signaling loads.
Connectionless networks traditionally offer an unreliable datagram service, yet it seems that in a signaling network one would want the service to be as reliable as possible. On the other hand, connectionless networks are efficient. Together, these two facts led designers to choose a connectionless network, providing high performance in the face of heavy signaling loads. A related issue is the desirability of insuring that signaling messages relating to the same call should all arrive in sequence at the destination. Once again, datagram networks do not traditionally guarantee in-sequence delivery because individual datagrams can take different routes through the network. However, the MTP protocol is designed to achieve this end. First of all, standard techniques try to assure reliable delivery of correct messages (i.e., no bit errors). Additionally, to try to insure sequential delivery, all signaling messages associated with the same call are sent over the same network path. All such messages carry the same number, and messages carrying that number are all routed the same way.
The message handling function consists of three major components: message discrimination, message routing, and message distribution. Message routing and message distribution depend on the outcome of the first step, message discrimination. Once a message has been received and error checked at MTP Level 2, it is handed to MTP Level 3, which checks the routing label to determine whether the destination address is the current node. If the destination address is not the current node, the message is passed to the message routing process, which determines the correct link on which the message should be sent.
If the destination address is the same as the current node, the message is passed to the message distribution process, which determines whether it should be passed to the network management process or handed to a User Part. The Service Information Octet (SIO) is examined to identify the User Part to which the message should be handed. It also identifies whether the message conforms to an international or national standard.
The flow chart on the visual illustrates the sequence of decisions necessary to process each incoming message.