Message Transfer Part Functions and Protocol

Message Transfer Part (MTP) provides services for transferring the messages of user parts (such as TUP, ISUP, and SCCP) to destinations through SS7. If SS7 experiences a fault, the messages can be transmitted correctly through the signaling network management function.
The overall objectives of the MTP are provided the means for:

- The reliable transport and delivery "User Part" signaling information across the SS7 network.
- The ability to react to system and network failures that will affect, and take the necessary action to ensure that is achieved.

MTP1 Signaling Data Link Layer
Bidirectional transparent transmission channel (64kbit/s)

MTP2 Signaling Link Layer
MTP2 provide reliable signaling links for transmitting signal messages between two directly connected signaling points.
- It indicates the starting point and the ending point of a signal unit, namely, recognizing signal units from bit streams in a signaling data link.
- It is used to check whether signaling links in service is loss of alignment. Loss of alignment will cause a change in the mode of operation of the signal unit error rate monitor.
- It is used to check whether the bit streams of a signal unit are transmitted incorrectly.
- It is used to re-obtain correct signal units if some errors are found. There are two correction methods: basic method and preventive cyclic re-transmission method.
- The initial alignment procedure is appropriate to both first time initialization (e.g. after "switch-on") and alignment in association with restoration after a link failure.
Signaling Link Error Monitoring
It is used to monitor the errors of a signaling link to ensure high-quality service. It include signal unit error detection and the alignment error rate detection
- It is used to process the congestion problem detected by the second layer to lighten the congestion condition.
- It is used to mark or remove the faulty status of a processor.

MTP3 Functional Layer of Signaling Network
A normal signaling link sends an SLTM message every minute. The signaling point that receives the SLTM message sends an SLTA message. The value of test code in the SLTA message must be same to that in the SLTM message. Also, the number of SLTA messages must be equal to that of SLTM messages.

After MTP2 is aligned successfully, MTP3 sends an SLTM message and starts a six-second timer. If an SLTA message is received before the timer is time out, the link service is activated. If an SLTA message is not received, the SLTM message is transmitted again after the timer is timeout. If the SLTA message is not received within six seconds, the link service cannot be activated.

If a signaling link is unavailable, the changeover is performed. During the changeover process, signaling services are transferred from an unavailable signaling link to one or more replaced link and avoiding message loss, duplication or mis-sequencing. Figure 1-9 shows the changeover procedure.