Message Transfer Part 2 – MTP2 corresponds to OSI Layer 2 (the data link layer) and as such is the lowest protocol in the stack. Sitting on the physical layer, it provides a reliable means of transfer for signaling information between two directly connected signaling points (SPs), ensuring that the signaling information is delivered in sequence and error-free.
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MTP2 performs the following functions:
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The signaling information is transmitted in frames called signal units (SUs). SUs are of variable length, thereby requiring the start and end of each SU to be flagged in the data stream. MTP2 performs this function, which is called signal unit delimitation. The ability to correctly recognize signal units is achieved through signal unit alignment.
Error correction is implemented by retransmitting the signal unit(s) received in error. The link is also continuously monitored to ensure that error rates are within permissible limits. If the error rate becomes greater than predefined limits, MTP2 reports the failure to Message Transfer Part 3 (MTP3), which subsequently orders MTP2 to remove the link from service. Conversely, initial alignment procedures are used to bring links into service.
Link flow control procedures are provided to resolve congestion at the MTP2 layer. Congestion occurs if MTP3 falls behind in processing SUs from the MTP2 buffer.
This chapter describes each of the previously outlined functional areas of MTP2.
It is important to understand that the MTP2 protocol does not work end to end. Rather, it operates on a link-by-link basis (known in datacoms as point to point) between two SPs. Therefore, each signaling data link has an associated MTP2 at each end.