Signaling System No. 7 (SS7/C7) - Protocol, Architecture and Services (Full Book)
     
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Summary

MTP2 works point to point and "frames" signaling information into packets called signaling units (SUs). There are three types of SUs:

  • Fill-in Signal Unit (FISU)

  • Link Status Signal Unit (LSSU)

  • Message Signal Unit (MSU)

MTP2 uses flags (delimitation) to separate SUs.

FISUs are fillers that are sent when no LSSUs or MSUs are to be sent. LSSUs are sent to convey link status information between two adjacent signaling points (SPs). MSUs carry the real signaling content: messages for call control, network management, and TCAP query/response. MTP2 ensures that MSUs are received in sequence and without errors.

MTP2 provides monitoring functions to MTP3 by using error rate counters. If specified thresholds are exceeded, MTP3 asks MTP2 to put the link out of service. If instructed by MTP3 to do so, MTP2 attempts to put specified links in service by following an alignment procedure. MTP2 also provides status indications when it encounters congestion and when layers above MTP2 can no longer process MSUs because of failure.

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