MSU – Message Signalling Unit
The Message Signalling Unit provides the structure for transporting the higher layer protocol types in an SS7 network. These will include ISUP (ISDN User Part) TCAP (Transaction Capabilities Application Part) MAP (Mobile Application Part) and others. The difference between the MSU and signalling units such as LSSU (Link Status Signalling Unit) and FISU (Fill In Signalling Unit) is the MSU contains the SIO (Service Indicator Octet) and SIF (Service Information Field).
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