Functional Levels
The SS7 stack can be separated into four functional levels:
- Level 1 is the Signalling Data Link Functional Level (Data Link Level).
- Level 2 is the Signalling Link Functional Level (Link Level).
- Level 3 is the Signalling Network Functional Level (Network Level).
- Level 4 is the MTP User and consists of SCCP, ISUP, TUP, or any other MTP User.
Level 1 through Level 3 comprise the MTP, and Level 4 the MTP user. MTP Level 3 is sometimes abbreviated MTP3; MTP Level 2, MTP2. MTP and SCCP are together referred to as the Network Service Part (NSP).
There is no one-to-one mapping of MTP Levels 1 through 3 onto the OSI model. Instead, MTP provides the functionality of layers 1, 2 and part of layer 3 in the OSI model. The part of layer 3 of the OSI model that MTP does not provide, is provided by SCCP or other Level 4 parts (MTP users).
Read more about this topic: Message Transfer Part
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