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SS7 Protocol Layers
The SS7 protocol at signaling points is divided into several parts spread across seven "layers." These layers are collectively known as the SS7 protocol stack. The SS7 protocol stack was modeled after the seven-layer Open Systems Interconnection (OSI) model. It can be loosely mapped to the OSI model.
However it differs from OSI model is that User applications can be present starting from 4th layer onwards(i.e. SCCP , ISUP or TUP Layer). Its unlike other implementation of OSI models(e.g. TCP/IP) where user applications mostly link with highest layer i.e. Application Layer. You would hardly come across any application which directly operates at Presentation or Session layer.
Its reason , I have used term "loosely mapped".
Following Digram show mapping between both models :
Dividing the protocol stack into layers and assigning specific responsibilities to specific parts is a main advantage in that changes can be quickly implemented. If a change in technology occurs, only one part of the stack will likely be affected. In this way, it is easier to capitalize on new technology.