OSI model
OSI (Open Systems Interconnection) model is a conceptual model that standardizes the functions of a telecommunication or computing system into seven abstraction layers.
A layer serves the layer above it and is served by the layer below it. For example, a layer that provides error-free communications across a network provides the path needed by applications above it, while it calls the next lower layer to send and receive packets that make up the contents of that path.
OSI model.