OSI

The Open Systems Interconnection (OSI) is a standard for network communication of all computer systems. This model of communications between computers is proposed by the ISO and it describes the functionalities necessary for communication and their organization.

The model is essentially an architecture in defined and delimited layers with the notions of service, protocol and interface.

A service is an abstract description of functionality using primitives (commands or events) such as a connection request or data reception.

A protocol is a set of messages and exchange rules implementing a service.

An interface (“service access point” in the standard) is the concrete way to use the service. In a program, it is typically a set of library functions or system calls. In a hardware implementation, it is for example a set of registers at the input of a circuit.