OSI Open System Interconnection (OSI Reference Model or OSI Model) is an abstract description for layered communications and computer network protocol design.In its most basic form, it divides network architecture into seven layers which, from top to bottom, are the Application, Presentation, Session, Transport, Network, Data-Link, and Physical Layers. It is therefore often referred to as the OSI Seven Layer Model.

This is a commonly-used network protocol. The X.25 protocol allows computers on different public networks (such as CompuServe, Tymnet, or a TCP/IP network) to communicate through an intermediary computer at the network layer level. X.25's protocols correspond closely to the data-link and physical-layer protocols defined in the Open Systems Interconnection (OSI) communication model.

There are 7 Layers of the OSI model which are:-

1. . Application Layer (Top Layer)

2.  Presentation Layer

3.  Session Layer

4.  Transport Layer

5.  Network Layer

6.  Data Link Layer

7.  Physical Layer (Bottom Layer)

The International Standards Organization (ISO) Open Systems Interconnect (OSI) is a standard set of rules describing the transfer of data between each layer. Each layer has a specific function. For example the Physical layer deals with the electrical and cable specifications.