Open System Interconnection OSI is an ISO International Standards Organization standard for global communications that defines a networking framwork for implementing protocols in the seven Layers. ISO OSI is a proof of concept model only. Very simply speaking the OSI model describes how data and network information will flow from one computer to another through the network media. Each of the seven layers in the OSI reference model has a unique set of properties & functionality. Layering helps in breaking down the problem and addressing each network communication issue seperately. Control is passed from one layer to the other starting first at the Application Layer to the bottom most layer Physical layer. The Seven Layers of the ISO OSI Reference model are given below.

Layer 7 - Application Layer
Layer 6 - Presentation Layer
Layer 5 - Session Layer
Layer 4 - Transport Layer
Layer 3 - Network Layer
Layer 2 - Data Link Layer
Layer 1 - Physical Layer

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Database Replication , Single Sign On , Public Key Infrastructure