The Open Systems Interconnection (OSI) model is a product of the Open Systems Interconnection effort at the International Organization for Standardization. It is a prescription of characterizing and standardizing the functions of a communications system in terms of abstraction layers. Similar communication functions are grouped into logical 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. Two instances at one layer are connected by a horizontal connection on that layer.

The OSI is made up of 7 layers and they are as follows:

Layer 7- Application Layer

-Defines interface to user processes for communication and data transfer in network.  It provides standardized services such as virtual terminal, file and job transfer, and operations.

Layer 6-Presentation Layer

• Masks the differences of data formats between dissimilar systems
• Specifies architecture- independent data transfer format
• encodes and decodes data; encrypts and decrypts data; compresses and decompresses data

Layer 5-Session Layer

• Manages user sessions and dialogues
• Controls establishment and termination of logic links between users
• Reports upper layer errors

Layer 4-Transport Layer

• Manages end to end message delivery in network
• Provides reliable and sequential packet delivery through error recovery and flow control mechanisms
• Provides connectionless oriented packet delivery

Layer 3-Network Layer

• Determines how data are transferred between network devices.
• Routes packets to unique network device addresses.
• Provides flow and congestion control to prevent network resource depletion.

Layer 2-Datalink Layer

• Defines procedures for operating the communications links.
• Frames packets
• Detects and corrects packets transmit errors

Layer 1-Physical Layer

• Defines physical means of sending data over network devices.
• Interfaces between network medium and devices.
• Defines optical, electrical, and mechanical characteristics