OSI (Open Systems Interconnection) model was created by the International Organization for Standardization (ISO), an international standard-setting body.
It was designed to be a reference model for describing the functions of a communication system.
The OSI model provides a framework for creating and implementing networking standards and devices and describes how network applications on different computers can communicate through the network media.
The OSI model has seven layers, with each layer describing a different function of data traveling through a network. Here is the graphical representation of these layers:
So, what is the purpose of these layers?
They are most commonly used by vendors. They enable them to implement some functionality into a networking device, which then enables easier interoperability with devices from other vendors
Physical:
Defines how to move bits from one device to another. It details how cables, connectors and network interface cards are supposed to work and how to send and receive bits.
Data Link:
Encapsulates a packet in a frame. A frame contains a header and a trailer that enable devices to communicate. A header (most commonly) contains a source and destination MAC address.
A trailer contains the Frame Check Sequence field, which is used to detect transmission errors. The data link layer has two sublayers:
Logical Link Control – used for flow control and error detection.
Media Access Control – used for hardware addressing and for controlling the access method.
Network:
Defines device addressing, routing, and path determination. Device (logical) addressing is used to identify a host on a network (e.g. by its IP address).
Transport:
Segments big chunks of data received from the upper layer protocols. Establishes and terminates connections between two computers. Used for flow control and data recovery.
Session:
Defines how to establish and terminate a session between the two systems.
Presentation:
Defines data formats. Compression and encryption are defined at this layer.
Application:
This layer is the closest to the user. It enables network applications to communicate with other network applications.
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