OSI Reference Model

2.4 The OSI Reference Model

The International Organization for Standardization (ISO) is the organization responsible for standardizing the methods by which computers communicate with each other. In 1984 the ISO devised their model for network communication. This model is called the Open Systems Interconnection (OSI) reference model or more commonly the OSI model.

The OSI model divides network communications into seven layers. Each layer is responsible for carrying out specific functions when transmitting data on the network. Each layer of this 7 layer structure has a definite function.

These layers can be recalled using the following acronyms:

All People Seem To Need Data Processing. (Layers 7 down to 1)

People Do Not Throw Sausage Pizza Away (Layers 1 up to 7)

Each of the letters represents a different layer (note that the order of these layers is critical).

Layers of the OSI Model

The OSI reference model divides network communication into seven layers. Each layer covers different network activities, equipment, or protocols. The table below represents the layered architecture of the OSI reference model. The OSI reference model defines how each layer communicates and works with the layers immediately above and below it.

No

Layer Name

Layer Function

7

Application Layer

Network process to application

6

Presentation Layer

Data representation and encryption

5

Session Layer

Interhost communication

4

Transport Layer

End-to-end connections and reliability

3

Network Layer

Path determination and logical addressing (IP)

2

Data Link Layer

Physical addressing (MAC & LLC)

1

Physical Layer

Media, signal and binary transmission

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