History

  • In 1983, the International Standards Organization (ISO) developed a model called Open Systems Interconnection (OSI) which is a standard reference model for OSI communication between two end users in a network.
  • The model is used in developing and TCP/IP Models products and understanding networks.
  • This model is a prescription of characterizing and standardizing the functions of a communications system in terms of abstraction layers.
  • Here, similar communication functions are grouped into logical layers.
  • A layer serves the layer above it and is served by the layer below it.

Introduction

  • To send/receive and understand information in a computer network successfully, there exist a set of rules(TCP/IP) or standards (OSI) where standards ensure that varying devices and products can communicate with each other over any network. This set of standards is called a network reference model.
  • There are a variety of networked models currently being implemented but OSI and TCP/IP models are important one.

Features

  • Each layer of OSI reference model is responsible for a particular aspect of data communication.
  • Each layer has a set of specific functions that are to be performed by a specific protocol(s).
  •  The OSI reference model has a protocol suit for all of its layers.

Types of OSI Reference Models

(A) The layers of the OSI reference model are divided into two major  groups  : –

(a) The Host/Upper Layers :

    • This layer includes Application layer, Presentation layer, Session layer and Transport layer.
    • This layer focuses on user’s applications and how applications/ files are represented on the computers prior to send.

(b) The Media/Lower Layers :

    • This layer includes Network layer, Data Link layer and Physical layer.
    • The layer focuses on how the communication across a network actually occurs.

(B) There are seven layers of OSI reference model –

(i) Physical Layer (Layer 1)

(ii) Data Link Layer (Layer 2)

(iii) Network Layer (Layer 3)

(iv) Transport Layer (Layer 4)

(v) Session Layer (Layer 5)

(vi) Presentation Layer (Layer 6)

(vii) Application Layer (Layer 7)

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