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Basic Networking: OSI Model Part-1



OSI Model Overview:

The OSI model was created to help multiple vendor’s network devices in the form of protocols so that different vendor networks can easily work with each other.

The OSI model is the primary architectural model for the networks. It describes that how data and network information are communicated from an application on the one computer through the networking media to an application on the computer.

 The OSI model breaks this approach into multiple Layers.In the following section, we are going to explain the layered approach and how we can use this approach to help us troubleshoot our inter-networks.



Layered Approach

An OSI model addresses all the processes required for effective communication and also divides these processes into logical groupings, called layers. When a communication system is designed in this manner, called layered architecture.

Advantages of OSI Models:

The OSI model is hierarchical, and the same benefits and advantages we can apply to any layered model. OSI model is used to allow different vendors’ networks to interoperate.

Advantages of using the OSI model includes.


  1. ·         OSI Model allows multiple-vendor development through standardization of network components.
  2. ·         OSI Model encourages industry standardization by defining what functions occur at each layer of the model.
  3. ·         OSI Model allows multiple types of network hardware and software to communicate.
  4. ·         OSI Model prevents changes in one layer from affecting other layers, so it does not hamper development.

 OSI Reference Model

OSI specifications is to assist in the data transfer between disparate hosts meaning,
For example, they enable us to transfer data between a UNIX host and a PC or a Mac.

The OSI is not a physical model, though. It is a set of rules that application developers can use to create and implement applications that run on a network. It also provides a framework for creating and implementing standards networking, devices, and internetworking.

The OSI model has seven different layers, it divided into two groups. The top three layers define how the applications will communicate with each other and with users. The bottom four layers define, how data is transmitted in the end to end network.

The OSI model has the following seven layers:

·         Application layer (layer 7)
·         Presentation layer (layer 6)
·         Session layer (layer 5)
·         Transport layer (layer 4)
·         Network layer (layer 3)
·         Data Link layer (layer 2)
·         Physical layer (layer 1)

 Application Layer

The Application layer of the OSI model marks the spot where users can actually communicate to the computer. This layer comes into picture only when it is apparent that access to the network is going to be needed soon. For example, Internet Explorer (IE), FTP and TFTP.

 Presentation Layer

The Presentation layer gets its name from the it’s purpose: It presents data to the Application layer and it is responsible for data translation and code formatting.

The OSI has protocol standards that define, how data should be formatted. For Example, data compression, decompression, encryption, and decryption are associated with this layer.

Session Layer

The Session layer is used to setting up, managing, the sessions between Presentation layer entities. This layer also provides the dialog control between devices, or nodes. It is offering three different modes: simplex, half duplex, and full duplex.

Transport Layer

 The Transport Layer provides end-to-end data transport services of data and can establish a logical Connection between the sending host and destination host on a network.

For Example, TCP is a reliable service and UDP is not.

The following sections will provide you an overview on the connection-oriented (reliable) protocol of the Transport layer.

Flow Control

Transport layer maintains flow control and by allowing applications to request reliable data transport between systems. In the reliable data transmission using a connection oriented (TCP) communications session between systems, and the protocols involved ensure that, the following will be achieved.

The main purpose of flow control is to provide a method for the receiver to control the amount of data sent by the sender.

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