The seven layer osi model

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Ricki Lambert

Communications Technology

The seven layer OSI model – D3

Introduction

The OSI seven layer model is the method in which data is sent and received through a communications channel. As the name clearly states there are 7 processes that need to be undertaken in order for this to happen. However, for data to be sent and received successfully it needs to undergo the process twice, the second time in reverse order. An example of this can be seen below;

The first layer we come across, and the last layer when sending is application. Let’s have a look at the application layer a little further.

Application

It is important to consider what an application really can be when applying it to computing. Application is a software program designed to perform a specific task or group of tasks, such as word processing, communications, or database management. In the case of communication we have many applications, such as messenger software (yahoo or msn), chat rooms, most prominent though is the e-mail based software used on a daily basis by almost every internet user. An application of this type maybe outlook express, or a standard web based e-mail account. Application is considered very basic and is something which is visible to the user.

So as the user is using their machine they send information through the web based e-mail account or outlook express by clicking the send button, this could be any type of media or text based document.

Presentation

The purpose of the 6th layer, called presentation is to transform the data from the application layer and or the session layer (which will be described next) in to a useable format. It can do this is numerous ways, namely mime encoding. Mime encoding is multipurpose internet mail extension. It’s a system used that involves a process of tagging data so that it known which applications can be used to run them. Mime is therefore considered a protocol. Another feature used in the 6th layer of the model is data encryption. This is important to any file whether it be text based or media. Data encryption is encoding of bits when sent by using a mathematical process. This then means that the message can be made secure, and prevents from security issues such as hacking and so on. Not all forms of encryption take place on the 6th layer as some may take place further down the line. Compression is another process that can be undertaken by the presentation layer. The reason this is used is to increase the actual data throughput of the system. However, we have looked at the main functions of the presentation layer and two of them, compression and encryption play a massive part overall. But do we really need this? That is a very open ended question. Not all people wish to use these methods and hence the reason was to why they are optional. In many cases the 6th layer is skipped by the protocol stack implementation and communication goes directly between application and session.

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Session

The session layer of the seven layer model is where the first basis of communication happens. Session is considered a form of management utility and makes sure that the differing ports and devices can talk and listen to each other. The session layer technology uses a programming language called API, Application Programme Interface which allows programmers to develop network applications.

Session works with half duplex or duplex only as the information to be sent needs to be bidirectional, something that is not possible with simplex communication. For all sessions to be productive there must be ...

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