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Open Systems Interconnection (osi) Model

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OSI Model

This memorandum will attempt to explain the Open Systems Interconnection Model, known more simply as the OSI Model. The OSI Model has seven levels, and these levels will be discussed in detail. Particular mention will be made to which level TCP/IP functions with the OSI Model.

The seven levels of the OSI Model are as follows:

7) Application: Provides different services to the applications.

6) Presentation: Converts the information.

5) Session: Handles problems which are not communication issues.

4) Transport: Provides end to end communication control.

3) Network: Routes the information in the network.

2) Data Link: Provides error control between adjacent nodes.

1) Physical: Connects the entity to the transmission media.

In simple terms, the OSI model defines a networking framework for implementing protocols. Is defines seven levels to accomplish this. Control is passed from one level to the next, starting at the Application level (Level Seven) and working its way through the levels until it reaches and completes Level One, the Physical level. Once this cycle has been completed, control moves to the next station on the network and back up the hierarchy.

Since the process begins at the seventh level, the Application level, it will be detailed first. As its name implies, this Application level supports applications, but it also supports other end-user processes. User authentication and privacy on the network is considered at this level. This level also provides application services for file transfers (such as FTP and Telnet) and e-mail.

Once the Application level has been completed, control is passed to the sixth level, or Presentation level. What occurs at this level involves transforming the data into a form that the application layer can accept. Encrypted data is handled at this level. This layer formats and encrypts data to be sent across a network. Sometimes this level is known as the syntax level.

Control is then passed to the Session level, or level five. This level, as the name implies, deals with the different network sessions. This level establishes, manages and terminates connections between the different applications on the network. It also sets-up, coordinates and terminates exchanges and dialogue between the applications on either end.

The Transport level assumes control at this point. This level provides transparent transfer of data between end systems, or hosts, and is responsible



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