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OSI Reference Model

Introduction to TCP/IP

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Understanding Switching

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Understanding Routing

What Is Layer 3 Switching?

Understanding Virtual LANs

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OSI Reference Model

This lesson covers the OSI reference model. It is sometimes also called ISO or 7 layer reference model. The model was developed by the International Standards Organization in the early 1980's. It describes the principles for interconnection of computer systems in an Open System Interconnection environment.

      Contents
 - The Layered Model

 - Need of Layered Network Model

 - Layer Functions

 - Peer to Peer Communications

 - Layers 1 & 2: Physical & Data Link Layers

 - Network Layer

 - Operations of Network Layer Protocol

 - Static and Dynamic Routes

 - LAN-to-LAN Routing

 - Transport Layer

 - Session, Presentation, and Application Layers

The Layered Model

The concept of layered communication is essential to ensuring interoperability of all the pieces of a network. To introduce the process of layered communication, let’s take a look at a simple example.

In this image, the goal is to get a message from Location A to Location B. The sender doesn’t know what language the receiver speaks – so the sender passes the message on to a translator.
The translator, while not concerned with the content of the message, will translate it into a language that may be globally understood by most, if not all translators – thus it doesn’t matter what language the final recipient speaks. In this example, the language is Dutch. The translator also indicates what the language type is, and then passes the message to an administrative assistant.
The administrative assistant, while not concerned with the language, or the message, will work to ensure the reliable delivery of the message to the destination. In this example, she will attach the fax number, and then fax the document to the destination – Location B.

The document is received by an administrative assistant at Location B. The assistant at Location B may even call the assistant at Location A to let her know the fax was properly received.
The assistant at Location B will then pass the message to the translator at her office. The translator will see that the message is in Dutch. The translator, knowing that the person to whom the message is addressed only speaks French, will translate the message so the recipient can properly read the message. This completes the process of moving information from one location to another.

Upon closer study of the process employed to communicate, you will notice that communication took place at different layers. At layer 1, the administrative assistants communicated with each other. At layer 2, the translators communicated with each other. And, at layer 3 the sender was able to communicate with the recipient.

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