ISO OSI.Introduction

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Internetworking Models

When networks first came into being, computers could typically communicate only with computers from the same manufacturer. For example, companies ran either a complete DECnet solution or an IBM solution—not both together. In the late 1970s, the OSI (Open Systems Interconnection)

model was created by the International Organization for Standardization (ISO) to break this barrier. The OSI model was meant to help vendors createinteroperable network devices. Like world peace, it’ll probably never happen completely, but it’s still a great goal. The OSI model is the primary architectural model for networks. It describes how data and network information are communicated from applications on one computer, through the network media, to an application on another computer. The OSI reference model breaks this approach into layers.

The Layered Approach

A reference model is a conceptual blueprint of how communications should take place. It addresses all the processes required for effective communication and divides these processes into logical groupings called layers. When a communication system is designed in this manner, it’s known as layered architecture.

Think of it like this: You and some friends want to start a company. One of the first things you’d do is sit down and think through what must be done, who will do them, what order they will be done in, and how they relate to each other. Ultimately, you might group these tasks into departments. Let’s say you decide to have an order-taking department, an inventory department, and a shipping department. Each of your departments has its own unique tasks, keeping its staff members busy and requiring them to focus on only their own duties.

In this scenario, departments are a metaphor for the layers in a commu-nication system. For things to run smoothly, the staff of each department will have to both trust and rely heavily on the others to do their jobs and competently handle their unique responsibilities. In your planning sessions, you would probably take notes, recording the entire process to facilitate later discussions about standards of operation that will serve as your business blue-print, or reference model.

Once your business is launched, your department heads, armed with the part of the blueprint relating to their department, will need to develop practical methods to implement their assigned tasks. These practical methods, or protocols, will need to be compiled into a standard operating procedures manual and followed closely. Each of the various procedures in your manual will have been included for different reasons and have varying degrees of importance and implementation. If you form a partnership or acquire another company, it will be imperative for its business protocols—its business blueprint—to match, or be compatible with, yours.

Similarly, software developers can use a reference model to understand

computer communication processes and to see what types of functions need to be accomplished on any one layer. If they are developing a protocol for a certain layer, all they need to concern themselves with is the specific layer’s functions, not those of any other layer. Another layer and protocol will handle the other functions. The technical term for this idea is binding. The communication processes that are related to each other are bound, or grouped together, at a particular layer.

Advantages of Reference Models

The OSI model is hierarchical, and the same benefits and advantages can apply to any layered model. The primary purpose of all models, and especially the OSI model, is to allow different vendors to interoperate. The benefits of the OSI model include, but are not limited to, the following: