The OSI model as a foundation for understanding networks

Network Support Learning Module

The OSI Model

The OSI model helps us to understand network concepts and nomenclature by establishing standards of definition and functions.

The OSI model isn't a blueprint for implementing Network Systems. In fact, nobody builds an OSI model-based network. For several reasons it wouldn't be a good idea. However, the standard nomenclature and functionality helps us to understand at each level exactly what we are talking about and how it fits into the big picture. It is a descriptive model, not a specification.

LEARNING OBJECTIVES:---- What do we hope to accomplish here?

You will thoroughly understand the OSI model and be able to apply its terminology to real networks.

The OSI model. You should be able to converse with professional colleagues, understanding the concepts and terminology
Data packets, their reason, their organization, how they are nested, and how they flow through networks.
The organization of modern modular Network software from the application using a network, to the Network operating system, to the lower level protocols.
How to create an organized presentation on a technical subject.

BACKGROUND & DISCUSSION - Why is this important? - additional help
Although networks aren't designed to the OSI model, most developers use OSI terminology, nesting techniques, and organization in defining their networks. If you don't understand OSI, you will be limited, especially in comparing network hardware and software.

INSTRUCTIONS: How to do the exercise
You will research the subject and produce either an oral presentation or a written report.

EVALUATION CRITERIA: How well have you done?
You will have to do two things with your report. Convince me of the depth of understanding, and produce a report that will explain the model to a new student.

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Background & Discussion ------ Some help in getting started. Why is this important?

Basically, I am asking you to apply an existing standard to real networks. If you are going to understand how data gets from one place to another, you need to understand the way it happens and the components of the system. If you have to compare alternatives in creating or updating a network, you need to know how the parts fit together; how they interface; whether they interface. The OSI model is a good tool to use in understanding data communications.

Packetizing: Before data was routinely packetized and treated in a standard way, telecommunications was utter chaos. Very little attempt was made to standardize or to automate anything. Basically, a person created data on one end of a channel and another person did something with it at the other end. If a computer were at the other end, the programmer had to use or adapt the proprietary rules of the computer.

Data integrity: Packetizing allowed people to identify data and protect it from error in standard ways.

Standardization: In the bad old days there was no standardization. If machine A had to talk to machine B.... forget it! Or pay lots for specialized processing to be programmed. We now have a set of standards that is both controlling and permissive. You can investigate TCP/IP, Ethernet, Token Ring, RS-232, X.25 and other standards, relating them to OSI.

Nesting: Think about what nesting packets within packets does for us. At each layer, we can forget about the needs of the other layers. We can TUNNEL if necessary without losing the integrity of the original packetization.

Bridging/routing: You should look closely at how the various chunks of equipment and software relate to the OSI model. A repeater becomes more understandable if you recognize that it operates only at layer 1. At what layers do bridges, routers, and gateways operate?

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Exercise Description -------- Procedures for completing this exercise

You may use any resources you like, including each other. In fact, I would prefer that you do this project with one or two of your fellow students. I have several textbooks and periodicals you may use. The Web should also be a good resource.

I want you to read several descriptions of the OSI model in different resources. You should also pick at least two network systems and compare their modularity with the OSI model. Pictures should be a big help in visualizing where everything fits in. Why did they choose to draw the module boundaries where they did instead of exactly following OSI layers?

You should look very closely at two fundamental principles of the OSI model: Packetization of the data stream and nesting of packets as they travel down the layers. Why are these good ideas? What are the pro and con consequences? Generally, what are the impacts on efficiency? Remember both kinds of efficiency: Throughput and ease of design.

You already should know a lot about the OSI model from your other classes. Except for looking at real networks and investigating the efficiency aspects, you should be able to jump right in and write your report. Spend about four hours in research (a bit over half your outside-of-class commitment for a week) and as much time as you need to pull the report together.

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Evaluation Criteria ------- How do you know when you are done? How well have you done?

Your depth of understanding will be demonstrated by the depth of your presentation on this subject. You will have to meet the following criteria:

Demonstrate your knowledge by producing a comprehensive explanation of the subject. Please don't stop at just copying sterile words from a textbook. Just what is the difference, in practical terms, between the session and application layers? What is the interface? How is it normally implemented? Demonstrate your understanding.
Your target audience is a network users in an office who knows only what they might have learned about OSI in a one credit telecommunications & networking class. The user needs to know about the OSI model to participate in a project to network the 15 computers in the office. They aren't technical people but are competent in using computers. If you can get another student to review or listen to your presentation, it will be a big plus.
A written report will be as long as it needs to be. I would expect about four pages of text and a like amount of graphics. Oral reports should be at least 10 minutes long with about 10 good slides, delivered on a computer projector.
The document or presentation must be done professionally. You should be proud to show it to a prospective employer as an example of your work.

(By the way, your employer probably could learn from your presentation.)

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