Tutorial Class 5: The OSI Reference Model

In our last class, we got to understand the basics of an internetwork through examples. In this tutorial class, I’ll be introducing you to internetworking models. This simply talks about the Open Systems Interconnection (OSI) reference model which was created by the International Organization for Standardization (ISO) to break the barrier of computers typically communicating only with computers from the same manufacturer.
The OSI model is the primary architectural model for networks. It describes how
data and network information are communicated from an application on one computer through the network media to an application on another computer by breaking it into layers.

What is a reference model?
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.
To understand it better, let’s take the following scenario:

You and some friends want to start a company. One of the first things you’ll do is sit down and think through what tasks must be done, who will do them, the order in which they will be done, and their relationship with each other. These tasks will be grouped into departments. Each of your departments has its own unique tasks, keeping its staff members busy and requiring them to focus on only their own duties.
These departments are simply the layers of the communication system. For things to run smoothly, the staff of each department will have to trust and rely heavily upon 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 blueprint, or reference model.

Similarly, software developers can use a reference model to understand computer communication processes and 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 that 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 such models, especially the OSI model, is to allow different vendors’ networks to interoperate.

Advantages of using the OSI layered model include, but are not limited to, the following:
•    It divides the network communication process into smaller and simpler components, thus aiding component development, design, and troubleshooting.
•    It allows multiple-vendor development through standardization of network components.
•    It encourages industry standardization by defining what functions occur at each layer of the model.
•    It allows various types of network hardware and software to communicate.
•    It prevents changes in one layer from affecting other layers, so it does not hamper development.

The OSI Reference Model

One of the greatest functions of the OSI specifications is to assist in data transfer between hosts from different vendors—meaning, for example, that they enable us to transfer data between a Unix host and a PC or a Mac.
The OSI isn’t a physical model, though. Rather, it’s a set of guidelines that application developers can use to create and implement applications that run on a network. It also provides a framework for creating and implementing networking standards, devices, and internetworking schemes.

The OSI has seven different layers, divided into two groups. The top three layers define how the applications within the end stations will communicate with each other and with users. The bottom four layers define how data is transmitted end to end. Figure 1 shows the three upper layers and their functions, and Figure 2 shows the four lower layers and their functions.

The user interfaces with the computer at the Application layer and also the upper layers are responsible for applications communicating between hosts.

The four bottom layers are responsible for networking or network addresses. They define how data is transferred through a physical wire or through switches and routers. These bottom layers also determine how to rebuild a data stream from a transmitting host to a destination host’s application.

The following network devices operate at all seven layers of the OSI model:
•    Network management stations (NMSs)
•    Web and application servers
•    Gateways (not default gateways)
•    Network hosts

The OSI reference model has seven layers:
•    Application layer (layer 7)
•    Presentation layer (layer 6)
•    Session layer (layer 5)
•    Transport layer (layer 4)
•    Network layer (layer 3)
•    Data Link layer (layer 2)
•    Physical layer (layer 1)

So, I guess with this short class, you’ve gotten a brief introduction to the OSI model—the seven-layer model used to help application developers design applications that can run on any type of system or network. Each layer has its special jobs and select responsibilities within the model to ensure that solid, effective communications do occur.

In our next tutorial class, you’ll get to know the functions defined at each layer of the upper layer of the OSI model.

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