The OSI Reference Model

One of the greatest functions of the OSI specifications is to assist in data transfer between disparate hosts—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 1.6 shows the three upper layers and their functions, and Figure 1.7 shows the four lower layers and their functions.
When you study Figure 1.6, understand that the user interfaces with the computer at the Application layer and also that the upper layers are responsible for applications communicating between hosts.
Remember that none of the upper layers knows anything about networking or network addresses. That’s the responsibility of the four bottom layers.

In Figure 1.7, you can see that it’s the four bottom layers that 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

Basically, the ISO is pretty much the Emily Post of the network protocol world. Just as Ms. Post wrote the book setting the standards—or protocols—for human social interaction, the ISO developed the OSI reference model as the precedent and guide for an open network protocol set. Defining the etiquette of communication models, it remains today the most popular means of comparison for protocol suites.

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

Figure 1.8 shows a summary of the functions defined at each layer of the OSI model. With this in hand, you’re now ready to explore each layer’s function in detail.

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:

1.It divides the network communication process into smaller and simpler components, thus aiding component development, design, and troubleshooting.
2.It allows multiple-vendor development through standardization of network components.
3.It encourages industry standardization by defining what functions occur at each layer of the model.
4.It allows various types of network hardware and software to communicate.
5.It prevents changes in one layer from affecting other layers, so it does not hamper development.

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’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 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, I’m using departments as a metaphor for the layers in a 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.

Once your business is launched, your department heads, each armed with the part of the blueprint relating to their own 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 that its business protocols—its business blueprint—match yours (or at least be compatible with it).

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.