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A reference model is a conceptual blueprint of how communications should take place. It addresses all the processes required for effective communication and divides them into logical groupings called layers. When a communication system is designed in this manner, it's known as a hierarchical or layered architecture. In this section two models covered on the exam are compared and contrasted.

The OSI Reference Model

The OSI model is hierarchical, and there are many advantages that can be applied to any layered model, but as I said, the OSI model's primary purpose is to allow different vendors' networks to interoperate.

Here's a list of some of the more important benefits of using the OSI layered model:

■ It divides the network communication process into smaller and simpler components, facilitating component development, design, and troubleshooting.

■ It allows multiple-vendor development through the standardization of network components.

■ It encourages industry standardization by clearly 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 to expedite development.

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 as well as with users. The bottom four layers define how data is transmitted end to end.

Figure 1.1 shows the three upper layers and their functions.

Figure 1.1 The upper layers

When looking at Figure 1.1, understand that users interact with the computer at the Application layer and also that the upper layers are responsible for applications communicating between hosts. None of the upper layers knows anything about networking or network addresses because that's the responsibility of the four bottom layers.

In Figure 1.2, which shows the four lower layers and their functions, you can see that it's these four bottom layers that define how data is transferred through physical media like wire, cable, fiber optics, switches, and routers. These bottom layers also determine how to rebuild a data stream from a transmitting host to a destination host's application.

Figure 1.2 The lower layers

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)

■ Servers

■ Network hosts

The OSI reference model has the following 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)

Some people like to use a mnemonic to remember the seven layers, such as All People Seem To Need Data Processing. Figure 1.3 shows a summary of the functions defined at each layer of the OSI model.

Figure 1.3 OSI layer functions

I've separated the seven-layer model into three different functions: the upper layers, the middle layers, and the bottom layers. The upper layers communicate with the user interface and application, the middle layers do reliable communication and routing to a remote network, and the bottom layers communicate to the local network.

TCP/IP and the DoD Model

The DoD model is basically a condensed version of the OSI model that comprises four instead of seven layers:

■ Process/Application layer

■ Host-to-Host layer or Transport layer

■ Internet layer

■ Network Access layer or Link layer

Figure 1.4 offers a comparison of the DoD model and the OSI reference model. As you can see, the two are similar in concept, but each has a different number of layers with different names. Cisco may at times use different names for the same layer, such as both “Host-to-Host” and Transport” at the layer above the Internet layer, as well as “Network Access” and “Link” used to describe the bottom layer.

Figure 1.4 The DoD and OSI models

Exam Essentials

List the layers of the OSI and TCP/IP models. List the layers in order, and describe the function of each layer.

Compare and contrast the layers of the TCP/IP and OSI models. Identify the layers in each model that perform like functions.