CCENT ICND1 Study Guide. Lammle Todd

to software developers too. They often use a reference model to understand computer communication processes so they can determine which functions should be accomplished on a given layer. This means that if someone is creating a protocol for a certain layer, they only need to be concerned with their target layer’s function. Software that maps to another layer’s protocols and is specifically designed to be deployed there will handle additional 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 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 Reference Model

      One of best gifts the OSI specifications gives us is paving the way for the data transfer between disparate hosts running different operating systems, like Unix hosts, Windows machines, Macs, smartphones, and so on.

      And remember, the OSI is a logical model, not a physical one. It’s essentially a set of guidelines that developers can use to create and implement applications to 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 as well as with users. The bottom four layers define how data is transmitted end to end.

Figure 1.7 shows the three upper layers and their functions.

Figure 1.7 The upper layers

      When looking at Figure 1.6, 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 1.8, 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 1.8 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

      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 the most popular means of comparison for protocol suites today.

      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 1.9 shows a summary of the functions defined at each layer of the OSI model.

Figure 1.9 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.

      With this in hand, you’re now ready to explore each layer’s function in detail!

       The Application Layer

      The Application layer of the OSI model marks the spot where users actually communicate to the computer and comes into play only when it’s clear that access to the network will be needed soon. Take the case of Internet Explorer (IE). You could actually uninstall every trace of networking components like TCP/IP, the NIC card, and so on and still use IE to view a local HTML document. But things would get ugly if you tried to do things like view a remote HTML document that must be retrieved because IE and other browsers act on these types of requests by attempting to access the Application layer. So basically, the Application layer is working as the interface between the actual application program and the next layer down by providing ways for the application to send information down through the protocol stack. This isn’t actually part of the layered structure, because browsers don’t live in the Application layer, but they interface with it as well as the relevant protocols when asked to access remote resources.

      Identifying and confirming the communication partner’s availability and verifying the required resources to permit the specified type of communication to take place also occurs at the Application layer. This is important because, like the lion’s share of browser functions, computer applications sometimes need more than desktop resources. It’s more typical than you would think for the communicating components of several network applications to come together to carry out a requested function. Here are a few good examples of these kinds of events:

      ■ File transfers

      ■ Email

      ■ Enabling remote access

      ■ Network management activities

      ■ Client/server processes

      ■ Information location

      Many network applications provide services for communication over enterprise networks, but for present and future internetworking, the need is fast developing to reach beyond the limits of current physical networking.

       The Application layer works as the interface between actual application programs. This means end-user programs like Microsoft Word don’t reside at the Application layer, they interface with the Application layer protocols. Later, in Chapter 3, “Introduction to TCP/IP,” I’ll