Layered Approach
Understand that 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.
Think of it like this: You and some friends want to start a company. One of the first things you’ll do is sort out every task that must be done and decide who will do what. You would move on to determine the order in which you would like everything to be done with careful consideration of how all your specific operations relate to each other. You would then organize everything into departments (e.g., sales, inventory, and shipping), with each department dealing with its specific responsibilities and keeping its own staff busy enough to focus on their own particular area of the enterprise.
In this scenario, departments are a metaphor for the layers in a communication system. For things to run smoothly, the staff of each department has to trust in and rely heavily upon those in the others to do their jobs well. During planning sessions, you would take notes, recording the entire process to guide later discussions and clarify standards of operation, thereby creating your business blueprint – your own reference model.
And once your business is launched, your department heads, each armed with the part of the blueprint relevant to their own department, will develop practical ways to implement their distinct tasks. These practical methods, or protocols, will then be compiled into a standard operating procedures manual and followed closely because each procedure will have been included for different reasons, delimiting their various degrees of importance and implementation. All of this will become vital if you form a partnership or acquire another company because then it will be really important that the new company’s business model is compatible with yours!
Models happen to be really important 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