Understanding Network Layers
Before discussing the specifics of building and using a TCP/IP network, it’s important to learn a bit of the background on what a network really is and how it works. Fortunately, a number of real-world analogies can help us gain that understanding. We know, for example, that a car moves us from place to place. A network, similarly, moves data from place to place. A car also consists of several components, which, depending on our automotive background, can be abstracted in different ways. There’s an engine, which provides power for the transportation, a passenger compartment that provides the storage for moving people and their belongings around, a transmission to control the speed, and braking system to prevent errors from occurring. A network behaves in much the same fashion and can be described much like our abstraction of a car. The trick is to come up with a network model that can be applied to any situation, whether you’re building a TCP/IP network or a network using protocols that haven’t even been created yet. There have been several attempts at providing such a model, but most have failed to cover all the aspects of defining all the components that can be considered part of a network. Luckily for us, there is one model that has been widely accepted internationally and can be used to describe both present and future network models in an abstract manner. OSI by the ISO
What is the ISO?The ISO, or International Standards Organization, is a group comprised of members from over 75 countries who work together to compile and maintain engineering standards from around the world. Interestingly enough, ISO is not an acronym; the ISO was named for the Greek word iso, which means “equal,” so like chemical elements, the name ISO is consistent in all languages. The ISO created the network model we’ll use for examining TCP/IP.
The International Standards Organization (ISO) recognized the need for a model that could be used when designing networking protocols. The Open Systems Interconnect (OSI, not to be confused with ISO) network model provides us with an abstract view of how a network functions, from the wiring that connects the computers to the programs we use to communicate. Layers are the key component to the OSI model. A layer in the network model is simply a functional piece of the whole network, like the braking system or engine is in our car model. Breaking the network down into layers provides us with a starting point for our definition of a network, and allows us to build up the components we need to create a network that does what we want. From a development standpoint, defining networking based on a layered model allows for the development of new technologies that take advantage of existing hardware and software by using underlying layers that are already in place. As you read about the OSI model, try to picture the relationship between each of the layers. I think you’ll find that each layer has a distinct and necessary purpose. Together, the whole is truly greater than the sum of its parts.
OSI Network Model Overview
A total of seven layers are used in the OSI model to describe a network from the ground up. These layers, in order, are the Physical, Data Link, Network, Transport, Session, Presentation, and Application layers. Each layer builds on the next and would be completely useless by itself, or if one of the preceding layers was missing. It follows that each layer cannot exist in a vacuum; it must possess some knowledge of the layers that surround it and have a method of communicating to those layers. In its specification of the OSI model, the ISO does not bind any particular network standard, such as TCP/IP, to the model. By basing the layers on their function, rather than specific existing network standards, the ISO has provided a model that is robust, open, and can be used to explore existing network specifications and design the standards of the future (see Figure 1.1).
Inside the OSI Layers
As we look at a general overview of each layer, we’ll compare it to a real world model when appropriate. You’ll see how easily the OSI model lets us view the complex creature known as a network.
The Hardware/Physical Layer
This first layer provides the foundation that the following layers will build on. Hardware refers to the computer, network cable, satellite dishes, or any other physical devices you choose to use when linking two or more computers. This concept includes the actual physical wiring and the electrical signals that travel through them. For satellite hardware, the term would include the radio waves that pass to and from the satellites. The hardware must also have the capability of determining when a signaling problem has occurred and notifying upper levels of the trouble.
These tasks may sound a bit daunting, but they happen at a level that most of us don’t need to worry about. The signaling properties are handled by the hardware we choose to link the computers; we don’t need to design our signaling methods each time we set up a network. In a car, this action is equivalent to the engine’s ability to translate fossil fuels into kinetic energy, which turns the tires. We don’t necessarily need to know all the specifics of combustion to understand how it works; we just need to know that without an engine, there will be no motion. Furthermore, we don’t need to be civil engineers to understand that without the road, our car isn’t going to be traveling very far. Likewise, it’s quite safe to say that if there isn’t a Physical Layer, you’re going to be having some serious problems getting a network up and running.
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