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Figure 2-10 Physical agreements
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selected to form functional groupings, are what make data communications work properly. Perhaps the best-known family of protocols is the International Organization for Standardization s Open Systems Interconnection Reference Model, usually called the OSI Model for the sake of simplicity. Shown in Figure 2-11 and comprising seven layers, it provides a logical way to study and understand data communications and is based on the following simple rules. First, each of the seven layers must perform a clearly defined set of responsibilities that are unique to that layer and guarantee the requirement of functional modularity. Second, each layer depends upon the services of the layers above and below to do its own job, as we would expect, given the modular nature of the model. Third, the layers have no idea how the layers around them do what they do; they simply know that they do it. This is called transparency. Finally, there is nothing magical about the number seven. If the industry should decide that we need an eighth layer on the model, or that layer five is redundant (and there are those who think it is), then the model will be changed. The key is functionality. There is an ongoing battle within the ranks of OSI Model pundits, for example, over whether there is actually a requirement for both layers six and seven, because many believe them to be so similar functionally that one or the other is redundant. Others question whether there is really a need for layer five, the functions of which are considered by many to be superfluous and redundant. To these people I recommend the purchase of a dog. Whether the addition or elimination of a layer ever actually happens is not important. The fact that it can is what matters. It is important to understand that the OSI Model is nothing more than a conceptual way of thinking about data communications. It isn t hardware; it isn t software. It merely simplifies and groups the processes of data transmission so that they can be easily understood and manipulated. Let s look at the OSI Model in a little more detail (see Figure 2-12). I tend to think of it as a seven-drawer chest. In each drawer a collection of standards is stored, and when network implementers set up a network they rummage through each drawer, select the most appropriate standard for their requirements, and set up the network. As we mentioned earlier, the model is a seven-layer construct within which each layer is tightly dependent upon the layers surrounding it. The Application Layer, at the top of the model, speaks to the actual application process that creates the information to be transported by the
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Figure 2-11 OSI Model
2
Application
Presentation
Session
Transport
Network
Data Link
Physical
network; it is closest to the customer and the customer s processes, and is therefore the most customizable and manipulable of all the layers. It is highly open to interpretation. On the other end of the spectrum, the Physical Layer dwells within the confines of the actual network, and is totally standards dependent. There is minimal room here for interpretation; a pulse is either a one or a zero there s nothing in between.
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