|The Unix and Internet Fundamentals HOWTO, by Eric Raymond|
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OSI Reference Model
ITU-T Recommendation X.200: The Basic Reference Model for Open Systems Interconnection or Open Systems Interconnection Reference Model, the OSI Reference Model, or even just the OSI Model. The text of ITU-T Recommendation X.200 was approved on 1st of July 1994. The identical text is also published as ISO/IEC International Standard 7498-1. ITU ( International Telecommunication Union ) is the United Nations Specialized Agency in the field of telecommunications. One of the ways in which networking technology is made easier to understand is by splitting it into pieces, each of which plays a particular role, or is responsible for a specific job or function. However, if this is to be done, we must have a way of ensuring that these various pieces can interoperate; that is, each must know what is expected of it, and also what it can expect from the other pieces. This is one of the important roles of networking models. They split the multitude of tasks required to implement modern networks, into smaller chunks that can be more easily managed. Just as importantly, they establish “walls” between those pieces, and rules for passing information over those walls. Networking models represent a framework for dividing up the tasks needed to implement a network, by splitting the work into different levels, or layers. Hardware and software running at each layer is responsible for interacting with its corresponding hardware and software running on other devices at the same layer. The Open Systems Interconnection Reference Model (OSI Reference Model or OSI Model) was originally created as the basis for designing a universal set of protocols called the OSI Protocol Suite. This suite never achieved widespread success: the rise in popularity of the Internet and its TCP/IP protocols met the OSI suite head on, and in a nutshell, TCP/IP won. A lot of networking books and other resources gloss over the OSI Reference Model, including only passing mention of it, or relegating it to an appendix. The usual stated reason for this is that the OSI model is “too theoretical” and “doesn't apply to modern networking protocols like TCP/IP”. While it is certainly true the OSI model is primarily theoretical, and that networking protocols aren't always designed to fit strictly within the confines of its layers, it is difficult to read about networking technology today without seeing references to the OSI model and its layers. Some protocols are even named specifically in terms of their place in the OSI Reference Model! For an example, consider the Layer Two Tunneling Protocol. Also, switches are now commonly categorized as being layer 2, layer 3 or even higher-layer switches. The OSI Reference Model is not the only model used to describe the structure of networks. There are several other models and systems that are used to describe various sets of networking technologies that work together. These are generally not theoretical models, but rather describe groupings of protocols that are actively used in actual networks. They are, therefore, more often called networking architectures and protocol suites than models. Regardless of what the individual layers and technologies are called, networking protocol suites all try to accomplish the same goals in implementing a network. Thus, even though the layers are not the same, they are often comparable. The most important OSI Reference Model concept is that of networking layers. It’s not an exaggeration to say that layers are really the heart of the OSI model: the entire point of the model is to separate networking into distinct functions that operate at different levels. Each layer is responsible for performing a specific task or set of tasks, and dealing with the layers above and below it. The OSI Reference Model is comprised of seven conceptual layers, each assigned a “ranking” number from one to seven. The layer number represents the position of the layer in the model as a whole, and indicates how “close” the layer is to the actual hardware used to implement a network. The first and lowest layer is the physical layer, which is where low-level signaling and hardware are implemented. It is the domain of hardware engineers and signaling experts. The seventh and highest layer is the application layer, which deals with high-level applications employed by users: both end users and the operating system software.
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