Protocols: Horizontal (Corresponding Layer) Communication
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Each layer in the OSI Reference Model has a particular role (or roles)a set of general tasks for which it is responsible. On each system on the network, hardware and software are running at many of the different levels in the model. The routines doing a particular job on machine A are designed to communicate with similar or complementary ones that are running on machine B. This horizontal communication is the very heart of what networking is about. It is what enables Web browsers and Web servers to talk, email applications to exchange messages, and so much more.
Of course, all types of communication function only if everyone agrees to the same methods of accomplishing it. Each set of rules describing one type of communication is called a protocol. You can think of a protocol as being like a language or a set of instructions. Each function or service of a network has its own language; like human languages, some are similar to each other while others are quite unique.
If you've done any reading at all about networks, you have probably seen the term protocol many, many times. Like the word interface, the word protocol can have many meanings. In fact, it is so fundamental to networking, and used in so many different ways, that I have a topic devoted to it in the chapter on Networking Fundamentals.
All that aside, we must remember that the OSI Reference Model is intended to be a formal way of describing networks. As such, the term protocol has a formal meaning in the context of the model. It refers specifically to a set of communication rules, instructions and/or procedures that describe communication between specific software or hardware elements running at the same layer on different machines within a network.
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