For any communication to take place, a sequence of agreements must facilitate or allow the communication process to proceed. Such agreements are called protocols. The study of communications is, in general, the study of protocols.
All communications require protocols, not just those associated with computer networks. For example, for two people to communicate successfully they must first agree on a language for that communication. They must agree on a medium for that communication. If two people do not align in the various protocols they use, then their communication will fail.
Consider the two people who meet to plan an activity. One begins to speak English but the other speaks only French. Their planning will fail because they will fail to communicate. The same two people may agree to speak English, but one is using the spoken word (e.g., acoustical energy) and the other is writing everything down on a piece of paper. The presence of two different media is not a problem as long as both receivers have the necessary receiving equipment. However, if the person speaking is communicating with an individual that is deaf, the communication will fail (ignoring things like lip reading). If the person writing is communicating with an individual that is blind, the communication will likewise fail. In this case, even though they have agreed on one protocol, they have not agreed on the underlying medium for its delivery.
In data communication networks, the number of protocols that must exist is relatively large. Rather than considering all protocols at once, we tend to partition the set of agreements into relatively small subsets called layers. Our communications model is defined as a series of these layers that perform the various tasks required to make successful communication possible.
All of the protocols in use in a particular networking environment are referred to collectively as a protocol suite.
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