A protocol is a set of rules. The OSI model provides a framework for the communication protocols used between computers. Just as we need rules of the road—for example, so that we know that a red light means stop and a green light means go—computers also need to agree on a set of rules to successfully communicate. Two computers must use the same protocol to communicate. Computers that try to use different protocols would be analogous to speaking in Italian to someone who understands only English—it would not work.
Many different networking protocols are in use, in a variety of categories. For example, LAN and WAN protocols (at the lower two OSI layers) specify how communication is accomplished across various media types. Routed protocols (at Layer 3) specify the data's format and how it is carried throughout a network, and routing protocols (some of which also operate at Layer 3) specify how routers communicate with one another to indicate the best paths through the network.
Many protocol suites define various protocols that correspond to the functions defined in the seven OSI layers, including routed protocols, a selection of routing protocols, applications, and so forth. Protocol suites are also known as protocol stacks.
The most widely used network protocol suite today is the TCP/IP suite, named after two of the protocols within the suite. This network protocol suite is used in many places, including the backbone of the Internet and within organization's networks. Novell's NetWare, Apple Corporation's AppleTalk, and IBM's System Network Architecture are other examples of network protocol suites.
The OSI protocol suite is yet another suite. Although the OSI protocol suite uses the same names for its seven layers as the OSI seven-layer model does, the two OSI items are different—one is a protocol suite, and the other is the model that is used as a point of reference for all of the protocol suites.
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