In the area of communications and networking, layers are used to distinguish the types of network and application-based activities that are carried out. For example, perhaps the best- known layer model is the seven-layer OSI Reference Model for describing network activities. Its layers range from the physical layer, at which details of cable connections and electrical signaling are specified, to the application layer, at which details of the immediate interface between an application and network services are defined. Other layer models include IBM's SNA (Systems Network Architecture), Digital's DECnet, and the TCP/IP model used on the Internet.
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