Communication networks for computers may be organised as a set of more or less independent protocols, each in a different layer (or "level"). The lowest layer governs direct host-to-host communication between the hardware at different hosts; the highest consists of user application programs. Each layer uses the layer beneath it and provides a service for the layer above. For each layer, programs at different hosts use protocols appropriate to the layer to communicate with each other.
TCP/IP has five layers of protocols; OSI has seven. The advantages of layered protocols is that the methods of passing information from one layer to another are specified clearly as part of the protocol suite, and changes within a protocol layer are prevented from affecting the other layers. This greatly simplifies the task of designing and maintaining communication programs.
See also ISO seven layer model, protocol stack.
(07 Dec 1994)