from
The Free On-line Dictionary of Computing (8 July 2008)
protocol layer
layer
<networking> The {software} and/or {hardware} environment of
two or more communications devices or computers in which a
particular {network} {protocol} operates. A network
connection may be thought of 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. Each
networking component {hardware or software} on one host uses
{protocols} appropriate to its layer to communicate with the
corresponding component (its "peer") on another host. Such
layered protocols are sometimes known as peer-to-peer
protocols.
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 systems.
Examples of layered protocols are {TCP/IP}'s five layer
{protocol stack} and the {OSI} seven layer model.
(1997-05-05)