Source : Free On-Line Dictionary of Computing

Open Systems Interconnect
     
         (OSI-RM, OSI Reference Model, seven layer model)
        A model of network architecture and a suite of {protocols} (a
        {protocol stack}) to implement it, developed by {ISO} in 1978
        as a framework for international {standards} in heterogeneous
        computer {network} architecture.
     
        The OSI architecture is split between seven {layers}, from
        lowest to highest: 1 {physical layer}, 2 {data link layer}, 3
        {network layer}, 4 {transport layer}, 5 {session layer}, 6
        {presentation layer}, 7 {application layer}.
     
        Each layer uses the layer immediately below it and provides a
        service to the layer above.  In some implementations a layer
        may itself be composed of sub-layers.
     
        OSI is the umbrella name for a series of non-proprietary
        protocols and specifications, comprising, among others, the
        OSI Reference Model, ASN.1 ({Abstract Syntax Notation 1}), BER
        ({Basic Encoding Rules}), {CMIP} and {CMIS} (Common Management
        Information Protocol and Services), {X.400} (Message Handling
        System, or MHS), {X.500} (Directory Service), {Z39.50} (search
        and retrieval protocol used by {WAIS}), and many others.
        Apart from its actual application to real protocols, it also
        serves as a useful teaching model.
     
        (2002-05-19)