Appendix C. The International Organization for Standardization: Seven-Layer Open Systems Interconnection (OSI) Reference Model


7 Application Layer User-level and application-specific services and procedures: distributed application support, e-mail, access security checking, information validation. Application dialog.

6 Presentation Layer Representation of information: formats, codes; transformation, encryption. Modification for syntactic compatibility.

5 Session Layer Session management: "conversation" initiation, initial screening, access security checking; sychronization, abnormal recovery. Dialog between workstations (processes, users, etc.).

4 Transport Layer Sending messages. End-to-end control across networks: session and network selection, virtual circuit management, flow control; message error checking, packet assembly/disassembly (PAD); enterprise-wide addressing; user-O/S interface. Creating a robust path.

3 Network Layer Sending packets or streams (of data). Network management: routing, switching, traffic monitoring, and congestion control; initiating an end-to-end pathway. Control of network logical topology.

2 Data Link Layer Sending blocks/frames (of bits). Link management: framing into transmission blocks; synchronization, bit error detection/ correction. Control of a single point-to-point, error-free, sequential channel.

1 Physical Layer Sending bits. Functional and procedural interface to the medium: activate, maintain, deactivate the physical connection; ...

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