Computer Security Basics, 2nd Edition

Appendix A. OSI Model

The Organisation Internationale de Normalisation (International Standards Organization, or ISO), which has been developing international standards since 1946, introduced the Open Systems Interconnection (OSI) model in an effort to define a conceptual model for accomplishing communication and interoperability between systems, regardless of the specific hardware or networking characteristics of the systems.

Adopted in 1984, the OSI Basic Reference Model defines a network architecture consisting of seven layers in the communications process, as shown in Figure A-1. In very rough terms, the top layer of the OSI model (layer 7) deals with the end user interface. The next three layers (layers 6, 5, and 4) define the characteristics of the systems at the two ends of the communication. The bottom three layers (layers 3, 2, and 1) define the network facilities necessary to transfer a message.

Figure A-1. Open Systems Interconnection (OSI) model

At the highest layer (the application layer), an application program in one computer communicates with an application in another computer. For example, you might send an electronic mail message to a user or to another computer. The mail program on the sending and receiving computers each resides in its own application layer.

Within the transmitting computer, the information to be transferred is passed down from one layer to the next ...