THE OSI LAYERED MODEL

The Open Systems Interconnection (OSI) Model was developed by several standards organizations and is now a widely used model for the design and implementation of computer networks. The ITU-T and the ISO are the two organizations that have led the effort. The OSI Model has been in development for about ten years. The ITU-T publishes its OSI Model specifications in the X.200-X.290 Recommendations. The X.200 documents contain slightly over 1100 pages. The ISO publishes its OSI Model in several documents, but does not use a common numbering scheme.

The Model is organized into seven layers, shown in Figure A–1. Each layer contains several to many protocols and are invoked based on the specific needs of the user. Each protocol ...

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