Appendix A. Overview of the OSI Seven-Layer Networking Reference Model

SOME OF THE MAIN TOPICS IN THIS APPENDIX ARE

It's Only a Model!

When discussing different network devices or software components of a network, it is common to use, as a reference point, a model that the International Organization for Standardization (ISO) created. The Open Systems Interconnection (OSI) Seven-Layer Networking Reference Model was originally developed as a blueprint for creating the additional network protocols that the ISO developed. However, these protocols were written during a time when a lot of computer horsepower was required to implement them, and they were never widely adopted. Some companies, such as Digital Equipment Corporation's DECnet, did move toward OSI standards.

With the explosion of the Internet, and also of its requisite protocols, TCP/IP became the overall standard for most local area (and wide area) networks instead of the OSI protocols. TCP/IP is based on a network model that has fewer layers , as discussed in Chapter 25, "Overview of the TCP/IP Protocol Suite." This appendix looks at the OSI reference model because it is still generally referred to when discussing networking technology.