TABLE OF CONTENTS Communications systems include many devices ranging in complexity from small handheld phones to large, central office switching devices. The earliest communications devices, such as phones, were all electrical and did not possess software, but new communications devices incorporate software based on the function that each device performs in the network. For example, a cellular phone has a microprocessor running a protocol stack to communicate with the cellular network. Frequently, it has additional capabilities, such as the ability to download software upgrades from the network or connect with the Internet.
Understanding issues common to communications equipment is the first step in developing a communications software strategy. Hardware variations then need to be considered in relation to these common issues. In some systems, the size of the code may play a more important role than performance, while in others, complete protocol functionality may not be required. Through a review of the Open Systems Interconnect (OSI) seven layer model, this chapter provides an introduction to the various types of communications systems and specifies a context for the software functions for each of these layers.
The Open Systems Interconnect (OSI) model (see xrefparanum) was created by the International Organization for Standardization (ISO) to form the basis for communications systems. The OSI seven-layer model for communication protocols provides a modular separation of functionality into seven layers, which can be implemented in hardware and/or software. Each layer works independently, yet builds upon the lower ones.
The seven-layer model is useful for educational...
