 (also called as a device). Second, standards contain less detail than specifications. Any component can be viewed as being surrounded by standards, each of which de- fines its interface to one other system component. The entire set of standards for a com- ponent's interface defines all the interfaces it must support to function well within the system. As a result, a component functions well within the system when it meets standards for every interface. In such a case, the interface standards for all interfaces of a component define the functional standard for the entire component. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. General Cellular Standards Standards for Cellular Systems The challenge design engineers face is to understand the standard, to ensure that their understanding is correct, and then creatively to specify and to design equipment that meets or exceeds the standard in the best way. Problems arise when different en- gineers interpret standards differently. Then components created by different compa- nies do not work together. A full specification is far more detailed than a standard. Even for a given interface, standards answer some questions and define the remaining ones. One view of this is that a good standard defines what each component will do at an interface, but the ques- tion of how the component will do it is left to the design specification. In addition, some devices do not have all interfaces defined by standards. A cellular telephone is a good example. The air interface is defined, but the human interface is not. Each vendor is free to design keypads, screen visuals, voice recognition, and bells and whistles for the user without following any particular standard. As written here, it seems that there is a linear progression from standard to specifi- cation to design to production to deployment. This picture is 