Introduction

Introduction Communicat ions systems include many devices ranging i n complexi ty f rom small hand­ held phones to large, central office switching devices. The earliest communications devices, such as phones, were all electrical and d id not possess software, but new com­ munications devices incorporate software based on the function that each device per­ forms in the ne twork . For example, a cellular phone has a microprocessor running a protocol stack to communicate w i t h the cellular ne twork . Frequently, i t has addi t ional capabilities, such as the abi l i ty to download software upgrades f rom the ne twork or con­ nect w i t h the Internet.