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information. The transfer cell provides a basic unit of capacity. A particulartransfer cell carries within it a header for channel identification, and the otherportion of the cell is used for carrying the useful information.If an asynchronous channel uses an average of 1 out of every K cells, itsaverage data rate hr b i can be expressed ashr b i¼ R rðL a hÞbit=sec ð8:5aÞKL aand the average time between successive channels iswhereT c ¼ 8 KL aR rsec ð8:5bÞK ¼ number of cells.L a ¼ number of octets in a cell.R r ¼ transmission rate (bits=sec).h ¼ number of octets in the header.The total capacity of the channel structure must be sufficient to at leastaccommodate the application with the highest data rate that must be supportedover the user-to-network interface.Unlike the synchronous channel structures with fixed channel arrangementand known capacity, the component channel of the asynchronouschannel structures dynamically provides a data rate matching the requirementsof the applications. This arrangement offers flexibility in efficiently supportinga wide and dynamically varying spectrum of applications with different datarates and usage patterns.In general, the channel structures provide the transmission capacity forthe exchange of information between functional groups on the two sides of theuser-to-network interface. The principle of functional layering inherent in theopen systems interconnection (OSI) leads to a more general representation ofthe total communications capacity of a functional group. Total communicationsof a functional group mean, for example, the interchange of informationbetween the user and the network elements, between network elements of thesame network, and between different networks. Since the OSI reference modelwas designed to allow various ‘‘open’’ systems to communicate, more is saidlater on the OSI reference mode. A system that communicates with othersystems is defined as open. As such, an open system is standards-(protocols)-based instead of propriety-based. The ISDN protocol reference model is thebasis for a set of ISDN protocol configurations. There is a functionalCopyright © 2002 by Marcel Dekker, Inc. All Rights Reserved.