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View File - University of Engineering and Technology, Taxila

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network <strong>and</strong> for networks to communicate with other networks using differentarchitectures.The ISDN Protocol Reference ModelAs previously defined, a protocol is a set <strong>of</strong> necessary procedures or a formatthat establishes the timing, instructs the processors, <strong>and</strong> recognizes themessages. A ‘‘protocol reference model’’ is a framework for the hierarchicalstructuring <strong>of</strong> functions in a system <strong>and</strong> its interaction with another system.The ISDN protocol reference model is based on the layered structures <strong>and</strong>signalling system number 7 (SS7) for both packet switching <strong>and</strong> circuitswitching with type B- <strong>and</strong> D-channels, which have already been describedin this chapter.Conceptually, the ISDN protocol reference model may be constructed bytaking into considerations three planes <strong>of</strong> communication in a particularnetwork: the user messages (including digitized voice, data, etc.), controlinformation (such as access control; network usage control including in-b<strong>and</strong>signalling, e.g., X.21 <strong>and</strong> X.25; <strong>and</strong> out-b<strong>and</strong> signalling, e.g., SS7), <strong>and</strong> localstation management aspects. The basic protocol building blocks <strong>of</strong> the threeplanes <strong>of</strong> communication are shown in Fig. 8.6. In the figure, the user messageFIGURE 8.6 A protocol reference model building block (U, message; S, control; M,management).Copyright © 2002 by Marcel Dekker, Inc. All Rights Reserved.

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