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plane is denoted by U, the control information plane by S, and the managementplane by M.The primary function of the U-plane is to convey user information flows,user-to-user signalling flows, and user management information flows. Interms of channel structures, the U-plane is associated with the B- or H-channels but may also involve the D-channels. The U-plane is, in general,incapable of changing the state of the connection itself.The main function of the S-plane is to control information flow as wellas the management information flows between a user and a network managementfacility. The S-plane is normally associated with the D-channels. Boththe U- and S-planes are further decomposed into seven layers: numbered fromU-1 to U-7 and S-1 to S-7, respectively. These layers’ flows are conveyed in alogical sense between peer entities in the appropriate plane of communicatingfunctional groups.Physically all the layers’ subflows are transmitted as a single flow overthe physical transmission medium that interconnects the functional groups. Toidentify which layer belongs to what, names such as ‘‘network layer’’ or ‘‘datalink layer’’ are used. The functions of each of these layers may be contained inone or more functional groups that interact with each other across the user-tonetworkinterface for the purpose of transferring information flows.Most interactions between the U and S layers go through the managementplane, M; the exceptions are in the higher layers. The main function ofthe M-plane is to manage the U and S resources, facilitate prompt exchangesof information flows between them, and convey management information toanother system the management application process (MAP).The preceding discussion allows us to say that a logical transfer of thesubflows (i.e., at the layers level) between functional groups exists and isgoverned by peer-to-peer protocols between pairs of U-plane or S-planeentities. Additionally, the protocol of each layer is independent of the protocolof the other layer. Consequently, a general ISDN protocol reference model canbe developed with the aid of Fig. 8.7 as follows:For the reference model within the network segment, different mediaconnections may be used between two inner building blocks.The U-plane can be represented by the three lower layers for networkelements that perform information exchange and transfer functions.The seven layers are used for network elements that perform end-systemfunctions.The notation TE in Fig. 8.7 denotes terminal (or station) equipment: afunctional group that includes functions necessary for protocol handling,Copyright © 2002 by Marcel Dekker, Inc. All Rights Reserved.