WPCL 2BJ|x p   6'6'   x|@  Fascicle III.2 Rec. I.110 u x  x|@  Fascicle III.2 Rec. I.110 u Recommendation I.110 O PREAMBLE AND ă U = GENERAL STRUCTURE OF THE ISERIES RECOMMENDATIONS ă U R FOR THE ă U @ INTEGRATED SERVICES DIGITAL NETWORK (ISDN) ă Q PREAMBLE ă Introduction  An ISDN is a network, in general evolving from telephony Integrated Digital Network (IDN), that provides endtoend digital connectivity to support a wide range of services, including voice and non voice services, to which users have access by a limited set of standard multipurpose user network interfaces. This concept requires a family of CCITT Recommendations.  The ISeries Recommendations will provide principles and guidelines on the ISDN concept, as well as a detailed specification of the UserNetwork and Internetwork interfaces. They will further contain suitable references so that the detailed Recommendations on specific elements within the network can continue to be developed in the appropriate Recommendation series.  Figure 1 produces a broad outline of the structure of the ISeries of Recommendations and their relationship to other Recommendations.  ( X( x|@  X.   X.   X.   X.   X.   X.   X.   X.   X.   X.    x  MFIGURE 1/I.110 T = Structure of ISeries Recommendations and their ă @ relationship with other Recommendations  XAs shown in Figure 1 above, the current structure of the ISeries documentation is subdivided into seven major parts. Other ISeries documents may be added as the need arises. In addition, to support the implementation of the ISDN concepts, Recommendations have been produced and others will be produced in other Series by the appropriate specialist group (see Recommendation I.111). Basis of the ISeries approach  XIn order to standardize all necessary aspects of ISDN the CCITT has divided the issues into a number of distinctive (but obviously related) areas (see Figure 2). Three of these areas are the following: X1)( Services (I.200Series of Recommendations); X2)( Network aspects (I.300Series of Recommendations); X3)( Usernetwork access and interfaces (I.400Series of Recommendations).  XNetwork aspects are further supported by other Recommendations both inside and outside the I Series. XThe ISeries Recommendations are directed at the following principles:  XXhXXa)( the standardization of services offered to subscribers, so as to enable services to be internationally compatible;  XXhXXb)( the standardization of usernetwork interfaces, so as to enable terminal equipment to be portable (and to assist in a));  XXhXXc)( the standardization of network capabilities to the degree necessary to allow usertonetwork and networktonetwork interworking, so as to achieve a) and b) above.  XThe distinction that has been made in this approach between services and network capabilities is perhaps the most important. In the past, each service which was a candidate for standardization was treated in isolation and the necessary standards developed. For the ISDN, a wide range of services has to be considered in a coordinated manner. In addition, there has not been a conscious decision in the past to separate the definition of standards required for services from the definition of the standards for the network capabilities to support these services.  XThe approach, in the development of the ISeries, has been, firstly, to establish the broad concepts of the two areas of standards, secondly, to uniquely define these two concepts, and thirdly, to show the relationship between them.  XThe fourth area shown in Figure 2, is user equipment. The ISeries includes reference configurations which identify key functional groupings and their physical relationship. The interfaces with the network are explicitly defined, however the ISeries does not provide a detailed description of any specific terminal element.  ( X*   X*   X*   X*   X*   X*   X*   X*    x  MFIGURE 2/I.110  XFigure 3 illustrates the relationship between services and network capability areas from a firstorder viewpoint, even so the relationship can be seen to be recursive. The figure shows that the driving forces are: Xa)( what the User wants or is prepared to purchase; Xb)( the availability of the necessary technology;  Xc)( the economics of developing and enhancing services and the network capabilities.  (  X'   X'   X'   X'   X'   X'   X'     x MFIGURE 3/I.110