An ISDN is a network, in general evolving from a telephony Integrated Digital Network,that provides end-to-end digital connectivity to support a wide range of services,including voice and non-voice services,to which users have accessby a limited set of standard multi-purposeuser network mterfaces.In contrast,existing dedicatednetworks have always been developedfor specific(types of) services.Therefore,especiallyin the initiaiphase,the ISDN may support many services which in principle are still existent in dedicated networks.Thus,it is necessary to provide interworking between ISDN and dedicated networks to allow communication between terminals belonging to equivalent services offered through different networks.There will be a need for interworking functions between ISDN and dedicated networks to cope with the different environments given by the various networks.The structure of these IWFs showing the functions necessary for the mapping should be uniform to permit,if possible,a common use of functional parts in several IWFs.Detaileddescription of these IWFs, which (as far as is possible),should permit conveyanceof ISDN fatures through existingnetworks, is given m the 1.500-Seriesof Recommendations.The 1.50O-Series of Recommendationsdeal with networkaspects of interworking.
ITU-T I.500-1993 history
1993ITU-T I.500-1993 General Structure of the ISDN Interworking Recommendations (Study Group XVIII) 7 pp