sers connect to the ISDN, rep- resent vertical partitions of the ISDN (the only interfaces that are standardized in ISDN). The network interface between users and the ISDN represents horizontal partitioning. This interface is, however, not standardized, i.e., each supplier of terminals designs it in its own way. ISDN users use primarily media connectivity services (in this model we have assumed that a voice service is used). A user may be a person using a telephone or a machine (e.g., a voice-message-storing or voice-answering machine), i.e., the horizontal network interface (H-NI) is a man-machine or a machine- machine interface. This H-NI comprises two logical interfaces: one is used by the user in his requester/acceptor role, one in his service using role. In man-machine interfaces the person using the interface takes both roles. In machine-machine interfaces, there may exist two interfaces, one for requesting the service, another for using The layers and logical networks model in the middle reveal the distinct separa- tion in ISDN between control and signaling layers on the one hand, and layers that transport user data on the other (in the ISDN standard referred to as the "control plane" and the "user plane"). This structure is characterized by the requirement that, before a user can use a media route, it must first be established by the call-handling layer (CH) that con- trols all switching layer elements and all media resources. A destination network address (in the scenario called na1) is only used by service requesters in com- munication with CH elements in terminals. The CH layer handles no APi and CEi, i.e., all media routes are single-channelled. The CH layer establishes a media route in two phases (sometimes referred to as "call handling" and "connection handling"): First it decides on which kind of media resources are needed in terminals (if a choice is possible), and if any transcoding resources are needed in between (as the A/u-law transcoding ele- ment in the scenario). As to the latter, the CH la