yer must also find out where such a resource is available in the network (in this scenario in LE2). In a second phase it creates circuit-switched routes between media resources. This scenario shows clearly the distinction between a "circuit" and a media connection, since two circuits were needed to create the media connection in this case. The effect of this structure is that when media routes and circuit-switched routes are used, there is no need for identifying any of these routes by an REi, or deliver any addressing parameters. Both types of routes appear as single-channel link routes, although they are created on-demand. ISDN is also an example of a network system that (with the exception for PABX 3.4 Route Type Examples systems) does not allow terminals to act as gateways, i.e., routing elements. The model reveals that in two ways: the destination network address is not delivered to terminating CH nodes; these nodes do not store their own network addresses or any T-RO. Thus, the network defines (through its routing tables) which node is a routing node and which is a terminating node. As has been said before: logical network structures can only be defined if the realization of layer interfaces are standardized (which implies the specification of H-NIs). Since there are no H-NI interfaces defined in ISDN, the whole ISDN is a single logical network, in relation to logical networks defined by user ele- ments. In the model at the bottom of Fig. 3.25 we have, however, indicated a possible path to an internal logical-network structure for ISDN. To turn this model into a correct logical-network structure, one must define H-NIs for all types of control points, in which case all control point will vanish from this model. Layers that are defined by control point protocols (i.e., interlayer protocols), as well as some new layers for supporting these protocols, will appear instead. We will discuss this subject in details in Sect. 4.3. Such a modification of the existing ISDN would lead to a new architecture, exhibiting a