f control signals) and could be used not only inside operator networks but for terminal access as well. OSI RM, ISDN, the Internet and UMTS show possible, but different, ways to build layer structures for network systems. What distinguishes them are the funda- mental principles that are applied on the highest level of functional structure: 1. The OSI RM emphasizes technology separation. It presents an abstraction hier- archy where transmission services are refined to more useful communication services step by step by adding specific layers. OSI RM does not define any other separation principle for the structure of network systems. 2. The ISDN emphasizes separation of control-data streams from user-data streams. It supports neither call-connection separation nor service-resource sep- aration however. On the contrary, all services are controlled from a common "control plane." 3. The Internet network system (as well as the UMTS) supports both call-connec- tion separation and service-resource separation. Note that, except for the OSI RM, none of the network systems mentioned are based on a generic set of layers (as the seven layers in OSI). This partly explains why OSI networks did not survive. Both the ISDN and Internet define important "higher level" separation principles that do not exist in OSI RM, which is another reason for the disappearance of OSI networks. The success of the Internet indicates that the call-connection and service-resource separations should be the primary principles for functional structures. The same principles now controls the evolution of public telecom networks into network systems such as the UMTS. AMLn sup- ports all principles mentioned, each on a specific level in the model of a network system. Many of the new concepts that were introduced by the mentioned network sys- tems were commonly used without first being unified, i.e., formally defined and agreed upon in the telecommunications domain in general. As a consequence, the usage of a concept in descriptions of network systems other 