tion-oriented communication. More important, however, is that TCAP devised an architecture where a single protocol could support many different and independent network functions in parallel. AMLn has inher- ited this technique by defining the "common-agent layer" concept as part of the language. 4. TMN (Telecom Management Network) introduced a generalized management network solution (based on a framework for management that was defined within OSI) in the late 1980s. This solution was based on the notion of a "man- aging system" separated from a "managed system," (or "traffic system,") and on "managed objects." The managed object approach was of course very much influenced by the development of object-oriented systems during the 1980s. The same can be said about AML, which defines object oriented communication in AMLs. 5. SDH (Synchronous Digital Hierarchy) introduced the notion of a "stratum" in the early 1990s. A stratum was a functional element in a network that could be operated as a network of its own (i.e., it had its own internal layer structure and management plane). To the environment, however, an SDH stratum appeared as just an OSI physical layer. 1 Introduction to Network System Modeling 6. The Internet and the UMTS (Universal Mobile Telecom System) introduced a new paradigm for network architectures, which we may call "service-resource separation." While in the OSI RM and ISDN, a layer must include the resources it needs (e.g., multi-party or multi-media bridges) for the services it provides, the new paradigm allows service control layers to be spatially separated from resources, which are pooled in special "media gateway" nodes. 7. Both the Internet and the UMTS also introduce a clear separation (in different strata) between functions that handle end-user services and functions that pro- vide raw connectivity (sometimes denoted the "call-connection separation"). This evolution developed from new transport technologies, such as IP and ATM, that could support all kinds of services (including transport o