types of control information flows in the model: service control (remote and local), resource control and operation and maintenance control (OM). Con- trol flows concern the units of the process control system. User-data flows provide connectivity services. The two main types of such services are teleservices and data services (or "bit-transport services"). Tele- services provide information connectivity (voice, audio, text, images, video, file transfer, etc.). Data services provide data transfer, e.g. as means for supporting teleservice. To specify the properties of data services is an important part of modelling telecom systems. These properties are often referred to as quality of service (QoS) parameters. The difference in QoS between different data services lies primarily in the type of switching that is provide by the network, which could be circuit switched or packet switched. Service access points: User-information is transported between (remote) TEUs, over a number of logical nodes, both on the terminal and the network sides. The 2 Foundations of Sysnet Modelling basic service that a telecommunication system must provide is teleservice between the interfaces where TEUs access the telecom system. These interfaces are teleservice service access points (TE-SAP). When an association is estab- lished between two TE-SAPs, they are referred to by the type of teleservice they provide and by an address that defines the particular network termination. This address is called service access point identifier (SAPI). A SAPI corresponds to an OSI "network address". The "subscriber number" in public networks also acts as SAPI, although it has other functions as well. A SAPI is the identifier that is used by the network to route user data to the right network termination. In networks where neither the network termination nor the TEUs are mobile (as in the old telephony network), this is relatively easy. Today, public networks support both mobile TEUs (by services like call- forward) and mobile network termination (cellula