cally connects network terminations with the phys- ical boundary of the network side, see Sect. 2.2.4. 2 Foundations of Sysnet Modelling By introducing the ATD and ATC, additional types of SAPs are identified in the model, called A-SAPs (access-SAP) in Fig. 2.2. User-data terminations, terminal process controller, network process controller and telecom resources are all users of these A-SAPs. A-SAPs of the ATC and the ATD must be distinctly kept apart in models, because they are used by different functional entities, both on the terminal side and on the network side. Often they also have completely different properties. The concepts SAP and SAPI were defined by the OSI RM, but OSI RM did not make the distinction between ATC and ATD. This meant that, in the OSI world, a network termination could not have separate SAPs for user-data termination and terminal process control. This also caused problems when ITU-T specified Inte- grated Services Digital Networks (ISDN) in the mid-1980s. ISDN gives an exam- ple of an ATN that has a completely separate ATC. The ATC function in ISDN is called LAPD, see ITU-T recommendation ITU-T Q.921. The teleservice access protocol is called the "ISDN user-network interface Layer 3", see ITU-T recom- mendation ITU-T Q.931. When the ISDN access interface was standardized, ITU-T used the OSI RM as an architectural model as much as possible. However, since ISDN had the architec- tural properties of Fig. 2.2, and OSI did not define separate A-SAPs for the terminal process controller and the user-data termination, there was a problem. It was solved by creating a special ISDN architecture, where the ATC function was modelled as the "control plane", and the ATD function was modelled as the "user plane". This architecture is described by ITU-T recommendation ITU-T 1.320, "ISDN Protocol Reference Model". To finalize the functional model of a telecom system, we must add the system for supporting communication between logical nodes on the network side. This system is called the network transport n