annot see interfaces between network termination entities and TEUs. The realization of these interfaces is therefore of no concern to a telecom system, and they may be realized as physical interfaces or software interfaces, or the two entities on either side could be integrated com- pletely. Note that this implies that the boundary of a telecom system is a logical boundary, not necessarily a boundary between network elements. In the days of the "plain old telephony system" (POTS), the interfaces between network termi- nations and TEUs were also fully integrated in equipment (telephones) that were normally supplied by the same vendor who delivered the telecom network nodes. However, with the advent of data networks, interfaces for connecting TEU equipment to data communication networks became defined as physical, standardized interfaces, in order to increase openness to TEU equipment from different manufactures. Examples of such standards are: - The ITU-T recommendations V.24 for connecting a single terminal to a circuit switched data network over the general telephony network. The network termi- nations in this case are fully standardized analogue modems that can be supplied by many manufactures. - The ITU-T recommendations X.24 for connecting a single terminal to circuit switched, digital data networks. The network terminations in this case are base band modems that can be supplied by many manufactures. - The ITU-T recommendations in the I.400 series for connecting multiple termi- nals to ISDN networks. The network terminations in this case are special equipements (NT1 and NT2), normally supplied by the same manufacture as the one who delivered the ISDN nodes. Functional separation: The Resource System is separated into two groups of resources: the resources on the network side are telecom resources like switches, network internal transmission lines and other types of devices. The other group 2.2 Focus on Telecom Systems and Networks are resources that terminate user data flows on the terminal side, called teles