needs. There are basically two types of internal data communication systems needed: one system for supporting communication between terminal side and network side, and one system for supporting communi- cation between distributed entities on the Network side. For these systems, the log- ical nodes of the telecom system are the users. 2.2 Focus on Telecom Systems and Networks We will now extend the previous functional model with these internal communi- cation systems. In Fig. 2.2 the access transport network (ATN) is added. This sys- tem supports all communication needs (inside the telecom system) between the terminal side and the network side. Teleservice Access Protocol TE-TPC A-SAP A-SAP Resource control Access Transport Network (ATN) A-SAP A-SAP TE-NPC TE-UDT TE-SAPs Teleservice protocols TE-UDT TE-TPC Fig. 2.2 The access transport network supports communication between logical nodes on the net- work and the terminal side The internal structure of the access transport network depends on whether the connectivity service that is provided by the telecom system is circuit switched, packet switched, connection-less or connection-oriented, and if service control data and user data use different or the same transport resources (channels) inside the ATN. Logically, however, the ATN can always be regarded as consisting of two separate communication functions. One function, the ATD (access transport, data), transports user data between Network Terminations and the telecom resources inside the network (switching points). The other function, the ATC (access transport, control), transports service con- trol data between terminal process controllers and the network process control- The data transport properties of the ATD and the ATC may be very different (depending on the type of network); this is another reason for keeping terminal process controllers apart from user-data terminations. Inside the ATN, these func- tions are almost always integrated to a common access mechanism over a common transmission function that physi