networks. Note that access nodes, by definition, do not switch user-data flows between TE-UDTs. They only relay flows between terminal systems and the core network. The actual switching is done in the core network. Possible physical structures on the network side depend on the functions and structure of the network transport network (NTN). The internal structure of physi- cal nodes of the core network is created by connecting distributed network process controllers, resource controllers and telecom resources to the NTN. As shown in Fig. 2.7, control flows and user-data flows can be handled completely independ- ently from each other in the NTN, since the separation and integration of flows is done in access nodes. Core Network Resource Control Protocol Service e.g. CCS Distribution Protocol Fig. 2.7 A possible physical structure of the core network The NTN consists of two separate logical networks, NTC and NTD. In public networks, the NTC logical network corresponds to the lower levels of the ITU-T CCS signalling system (SCCP and MTP). The NTC logical network is also some- times used for distributing TRCs from the network process control function. This is shown in the model by changing a local control interface to a remote control inter- face, called resource control protocol. Anther use of the NTC/CCS (not shown in Fig. 2.7) is to further separate network process control functions into a collabora- tion of more specialized logical nodes, like HLR-function, SSF and SCF2. Finally, there are numerous solutions to the physical structure of terminal sys- items. Figure 2.8 shows some possibilities. 2.2 Focus on Telecom Systems and Networks Terminal Systems Operator Network End-users Network Completely Termination Teleservice integrated Access protocol TEUs, TE-TCP Network Term. and ATN term. term. relay TE-UDT Access TE-TCP Integrated Network Term. and ATN term. term. relay TE-UDT TE-TCP TE-UDT ATN-termination a separate system, serving multiple Terminals term. relay (an NT1 solution in ISDN) TE-UDT TE-TCP Fig. 2.8 Pos