rce system and the proc- ess control system a step further, when compared to ISDN and X.25 networks. There is no discrimination performed by ATM access and transport links. Instead, process control data are relayed through the switch over virtual chan- nels in vpi0 to an internal link that terminates in the ATM-DNPC over an MTP node. The model in Fig. 3.66 shows the resource system part only and indicates how user data are transported. 3.4 Modelling Telecom Resource Systems ATM-IDU ATM-ICI ATM-SDU (48 octets) VPIVCIPTI CLP The ATM-CSP ATM access router ATM-PDU (a cell, 53 octets) GFCVPIVCIPTICLPHECuserData ATM-access link switch PDH/SDH connection ATM transport link CLP cell loss priority PTI payload type GFC general flow control SDH synchronous digital hierarchy HEC header error check SDU service data unit ICI interface control information VC virtual channel IDU interface data unit VCI virtual channel identification PDH plesiochronous digital hierarchy VP virtual path PDU protocol data unit VPI virtual path identifier Fig. 3.66 The ATM resource system on the access side A short description of the model in Fig. 3.66 follows: Fixed length ATM-SDUs (48 octets) are transferred between ATMUs and the ATM switch. An ATM-SDU is part of an ATM-IDU. The ATM-ICI consists of a virtual path identifier (VPI, 1 octet), a virtual chan- nel identification (VCI, 2 octets) and some extra control information that defines the type of SDU (PTI, 3 bits) and priority (CLP, 1 bit) of the SDU in rela- tion to other SDUs. The ATM-PDU also contains a flow-control parameter (GFC, 4 bits), which is only used on the access side of an ATM switch, and a checksum (HEC, 1 octet). The HEC is only used to control the correctness of the header information (eve- rything except the SDU). An incorrect HEC implies that the receiving ATM- UDT discards the whole SDU, i.e. there is no re-sending done over ATM links (as in X.25 networks). The correctness of the SDU is not checked since the ATM transport links have the benefits of modern transport systems wi