 of packet services, the TEU must connect an X.25 or IP terminal to the TE-SAPs and to the TEU-CSI. X.25 and IP functions are outside the ISDN concept, although the ISDN defines terminal adapters that can connect X.25 and IP terminals to the ISDN network. SCCP stands for Signalling Connection Control Part. It is part of CCS. SCCP is a general mechanism for supporting distribution of logical nodes in the network process control system. SCCP uses the network-level service of CCS that is provided through the MPT, Message Transfer Part (MTP is the CCS variant of IP). SCCP is described in ITU-T Q.711-Q.716. MTP is described in ITU-T Q.701-710. ISUP, Integrated Services digital network User Part, is the service control protocol in ISDN. It is described in ITU-T Q.761-766. 3.2 The Communication Service Provider Packet access (D) is a less expensive alternative for accessing packet services. The TEUs use SAPI 16 of the D-channel. The core network separates SAPI 16 data from other LAPD data. SAPI 16 data from all terminal systems are routed to packer handlers (PH) in the ISDN network. The PHs are, in turn, connected to packet-handling networks outside the ISDN. An ISDN user might have access to more than one of these services. The service control CSI is used by teleservice controllers (TEC) to request the type of service, to provide addresses and for user-to-user messages outside calls. Another important CSI is the one used by network operators. Services provided over this CSI are implemented by the operation and maintenance system (OM), which is part of the process control system. Note that a logical network structure model is not a primary model fragment of a sysnet model, since it can be derived from the set of logical network fragments that exist on integration level 3 in the sysnet model. A logical network structure is a derived model, while a logical network is modelled and stored as an individual primary model fragment of the sysnet model. For example, the logical network ISDN process control system will look as sh