tem Controller Network maintenance Audio Process system 64 K CS termination Control controller encoder Packet Handler converter network service Resource interface control types SAPI 16(D) 64 kbps SAPI 0 (D) SAP CSI The ISDN-CSP logical network structure Fig. 3.4 The logical network structure of the ISDN-CSP. All interfaces denote types of SAPs and CSIs. Interface symbols are defined in Fig. 3.23 The model also uses formal symbols for SAPs and control service interfaces (CSI). The reader is encouraged to compare the interface symbols used in Fig. 3.4 with the intuitive symbols used in Fig. 2.1. Formal interface symbols will be dis- cussed in detail in the following sections. Figure 3.4 shows that a logical network structure is a pure C-model. It does not show the topology of a network, and therefore no protocols, but it defines all types of network service interfaces that can exist in the CSP, including those used by tel- eservice users (TEU). The model is constructed by CSPs (here NTC, ATN and NTD) and by logical networks (here the process control system and the resource system). All major types of teleservices that an ISDN system can provide are indicated in the logical network structure of Fig. 3.4 as different TE-SAPs (telephony, audio, etc.). The large variation of teleservices gives an understanding of why ISDN stands for Integrated Services Digital Network. To provide one single network for all kinds of teleservices was the main objective for ISDN. The reason that ISDN never became a real success is primarily that is based on a rather expensive circuit switching technology for 64-kbps channels. We can now achieve the same objec- tive in a more cost-effective way by using emerging networks based on packet switching, like the Internet. This was, however, not the case back in the mid-1980s when ISDN was developed. 3 Logical Network Modelling TE-SAPs in ISDN differ if the service depends on a B-channel or on the D- channel, see the interfaces between the ATN and the resource system in Fig. 3.4. This primarily a