n operator. Another example is a card service for which a serviceperson dials a specific service number and security code and the network operator charges his or her employer instead of the tele- phone from which he or she is calling. One category of services implemented with the help of IN technology is value-added services. This term refers to the services that give additional value, not just point-to-point telephone conversation. Separate service pro- viders, not the telecommunications service provider, often provide these serv- ices. Examples of value-added services are telebanking, telephone doctor or lawyer services, and participation to TV games. IN technology provides flexi- ble routing and service-specific charging for these services. In previous sections we described the structure and operation of the telephone network and we have also looked at different network technologies that we need to provide different services. In the following section we look at how all of this fits together. 2.10.5 Public Switched Telecommunications Network Today The overview of the modern public switched telecommunications network is presented in Figure 2.20. The structure and functionality of the network are only reviewed here because most of the elements in the figure are discussed in The Telecommunications Network: An Overview Public switched To international telecommunications network Internet Router Mobile Internet exchange To other Leased regions and Digital Local Trunk Digital line data international lines exchange exchange circuit telephone (ISDN) Trunk network 1.5/2 Mbps Transmission exchange or ISDN network Local Local DSLAM exchange exchange (digital) Analog (analog) lines Centrex service Analog Modem Figure 2.20 Overview of the public switched telecommunications network. other sections of this book. Figure 2.20 presents a simplified diagram of a regional or national PSTN that has connections to the global Internet and PSTN. The network contains the public land mobile network (PLMN), which provides wireless access 