 network forms represent the circuit transfer mode. Today, the situation is different, voice can be carried by many transfer modes (VoIP, VoATM, VoFR) with many different bandwidths. In order to introduce new functions into the 'historical' PSTN it was necessary to upgrade the exchanges or build a more service-rich overlay network, such as an ISDN Understanding Changing Telecommunications - Building a Successful Telecom Business Table 3.1 The vertical networks and the targeted services Service Network Voice Data, wide area Data, local area Ethernet, Token Ring Data, 'Faster X.25' Frame relay Data + voice 'dream' ISDN, ATM Data internetworking Internet Voice with mobility GSM, TDMA, CDMA, etc. Voice + data + mobility Next generation network, 3G + information society (e.g. UMTS, WLAN) network. It is easy to realize that individual upgrades of exchanges is a very time- consuming way of improving network functionality. However, it was in the monopoly era, and the risk of losing subscribers was very low (and the waiting lists were often very long!). The IN architecture brought the first 'server' into the fixed network at the end of the 1980s. This was before the convergence between voice and data networks, and the term 'server' was not used by telecom staff. The IN concept with a service control point (SCP) node on top of a group of exchanges also became integrated into the mobile systems. The SCPs offer a centralized way to introduce new services all over the network, proving the advantage of a server-supported 'layered' network. The time to customer for the operator could now be shortened. However, the interfaces between telephone exchanges were not simplified. On the contrary, since the IN concept adds new signalling in the network, the overall interface complexity grew. Managing an IN network has also offered more problems than expected. It should be borne in mind that the IN services to a large extent are 'network ori- ented', often meaning advanced routing, advanced charging or advanced numbering. Such services 