itched network to communicate between them. The PSTN has been around for a long time, and its users are happy with it. People can talk to each other across oceans and still feel in proximity. Even in long-distance calls between two mobile phones, it is possible to understand what the other person is saying relatively well. Besides, the PSTN is a highly-reliable network. When somebody picks up a phone in order to make a call, most of the time the call goes through. Tele- phone switches rarely crash, and when they do, backup systems take over to continue providing service to the users. In short, people trust the PSTN. They have grown to expect trouble-free everyday use, and they are confident that in the case of an emergency, the PSTN will connect them to an ambulance, the police, or the fire department. Taking into account all these features of the PSTN, one might think that the procedures and mechanisms used by the telephone network would be emulated in any network providing a similar service. Since the PSTN works SO well, let us model any voice network after it. Specifically, let us use PSTN-like signalling protocols-they work well. This assumption is wrong, however, and we will see why for a different environment, such as the Internet, PSTN-like protocols are not suitable. Material differences in the PSTN architecture and the Internet architec- ture require completely new signalling protocols rather than the evolution of trusty, old ones. This chapter contains a brief PSTN-signalling history that explains how signalling protocols have evolved from analog to digital. We will see that beyond the differences in transport technology used by the PSTN and the Internet-the former is circuit-switched while the latter is packet-switched -differences in their respective paradigms for governance and operation make different signalling design mandatory. Signalling in the Circuit-Switched Network The Origins of Circuit-Switching A telephone network aims to provide its users with certain services. Many services fall into the