r of devices have emerged that have a number of names, including Voice over IP gateways, media gateways, next-generation switches, and softswitches. These elements are discussed in Chapter 9, "IP Services." These new devices in essence allow interoperability to exist seamlessly between the PSTN and packet-switched networks, whether IP or ATM or MPLS. Chapter The PSTN This chapter talks about the public switched telephone network (PSTN). It dis- cusses what comprises the PSTN, what sorts of technologies have been used to complete the connections, how the signaling systems operate, and the typical com- ponents and transmission capacities of the basic backbone architecture. This chap- ter also discusses what intelligent networks (INs) provide in terms of service logic and feature availability. Finally, this chapter describes some of the trends in the evolution of the PSTN that will support the new generation of applications. The PSTN Infrastructure Views about what a network should be designed to support and what the infra- structure should be composed of have changed quite a bit over the years, as appli- cations and technology have changed. Before discussing what is needed in a network today, this chapter takes a look at how the PSTN infrastructure evolved and where it is now. The traditional PSTN infrastructure was specifically designed to support only continuous, real-time voice communications. At the time this infrastructure was being designed, we had no notion of data communications or, indeed, notions of bursty communications or long-duration conversations. The length of calls is an important variable in the design of the PSTN. Most voice calls are quite short, SO the circuit switches in the PSTN are engineered for Chapter 4 The PSTN call durations of three minutes or less, and only a small percentage of subscribers off-hook at any time. The average Internet session, on the other hand, lasts around an hour. This means that increased Internet access through the PSTN has, in some locales, put a strain on the loc