This chapter provides an overview of the PSTN, as it existed before VoIP technologies emerged. The majority of the PSTN still appears as this chapter presents it. Many of the diagrams in telecommunications literature illustrating next generation technologies—such as soft switches, H.323, and Session Initial Protocol (SIP)—show interfaces to the PSTN. The diagrams refer to the PSTN discussed here, dominated by large, digital switches. The technologies introduced often replace some portion of the existing PSTN; however, they must also remain connected to the existing PSTN to communicate with the rest of the world. The VoIP-PSTN gateways provide this transition point, thus enabling a migration path from the traditional PSTN to the next generation architecture.
While the PSTN varies in its implementation from country to country, a number of common denominators exist. The PSTN is a collection of digital switching nodes that are interconnected by trunks. The network topology is usually a hierarchical structure, but it often incorporates some degree of mesh topology. The topology provides network access to residential and business subscribers for voice and data services. VoIP began another evolution of the PSTN architecture. The PSTN is a large infrastructure that will likely take some time to completely migrate to the next generation of technologies; but this migration process is underway.
