The Public Switched Telephone Network Summary

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.

Evolving the PSTN to the Next Generation

The expansion of the Internet continues to drive multiple changes in the PSTN environment. First, more network capacity is used to transport data over the PSTN. Dial-up Internet services use data connections that are set up over the PSTN to carry voice-band data over circuit-switched connections. This is a much different situation than sending data over a data network. Data networks use packet switching, in which many data transactions share the same facilities. Circuit-switched connections are dedicated connections, which occupy a circuit for the duration of a call. The phone networks were originally engineered for the three-minute call, which was the average length used for calculations when engineering the voice network. Of course, Internet connections tend to be much more lengthy, meaning that more network capacity is needed. The changes driven by the Internet, however, reach much further than simply an increase in network traffic. Phone traffic is being moved to both private packet-based networks and the public Internet, thereby providing an alternative to sending calls over the PSTN. Several different architectures and protocols are competing in the VoIP market to establish alternatives to the traditional circuit-switched network presented in this chapter. The technologies are not necessarily exclusive; some solutions combine the various technologies. Among the current leading VoIP technologies are:

• Soft switches

• H.323

• Session Initiation Protocol (SIP)

Each of these VoIP architectures use VoIP-PSTN gateways to provide some means of communication between the traditional PSTN networks and VoIP networks. These gateways provide access points for interconnecting the two networks, thereby creating a migration path from PSTN-based phone service to VoIP phone service. The core network interface connections for VoIP into the PSTN are the trunk facilities that carry the voice channels and the signaling links that carry SS7 signaling. PRI is also commonly used for business to network access. Figure 5-13 shows the interconnection of VoIP architectures to the PSTN using signaling gateways and trunking gateways. Chapter 14, “SS7 in the Converged World,” discusses these VoIP technologies in more detail.

Integration of SS7 into the PSTN

This section provides a brief overview of how the SS7 architecture is applied to the PSTN. Since SS7 has not been presented in great detail, the examples and information are brief and discussed only in the context of the network nodes presented in this section.

The PSTN existed long before SS7. The network’s general structure was already in place, and it represented a substantial investment. The performance requirements mandated by the 800 portability act of 1993 was one of the primary drivers for the initial deployment of SS7 by ILECs in the United States. IXCs embraced SS7 early to cut down on post-dial delay which translated into significant savings on access/egress charges. Federal regulation, cost savings, and the opportunity to provide new revenue generating services created a need to deploy SS7 into the existing PSTN.

SS7 was designed to integrate easily into the existing PSTN, to preserve the investment and provide minimal disruption to the network. During SS7′s initial deployment, additional hardware was added and digital switches received software upgrades to add SS7 capability to existing PSTN nodes. In the SS7 network, a digital switch with SS7 capabilities is referred to as a Service Switching Point (SSP). When looking at the SS7 network topologies in later chapters, it is important to realize that the SSP is not a new node in the network.

Instead, it describes an existing switching node, to which SS7 capabilities have been added. Similarly, SS7 did not introduce new facilities for signaling links, but used timeslots on existing trunk facilities. PSTN diagrams containing End Offices and tandems connected by trunks represent the same physical facilities as those of SS7 diagrams that show SSP nodes with interconnecting links. The introduction of SS7 added new nodes, such as the STP and SCP; however, all of the switching nodes and facilities that existed before SS7 was introduced are still in place.