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.

Figure 5-13. VoIP Gateways to the PSTN

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