The 5ESS Class 5 switch from Lucent Technologies reportedly offers some 3,500 features. The features are a function of Signaling System 7 (SS7) signaling (refer back to Chapter 2, "The Public Switched Telephone Network (PSTN)," for an SS7 tutorial). If the SS7 signaling network doesn't work, the 3,500 features will not be available to the subscriber. Almost every nation in the world has its own version of SS7 (or C7 outside the United States). Any international call must intermediate between two or more versions of SS7 in order for the call to be completed. In short, given the level of complexity of multiple SS7 versions, it's a miracle any international call is completed. The rise of VoIP with its own signaling protocols (H.323 and SIP) presents another challenge when interconnecting Public Switched Telephone Network (PSTN) and Voice over IP (VoIP) calls. The grand challenge for Internet telephony service providers (ITSPs) is the intermediation of Internet Protocol (IP) networks and the PSTN. Such intermediation must not only perform call setup and teardown, but it must also transmit the signaling necessary for call features to be used. This chapter will describe how SS7 is transported over IP networks to facilitate signaling between the PSTN and VoIP networks.

A VoIP network carries voice traffic cheaper than a switched-circuit telephone network because IP telephony networks make better use of available bandwidth. In a public switched telephone network, for example, a dedicated 64 Kbps end-to-end circuit is allocated for...