Overview

If the worldwide Public Switched Telephone Network (PSTN) could be replaced overnight, the best candidate architecture, at the time of this writing, would be based on Voice over IP (VoIP) and the Session Initiation Protocol (SIP). Much of the VoIP industry has been based on offering solutions that leverage existing circuit-switched infrastructure (such as VoIP gateways that interface a private branch exchange [PBX] and an Internet Protocol [IP] network). At best, these solutions offer a compromise between circuit- and packet-switching architectures with resulting liabilities of limited features, expensive-to-maintain circuit-switched gear, and questionable quality of service (QoS) as a call is routed between networks based on those technologies. SIP is an architecture that potentially offers more features than a circuit-switched network.

SIP is a signaling protocol. It uses a text-based syntax similar to the Hypertext Transfer Protocol (HTTP) like that used in web addresses. Programs that are designed for the parsing of HTTP can be adapted easily for use with SIP. SIP addresses, known as SIP uniform resource locators (URLs) take the form of web addresses. A web address can be the equivalent of a telephone number in an SIP network. In addition, PSTN phone numbers can be incorporated into an SIP address for interfacing with the PSTN. An email address is portable. Using the proxy concept, one can check his or her email from any Internet-connected terminal in the world. Telephone numbers, simply put, are not portable. They only ring at...