w Example Figure 7-7 illustrates SIP operation between two CMSs. Observe how the different SIP extensions implemented in PacketCable systems work together in a call. PacketCable implements the reliable delivery of provi- sional responses and QoS preconditions SO that when the callee picks up the phone, the network will always have enough available resources to meet call requirements. Chapter 7 Figure 7-6 PacketCable architecture. PSTN-to-SIP Interworking Telephony applications are among the most promising for taking SIP main- stream. In this arena, SIP is clearly an enabler for service builds. Tradi- tional telephony providers are now building IP networks for voice transmission in order to exploit Voice over IP's (VoIP's) service flexibility. When the IP network is properly configured and the appropriate QoS measures are taken, VoIP provides excellent voice quality that is compara- ble (or even better) than that achieved by the public-switched telephone net- Building Applications with the SIP Toolkit Figure 7-7 PacketCable uses (1) INVITE QoS preconditions and reliable (2) 183 Session Progress provisional responses. (3) PRACK (4) 200 OK (5) COMET (6) 200 OK (7) 180 Ringing (8) PRACK (9) 200 OK (10) 200 OK (11) ACK work (PSTN). At present, many VoIP providers offer cheaper international calls than the PSTN. Therefore, it seems inevitable that VoIP will be the future of the telephone network. However, no matter how efficiently SIP implements telephony services and no matter what level of quality it achieves, a hard-and-fast require- ment must be fulfilled by any new telephone system: it must able to inter- operate with the PSTN. The PSTN is the largest telephone network in the world and even if eventually gives way to IP networks in the future, it will still exist for many years. A huge telephony infrastructure cannot be thrown out and replaced wholesale by new technology. Chapter 7 Given the prominence of VoIP as a SIP driver, the community quickly realized that compatibility with the protocols used in the PSTN wa