TABLE OF CONTENTS Obviously, SIP-based networks will never be the only types of networks in existence. To begin with, circuit-switched networks will continue to be with us for a very long time. Therefore, an obvious need exists for SIP-based networks to be able to interwork with the circuit-switched networks of the PSTN. Furthermore, although SIP is viewed by many as the future of IP telephony, an embedded base of H.323 systems already exists, and more H.323 systems are being deployed. Therefore, a need also exists for SIP-based networks to interwork with H.323-based networks. The SIP specification, however, does not specifically address how such interworking should be achieved.
For interworking with the PSTN, gateways will be required to provide the conversion from circuit-switched media to the packet and vice versa. In fact, we have already seen that the H.323 architecture places great emphasis on the role of gateways. Not only must there be media interworking, but there must also be signaling interworking. After all, SIP is a signaling protocol. If calls are to be established between a SIP-based network and the PSTN, then the SIP network must be able to communicate with the PSTN according to the signaling protocol used in the PSTN. In most cases, this protocol is SS7. For the establishment, maintenance, and teardown of speech calls, the applicable SS7 protocol is the ISDN User Part (ISUP).
The usage of gateways and the control of those gateways in a SIP-based network are discussed in some detail in Chapter...
