185In this chapter, the basic telephony services and features will be discussed as
implemented in a SIP-enabled network. First, basic telephony will be covered,
followed by more advanced features. We will describe the basic and more
advanced telephony features for which there are Internet drafts published to
support inter-service-provider and intervendor product interoperability.
Basic Telephony Services
Basic telephony involves the establishment of sessions between endpoints.
The basics of telephony in an all-IP environment are covered in Chapter 6, “SIP
Overview.” This chapter will focus on SIP and PSTN internetworking for basic
telephony services.
SIP and PSTN Interworking
SIP and PSTN interworking occurs whenever a call originates in one network
and terminates in another network. To accomplish this, the signaling and
media transport protocols must be mapped between the two domains.
Gateways are the network elements bridging the two networks, as shown in
Figure 11.1. The gateway is, as a result, part of both the PSTN and SIP network.