Figure 2.3 Conference call hosted on a SIP endpoint, such as a SIP phone
Telephony Call Control Services
Telephony call control has been developed to an advanced degree of maturity
and complete standards documents, as well as many commercial implementa-
tions, exist for the emulation of Intelligent Network (IN) services, as well as of
PBX-and Centrex-style telephony features. The SIP standards for telephony
include the following:
■■ Basic SIP call flows [11]
■■ SIP-PSTN call flows [12]
■■ SIP service examples for PBX/Centrex style functions [13]Intelligent Network Services Using SIP:
ITU Services CS-1 and CS-2
Though the PSTN business has declined more rapidly than forecast in 2001
when the first edition of this book was published, it is still of interest to show
the capabilities of SIP for the control of ITU-T style legacy TDM switched net-
works. The extensive capabilities provided by the PSTN and ISDN Intelligent
Network services can be supported by SIP. Authors from Columbia University
and Lucent Bell Labs have published a detailed paper on Intelligent Network
services that can be provided by SIP [1, 4]. The capabilities also include mobile
telephony features.
Service Examples for PBX and Centrex-style IP Systems are given in Table 2.6.
SIP ServerGatewaySIP PhonesPSTNInternet Communications Enabled by SIP 25