Multimedia Telephony Services: PSTN/ISDN Simulation Services
We described earlier that the core IMS enables multimedia services and traditional telephony services. In this section we focus on the traditional telephony services that, in the context of the PSTN and ISDN, are globally known as PSTN/ISDN supplementary services. Traditional supplementary services are: Call Forwarding, Call Hold/Resume, Connected Line Identification Presentation/Restriction, etc.
In IMS, Applications Servers (ASes) are also able to provide PSTN/ISDN simulation services. These are telephony services similar in nature to the PSTN/ISDN supplementary services, but different in the realization since SIP is the call control protocol and is quite different to the PSTN/ISDN protocols. PSTN/ISDN simulation services are a simulated version of the PSTN/ISDN supplementary services. However, the aim of the service remains.
A number of these PSTN/ISDN simulation services are specified. Most of them assume an AS that is able to provide the service, either to the originating or to the terminating party. It is perfectly valid to combine several of these services into a single AS, a decision that is left to the implementors.
The PSTN/ISDN simulation services were initially developed by ETSI TISPAN in the context of services for fixed access to IMS. Later, especially when TISPAN IMS merged with 3GPP IMS, the TISPAN-created PSTN/ISDN simulation services were slightly improved for mobile access devices and renamed ...