Chapter 26 - 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.
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Chapter 26 - Multimedia Telephony Services: PSTN/ISDN Simulation Services Chapter 26Multimedia Telephony Services:PSTN/ISDN Simulation ServicesWe described earlier that the core IMS enables multimedia services and traditional telephonyservices. In this section we focus on the traditional telephony services that, in thecontext of the PSTN and ISDN, are globally known as PSTN/ISDN supplementary services.Traditional supplementary services are: Call Forwarding, Call Hold/Resume, Connected LineIdentification Presentation/Restriction, etc. In IMS, Applications Servers (ASes) are also able to provide PSTN/ISDN simulationservices. These are telephony services similar in nature to the PSTN/ISDN supplementaryservices, but different in the realization since SIP is the call control protocol and is quitedifferent to the PSTN/ISDN protocols. PSTN/ISDN simulation services are a simulatedversion 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 assumean 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 isleft to the implementors. The PSTN/ISDN simulation services were initially developed by ETSI TISPAN in thecontext of services for fixed access to IMS. Later, especially when TISPAN IMS merged with3GPP IMS, the TISPAN-created PSTN/ISDN simulation services were slightly improved formobile access devices and renamed as IMS multimedia telephony services. Through thischapter we used both terms, IMS multimedia telephony services and PSTN/ISDN simulationservices in an interchangeable manner. As for documentation aspects, 3GPP combines allof these services in a single specification, 3GPP TS 24.173 [36], which is an umbrella thatcontains pointers to each of the specifications that describe one or more related services.However, for historical reasons, ETSI published the set of PSTN/ISDN simulation services inthe ETSI namespace of specifications. Each service or group of related services is describedas a separate specification. Later, those ETSI specifications were contributed to 3GPP, andpublished under the 3GPP namespace. In general, the reader who is interested in moreinformation should be looking at 3GPP TS 24.173 [36]. PSTN/ISDN simulation services were developed with a clear focus on compatibility withexisting PSTN/ISDN supplementary services. Although PSTN/ISDN simulation serviceswere originally created in the context of fixed broadband access to IMS, they have beenembraced and are nowadays part of the full IMS, including wireless devices.The 3G IP Multimedia Subsystem (IMS): Merging the Internet and the Cellular Worlds Third EditionGonzalo Camarillo and Miguel A . Garc ıa-Mart´n ´ ı© 2008 John Wiley & Sons, Ltd. ISBN: 978-0-470-51662-1 CHAPTER 26. MULTIMEDIA TELEPHONY SERVICES530 Each PSTN/ISDN simulation service is based on the corresponding supplementaryservice in the PSTN/ISDN. However, in many cases the name of the service is changedin IMS, since in SIP the concept of a call can be broadened with messages, subscriptions,notifications, or multimedia sessions. Therefore, most of the PSTN/ISDN simulation servicesrefer to communication rather than calls. In Table 26.1 we take a brief look at each of these PSTN/ISDN simulation services. TheO/T column indicates whether the service is provided to the originating side of a call (i.e., thecaller) or the terminating side (i.e., the callee).26.1 Providing Audible AnnouncementsBefore we start looking into the different Multimedia Telephony Services, we ought todescribe a common building block that is used by a few of these services: providingan audible announcement. In effect, this is inherited from the PSTN/ISDN services,where the only mechanism to provide feedback to the user was to provide an audibleannouncement. In IMS, one could use other mechanisms to provide feedback, suchas sending an instant message, rendering a web page in the display of the phone, etc.However, the common minimum denominator, device-independent mechanism, is to providean audible announcement. This section describes the different ways to provide an audibleannouncement. These feature can be used by most of the Multimedia Telephony Servicesthat we describe in the following sections. The procedures for audible announcements are specified in ETSI TS 183 028 [142] andits equivalent 3GPP TS 24.628 [60]. Audible announcements can be provided at the time asession is being established, during an established session, or at the time the session is beingreleased. The procedures are slightly different in each case. Audible announcements can alsobe sent when a session attempt is rejected ...