Chapter 20 - The Presence Service in the IMS

Chapter 19 gave an overview of the presence service on the Internet, as defined by the IETF. This chapter focuses on the use of the presence service in the IMS. We explore the IMS architecture that supports the presence service and the applicability of presence to the IMS. 3GPP has defined, in 3GPP TS 24.141 [51], a presence service that runs over IMS, but mostly, 3GPP is just maintaining the specification, not actively progressing it.
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Chapter 20 - The Presence Service in the IMS Chapter 20The Presence Service in the IMSChapter 19 gave an overview of the presence service on the Internet, as defined by the IETF.This chapter focuses on the use of the presence service in the IMS. We explore the IMSarchitecture that supports the presence service and the applicability of presence to the IMS. 3GPP has defined, in 3GPP TS 24.141 [51], a presence service that runs over IMS, butmostly, 3GPP is just maintaining the specification, not actively progressing it. The presenceservice in IMS has since moved to OMA. OMA considers the presence service as an enabler,i.e., the set of specifications that enables a service. The main specification of the OMAPresence SIMPLE enabler is the OMA Presence SIMPLE specification [246]. This chapterdescribes this OMA Presence SIMPLE enabler.20.1 The Foundation of ServicesWhen we described the presence service on the Internet we unveiled a few of the powerfuland rich possibilities that the presence service can offer to both end-users and other services. On the one hand, end-users benefit from the presence service since they decide whatinformation related to presence they want to provide to a list of authorized watchers.Presentities can decide the information they want to publish, such as communication address,capabilities of the terminals, or availability to establish a communication. Watchers receivethat information in real time and decide how and when to interact with the presentity. All ofthese features enrich the end-user communication experience. On the other hand, presence information is not only available to end-users but also toother services. These other services can benefit from the presence information supplied.For instance, an answering machine server is interested in knowing when users are onlineto send them an instant message announcing that they have pending voicemails stored in theserver. A video server can benefit by adapting the bandwidth of the streaming video to thecharacteristics of the network where the presentity’s device is connected. For all of thesereasons we refer to the presence service as the foundation for service provision, as depictedin Figure 20.1.20.2 Presence Architecture in the IMSFigure 20.2 depicts the presence IMS architecture in conjunction with the Service configu-ration offered by the XDM architecture. The IMS terminal plays the role of both a watcherThe 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 20. THE PRESENCE SERVICE IN THE IMS444 Figure 20.1: Presence service: the foundation of all the servicesand a PUA (Presence User Agent), in addition to an XDMC). Central to the architecture isthe function of the Presence Server (PS), which acts as a Presence Agent (PA) and is locatedin the home network. The PS is complemented with a Content Server, a Presence XDMS,and with a Presence Content XDMS. All of these are Application Servers from the IMSarchitecture point of view. Typically presence requires the management of lists. A Resource List Server (RLS)provides the required functionality to host different lists. The RLS is complemented witha RLS XDMS and a Shared XDMS. All of these are Application Servers from the IMSarchitecture point of view. Finally, Application Servers can also act as watchers of presence information. Typicallya watcher in the presence service is implemented in connection to some other service thatneeds to keep track of the user’s presence. There are a number of interfaces in the Presence service, of which a non-comprehensiveview is provided in Figure 20.2. However, most of the interfaces are realizations of thegeneric ISC interface, which is based on SIP, and the Ut interface, which is based on XCAP.The PS can also implement the Sh interface, which is based on Diameter. SIP interfaces areused for real-time presence data whereas XCAP interfaces are used for configuration.20.3 Presence PublicationWhen the IMS presence application is launched, e.g., in an IMS terminal, the applicationpublishes the current presentity’s presence information. Figure 20.3 shows the flow and, aswe can see, there is not much difference from the mechanism we explained in Section 19.4.The IMS terminal sends a PUBLISH request (1) that contains an Event header set topresence and includes a Presence Information Data Format (PIDF) document that describesthe presence information. The S-CSCF receives the request (2) and evaluates the initialfilter criteria for the presentity. One of the initial filter criteria indicates that PUBLISHrequests containing an Event header set to presence ought to be forwarded to the PS wherethe presentity’s presence information is stored. So, the S-CSCF forwards the PUBLISH20.3. PRESENCE PUBLICATION 445 Figure 20.2: SIP-based presence architecture in the IMS Figure 20.3: The IMS terminal publishing presence informationrequest (3) to that Application Server. The PS authorizes the publication and sends a200 (OK) response (4). Apart from the IMS terminal, any other network entity can act as the source of presenceinformation and publish the user’s presence information. These entities are called PresenceNetwork Agents (PNAs) and the publish presence information on behalf of the user in a similarmanner as the PUA would: using the SIP PUBLISH method that contains a PIDF document.Any network entity can act as a PNA: A Mobile Switching Center (MSC) server, a GatewayGPRS Support Node (GGSN), a Serving GPRS Support Node (SGSN), a Home Location CHAPTER ...