Chapter 22 - The Instant Messaging Service in the IMS

Chapter 21 described the basic components of the instant messaging service. We learnt that there are two modes of operation: pager mode and session-based mode. In this chapter we analyze how these two modes are applied to the IMS. We explore the basic call flows and present examples of services that can be enriched with instant message capabilities.
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Chapter 22 - The Instant Messaging Service in the IMS Chapter 22The Instant Messaging Service inthe IMSChapter 21 described the basic components of the instant messaging service. We learnt thatthere are two modes of operation: pager mode and session-based mode. In this chapter weanalyze how these two modes are applied to the IMS. We explore the basic call flows andpresent examples of services that can be enriched with instant message capabilities.22.1 Pager-mode Instant Messaging in the IMSThe pager-mode instant messaging service was introduced with the first phase of IMS thatcame as part of Release 5 of the 3GPP specifications. 3GPP TS 23.228 [43] alreadycontained requirements for Application Servers (ASes) and S-CSCFs to be able to sendtextual information to an IMS terminal. 3GPP TS 24.229 [37] introduces support for theMESSAGE method extension. The specification mandates IMS terminals to implementthe MESSAGE method (specified in RFC 3428 [115]) and to allow implementation to bean optional feature in S-CSCFs and ASes (e.g., if required by the service). Obviously,pager-mode instant messages are subject to the constraints (e.g., message size, etc.) thatwe described in Section 21.3. The main purpose of a pager-mode instant message is to allow the S-CSCF or ASes tosend short instant messages to the IMS terminal. Since the MESSAGE method is alreadyimplemented in IMS terminals, users are able to send pager-mode instant messages to otherIMS users. The flow is simple, as depicted in Figure 22.1. An example of a service provided with the SIP MESSAGE method is shown inFigure 22.2. An AS is the controller of a voicemail system. The AS is interested to knowwhen the user successfully logs on to the IMS, so that the AS can inform the user that thereare pending voicemails to be retrieved. The service is implemented as follows: the userregisters with the IMS as usual, (1)–(20) in Figure 22.2. When the registration is completethe S-CSCF evaluates the initial filter criteria. One of the initial filter criteria indicates thatthe S-CSCF should perform a third-party registration with a particular AS. The S-CSCF thensends a third-party REGISTER request (21) to the indicated AS. The purpose of the third-party REGISTER request is not to register the user with the AS, but instead to indicate tothe AS that the user has just registered with the S-CSCF. Upon receipt of the REGISTERThe 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 22. THE INSTANT MESSAGING SERVICE IN THE IMS478 Originating Originating Terminating Visited Home Terminating Home Network Visited Network Network Network IMS IMS P-CSCF S-CSCF I-CSCF HSS S-CSCF P-CSCF Terminal #1 Terminal #2 (1) MESSAGE (2) MESSAGE Evaluation of initial filter criteria (3) MESSAGE (4) Diameter LIR (5) Diameter LIA (6) MESSAGE Evaluation of initial filter criteria (7) MESSAGE (8) MESSAGE (9) 200 OK Alert (10) 200 OK user (11) 200 OK (12) 200 OK (13) 200 OK (14) 200 OK Figure 22.1: Pager-mode instant messaging in the IMSrequest (21) the AS generates a MESSAGE request (23) that contains some informative text,maybe a link to a website that holds the transcription of the pending messages to retrieveor any other information that the service designer considers appropriate. The MESSAGErequest is forwarded via the S-CSCF and P-CSCF as any other SIP message.22.2 Pager-mode Instant Messaging to Multiple RecipientsIMS also allows IMS terminals to send MESSAGE request to multiple recipients. This isdone in cooperation with the help of a List Server, which takes the role of a URI-list server.In fact, the technology is a straight-forward application of MESSAGE U ...