Chapter 22 - The Instant Messaging Service in the IMS ppt

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.

22.1 Pager-mode Instant Messaging in the IMS

The pager-mode instant messaging service was introduced with the first phase of IMS that

came as part of Release 5 of the 3GPP specifications. 3GPP TS 23.228 [43] already

contained requirements for Application Servers (ASes) and S-CSCFs to be able to send

textual information to an IMS terminal. 3GPP TS 24.229 [37] introduces support for the

MESSAGE method extension. The specification mandates IMS terminals to implement

the MESSAGE method (specified in RFC 3428 [115]) and to allow implementation to be

an 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.) that

we described in Section 21.3.

The main purpose of a pager-mode instant message is to allow the S-CSCF or ASes to

send short instant messages to the IMS terminal. Since the MESSAGE method is already

implemented in IMS terminals, users are able to send pager-mode instant messages to other

IMS users. The flow is simple, as depicted in Figure 22.1.

An example of a service provided with the SIP MESSAGE method is shown in

Figure 22.2. An AS is the controller of a voicemail system. The AS is interested to know

when the user successfully logs on to the IMS, so that the AS can inform the user that there

are pending voicemails to be retrieved. The service is implemented as follows: the user

registers with the IMS as usual, (1)–(20) in Figure 22.2. When the registration is complete

the S-CSCF evaluates the initial filter criteria. One of the initial filter criteria indicates that

the S-CSCF should perform a third-party registration with a particular AS. The S-CSCF then

sends 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 to

the AS that the user has just registered with the S-CSCF. Upon receipt of the REGISTER

ıa- ´ Martın´

The 3G IP Multimedia Subsystem (IMS): Merging the Internet and the Cellular Worlds Third Edition

Gonzalo Camarillo and Miguel A. Garc

© 2008 John Wiley & Sons, Ltd. ISBN: 978- 0- 470- 51662- 1