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 ...
