The IMS: IP Multimedia Concepts And Services, Second Edition

5.3. Immediate messaging

Immediate messaging, or page-mode messaging, is the familiar instant messaging paradigm adopted in the IMS framework. It uses the Session Initiation Protocol (SIP) MESSAGE method (see Section 12.13.3 for details) to send messages between peers in near-real time. Figure 5.1 illustrates a typical message flow.

Figure 5.1. Immediate messaging flow.

In immediate messaging, the User Equipment (UE) simply generates a MESSAGE request, fills in the desired content – which typically consists of text, but can also contain snippets of multimedia such as sounds and images – and populates the request-URI (Uniform Resource Identifier) with the address of the recipient. The request is then routed via the IMS infrastructure similar to the manner used for an INVITE, until the immediate message finds its way to the UE of the recipient user.

There might, of course, be a reply to this message; in fact, a full dialogue of immediate messages back and forth between the two users is likely. However, in contrast to session-based messaging, the context of this session only exists in the minds of the participating users. There is no protocol session involved: each immediate message is an independent transaction and is not related to any previous requests.

If an immediate message is received while the IMS subscriber is offline, or in an unregistered state, the MESSAGE will route ...