Next Generation Wireless Applications: Creating Mobile Applications in a Web 2.0 and Mobile 2.0 World, 2nd Edition

14.10 IMS SERVICE CONCEPT

The IMS architecture envisions three layers, as shown in Figure 14.13. For most of this chapter, we have been discussing the session layer, which is the essential core of the IMS network – the SIP and CSCF servers and all the bells and whistles for billing, QoS, security and resource management. Then there's the access layer, which is mostly (and increasingly) outside the scope of IMS. Telephony sits within the session layer, including the required gateways to non-IMS networks, but we have already indicated that IMS is much more than just telephony. What remains is the applications layer.

The core concept of the applications layer is the Application Server (AS). SIP, as previously discussed, is a ‘rendezvous technology’, allowing companion applications to connect. The concept of an AS is that it sits in the SIP dialogue pathway and either involves itself in a dialogue directly with a SIP user agent or intercedes in a dialogue between two other agents. A classic example would be a voicemail AS. The SIP call is eventually routed via an S-CSCF, which attempts to forward the SIP Invite message to the called party's SIP device. However, if the receiving party doesn't answer, then the S-CSCF can route the SIP invite to a SIP-capable voicemail server sitting on the ISC interface.

The voicemail server accepts the SIP invite on behalf of the user (using the user's SIP URI address to associate the call with the appropriate mailbox⁷) and negotiates a media connection ...