In the strictest sense the application servers (AS) are not part of the IMS. They function in the application plane that rides on top of the IMS. However, since the application servers provide value-added services to the IMS their functions are briefly described in the IMS specification.
An AS can reside in the user’s home network, the user’s visited network, or a third-party network. An AS in the user’s visited network functions in the same manner as if it were in a third-party location, which can be as a stand-alone server or as part of a larger third-party network. This layout envisions that both IMS providers and stand-alone service providers would deliver services to the IMS user. In a similar manner, Amazon.com has a series of stand-alone servers from which users buy products.
The main functions of the application server are listed below.
- The ability to process and impact an incoming SIP session received from the IMS. The specification says that the AS can impact the nature of the session even though the SIP session is under the control of the S-CSCF. In its simplest form, the AS could deny the session or change some attribute of the session.
- The capability to originate SIP requests. That is, the AS can have a peer-to-peer dialog with the S-CSCF and if warranted by the request, the S-CSCF could forward the request all the way back to the UE.
Since the IMS originated in the mobile community, the application server is not limited to purely SIP-based services. As a mobile operator, access can be offered to services based on the Customized Applications for Mobile network Enhanced Logic (CAMEL) Service Environment (CSE) or the Open Service Architecture (OSA) for IMS subscribers. This means the IMS AS also refers to the OSA Service Capability Server (SCS) and the CAMEL IP Multimedia Service Switching Function (IM-SSF) as well the traditional SIP AS.
Briefly, the OSA allows an operator to create an IMS-like environment for its subscriber base. Perhaps more important is that the OSA allows for the creation of a secure environment to offer third-party services to the IMS subscriber base. The CAMEL service environment has been around for some time and its inclusion in the IMS is purely for support of legacy applications. Both of these areas are beyond the scope of our studies of the IMS; just understand that the IMS is a general purpose specification.
One last comment about the AS concerns the number of application servers that might be in session with the UE. In the simplest form, there is one service per AS. However, since the SIP session runs over the dynamic environment of the IP network, a subscriber can have many services in one session. Furthermore, a service might actually run on multiple applications servers. For example, an operator might use one AS to control the traffic flow based on session capabilities and another to provide the actual service to the subscriber.