Editing IP Multimedia Subsystem (section)

==== Application servers ====
{{see also|Next generation network services}}
SIP [[Application server]]s (AS) host and execute [[Service (systems architecture)|services]], and interface with the S-CSCF using SIP. An example of an application server that is being developed in 3GPP is the [[Voice call continuity]] Function (VCC Server). Depending on the actual service, the AS can operate in SIP proxy mode, SIP UA ([[user agent]]) mode or SIP [[B2BUA]] mode. An AS can be located in the home network or in an external third-party network. If located in the home network, it can query the HSS with the Diameter Sh or Si interfaces (for a SIP-AS).
* SIP AS: Host and execute IMS specific services
* ''IP Multimedia Service Switching Function'' (IM-SSF): Interfaces SIP to [[Camel Application Part|CAP]] to communicate with [[Customized Applications for Mobile networks Enhanced Logic|CAMEL]] Application Servers
* [[Open Services Access|OSA]] service capability server (OSA SCS): Interfaces SIP to the OSA framework;

===== Functional model =====
The AS-ILCM (Application Server - Incoming Leg Control Model) and AS-OLCM (Application Server - Outgoing Leg Control Model) store transaction state, and may optionally store session state depending on the specific service being executed. 
The AS-ILCM interfaces to the S-CSCF (ILCM) for an incoming leg and the AS-OLCM interfaces to the S-CSCF (OLCM) for an outgoing leg.
Application Logic provides the service(s) and interacts between the AS-ILCM and AS-OLCM.

===== Public Service Identity =====
Public Service Identities (PSI) are identities that identify services, which are hosted by application servers. As user identities, PSI takes the form of either a SIP or Tel URI. PSIs are stored in the HSS either as a distinct PSI or as a wildcarded PSI:
* a distinct PSI contains the PSI that is used in routing
* a wildcarded PSI represents a collection of PSIs.