6). The AS speaks SIP, as well
as DIAMETER, and interfaces directly with the various CSCFs within the network.
There are many uses for the AS, such as applications, service delivery, content delivery,
presence server, and even video conferencing. The AS can create a SIP dialog, which
means it can behave as a SIP user agent (UA). There are scenarios (depending on the
service being provided) where an AS may even communicate with other Application
Servers. For example, the AS can act as a redirect server generating requests and sending
responses to other SIP entities within the IMS.
The services provided on an AS are identified by service identifiers. Think of these as
addresses for the services. These in turn correspond to entries within the HSS where
users are registered. Each subscription is defined with all possible user identities and
with the service identifiers associated with each of the user identities. This is what
allows the S-CSCF to route service requests to the proper AS. The identifier is communicated
as part of the INVITE request.
If an AS has subscribed to event notification (via the SIP SUBSCRIBE method), the
S-CSCF will send the REGISTER to all of the Application Servers that have subscribed
to notification. This allows the servers to stay apprised of the users status at all times.
The AS does not generate a REGISTER message, however, as it does not need to register
with the S-CSCF.
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