Converting addresses may also be a role of the S-CSCF (as well as the P-CSCF and
the I-CSCF). Since SIP routes are based on SIP URIs, any TEL URIs must be translated
or converted to a SIP URI. The same is true in the opposite direction when routing
from the IMS to the PSTN. The S-CSCF is responsible for accessing an ENUM/DNS application
for converting these addresses into a SIP URI prior to forwarding the request
(or response) to its destination.
The ENUM/DNS application can be colocated within the same application server, or
it may be a stand-alone ENUM function. The purpose is to enable routing between the
legacy networks where E.164 numbers are still used to reach subscribers, while still
supporting the concept of the SIP URI.
S-CSCF operates as a stateful proxy and must always maintain the state of all registrations
and sessions under its control. Since the S-CSCF knows the state of each
session and registration, it is also capable of updating other entities that may have
subscribed to event notification using the SIP SUBSCRIBE method. Whenever the
state of a subscriber device changes within the network, the S-CSCF notifies all entities
involved of the state change.
This allows services such as presence to be notified if the status of a subscriber??™s
registration should change, while also preventing unauthorized access to the registration
status.
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