1 includes the public
user identity, the private user identity, and the P-CSCF network identifier. This information
is then stored in the HSS as part of the subscribers profile.
When the HSS receives this information, it must first determine if the subscriber is
already registered through another S-CSCF. If the subscriber is not registered, the HSS
checks the service profile to verify the subscriber is allowed to roam in the network
identified by the P-CSCF domain name.
The HSS then returns a response back to the I-CSCF containing the name of the
S-CSCF if previously registered. If not, then the HSS returns the capabilities for the
subscriber. This is an indication to the I-CSCF that it must assign the S-CSCF. The ICSCF
then forwards the REGISTER to the newly assigned S-CSCF.
Notice in Figure 5.1 that the S-CSCF and the HSS communicate once the REGISTER
has been received by the S-CSCF. This is so the S-CSCF can obtain any additional addresses
registered for the subscriber, as well as any security information. The HSS
stores the S-CSCF address for future queries from other network entities (such as during
session establishment). The S-CSCF may also identify any networks the subscriber
may be roaming in (identified in the P-VISITED-NETWORK header).
Once a subscriber has registered with the network, they must update their registration
prior to its expiration.
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