The S-CSCF that supports the session requirements and is available is then
assigned to the subscriber. When the subscriber changes locations and registers again,
that subscriber may very well be assigned to a different S-CSCF. Other criteria can be
used by operators for assignment of the S-CSCF.
Some operators may choose to assign the S-CSCF by geography (much as telephone
networks are designed today). If this is the case, then all S-CSCFs within a network
domain will support the same capabilities and be assigned according to a subscriber??™s
location during registration.
This is the more likely scenario for traditional operators, since their network resources
are already deployed according to geography. This also makes it simpler to
maintain the network in the long run.
Once the S-CSCF identity has been established, the REGISTER message is forwarded
to the S-CSCF for registration. The S-CSCF stores the address of the P-CSCF so that
it knows how to send any responses back to the subscriber. The S-CSCF will first challenge
the subscriber by returning a response 401 Unauthorized. The S-CSCF will then
query the HSS for authentication data. The S-CSCF only challenges a subscriber twice.
Likewise, a user device will only respond to the 401 Unauthorized response twice.
The HSS stores both a cipher key and an authentication key. Both of these keys are
used with specific algorithms to calculate the subscriber credentials and to identify how
to decipher encrypted messages.
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