Home Subscriber Server (HSS) While the S-CSCF acts as the core of the network, the
HSS (shown in Figure 1.5) serves as the central source of subscriber data. The HSS
stores user data such as the services a subscriber is allowed to access, the various identities
(the private user identity and all public user identities), the networks the subscriber
is allowed to roam to (in the case of wireless), and the location of the subscriber device.
When a subscriber registers with the network, the S-CSCF accesses the HSS to
obtain the user profile. The user profile is what identifies all of the private and public
identities associated with the subscription, as well as the service profiles for each of
the identities.
Whenever there is a change in the subscription of a subscriber device, the information
is pushed to the S-CSCF. The HSS will send all of the subscription data to the
S-CSCF, not partial data. This eliminates the possibility of data becoming corrupt or
out of synch with the HSS. The S-CSCF then replaces all subscription data it has with
the data the HSS sends.
The purpose of the registration is to provide a location for a subscriber device.
Location does not necessarily mean the exact location. In the case of wireless, the
location can be the GPS coordinates, but it usually means the cell site identifier. In
wireline networks, the location depends on where the P-CSCF that was used to access
the network is located.
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