This
provides a central location where the subscription can be controlled. If the subscriber
is roaming in another network, the barring of services still applies, since the network
they are roaming in will query the HSS via the S-CSCF to determine what services the
subscriber should be allowed.
One of the most critical functions of the HSS is to provide the encryption and authentication
keys for each subscription. When the subscriber device registers with the
network, the assigned S-CSCF challenges the device for the proper credentials stored
by the HSS. The S-CSCF queries the HSS to determine what the proper credentials
should be during registration. The subscriber device then returns a second REGISTER
message containing the proper credentials.
Only the network provider and the subscriber device know what these keys are, as
they are programmed into the device itself (more specifically, the SIM card of the device)
and the HSS. This is another reason for placing the HSS and S-CSCF functions
in the core network.
If you are familiar with GSM networks, then the HSS should be a familiar entity.
This is relatively the same as the HLR found in today??™s GSM networks. There are several
HSSs deployed within any one IMS, based on the number of subscribers that must
be supported within the network. Since this is where subscriber data is located, the
HSS must have the capacity to support the subscribers it will be assigned.
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