This is accomplished through the use of security
???keys??? that are programmed into the subscriber devices and shared with the network
when a subscriber activates their device.
This is analogous with how GSM networks exchange authentication data between
handsets and the HLR today. However, this mechanism also works within a wireline
model as well, as long as the device has some fashion of ISIM application that is inserted
into the device providing the authentication and subscription identifiers. IMS
defines the use of UICC within each device hosting the ISIM application.
When a subscriber purchases a device from his or her service provider, the device is
already equipped with a UICC hosting the ISIM application. The operator has therefore
already provisioned the ISIM with the appropriate authentication data.
If the subscriber purchases a device from someplace else, that subscriber still has to
have the proper UICC and ISIM application from his or her home service provider. This
is the only way for authentication to work properly.
156 Chapter 6
The service provider??™s Home Subscriber Server (HSS) is where security keys for each
subscriber are kept. When a subscriber activates his or her device and the registration
process begins, the S-CSCF assigned to the subscriber queries the HSS for the security
data. The S-CSCF will then ???challenge??? the subscriber device for this information.
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