This allows subscribers to purchase their phones from any source
and use the UICC to obtain access to the network of choice. This will be an important
concept in the IMS, as subscribers will be using a multitude of devices from many different
sources and will expect to be able to use these devices on any network they pay
to have access on.
This concept is what drives services of many different forms, because subscribers are
much more likely to purchase different devices from sources other than their home network
provider and use these devices to listen to music or play online games. Operators
will need to embrace the concept of the UICC and SIM application if they want their
services to be popular and widely used.
On the UICC is an application referred to as the IMS Subscriber Module (ISIM). This
is like the SIM card used in GSM phones today. Actually, the little circuit card itself is
the UICC, while the application residing on the card is the SIM, but the industry commonly
refers to the module as the SIM card.
Also stored on the ISIM is the subscriber??™s private user identity, also known only to
the network. When the subscriber device accesses the network and registers its location,
the S-CSCF and the HSS work together to ???challenge??? the subscriber device. This
challenge forces the device to return another registration with the correct credentials
based on data provided within the challenge itself (the 401 Unauthorized response
is the challenge), and the authentication keys embedded within the ISIM in the device.
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