For this reason the REGISTER messages also provide credentials that are embedded
in the device itself. Only the network provider and the device know these credentials. To
make service even more portable, a Universal Integrated Circuit Card (UICC) is used to
store the credentials and is inserted into the device by the subscriber. This allows subscribers
to use devices that they purchase from any store, without giving services away.
The credentials are provided using the REGISTER message only after the registrar
rejects the first REGISTER message. This means that the REGISTER message content
will not always be the same. In some cases the device only needs to update its location,
while in other cases the device needs to register when it is activated. The procedures
for registration are detailed in Chapter 5.
Subscribe SUBSCRIBE is used by application servers to request updates from the
S-CSCF and the HSS whenever a subscriber??™s registration changes. For example, a
Presence server needs to know anytime a subscriber changes locations or activates
another device in the network. The application server hosting the Presence application
will send a SUBSCRIBE to the S-CSCF to notify it that it would like notification
anytime a subscriber changes his or her registration.
The S-CSCF then uses the NOTIFY method to let the application server know of the
changed registration, accompanied by the registration changes.
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