When this occurs,
authentication is not required, because the subscriber has already registered
and has been authenticated already. If this parameter is set to NO, then the registrar
should challenge the subscriber device and authenticate the subscription.
This is implementation specific, as there are some service providers who may not
wish to use this capability. Indeed some service providers may wish to authenticate
each and every time, to ensure there are no registration hijacks, for example. The IMS
standard supports either method.
Once the device has connected to the network and has located the P-CSCF, it can send
a SIP REGISTER message. The REGISTER message will contain the domain name
of the subscriber (this identifies the home network that the subscription belongs to).
Establishing a Session in the IMS 111
The domain name is stored in the device, usually in the IP Multimedia Services Identity
Module (ISIM) as part of the user identity.
The ISIM is an application that runs on the device??™s Universal Integrated Circuit Card
(UICC), or what is more commonly known simply as the SIM card. The ISIM identifies
the subscriber by providing a private user identity and one or more public user identities.
These are used for the registration process and for authenticating the subscriber.
The ISIM also contains the authentication key and the cipher key.
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