Again it is this concept of using user identities in the form of URIs that helps support
mobility in the IMS. All that is needed is a subscribers identity, and calls (or e-mails, or
text messaging, or any other form of communications) can be sent to the subscriber no
matter what IP address that subscriber has been assigned. Only the network needs to
know the IP address for the subscriber at any point in time.
It should be noted here that there is no DNS query for subscriber identities. The URI
for a subscriber is resolved to an IP address by the S-CSCF and the HSS functions.
Addressing in the IMS 101
The IP address is updated anytime the device changes connections and re-registers
with the network. This would be the case for a mobile subscriber who is roaming in
other networks (or changes cell sites within his or her own home network).
The registration process is where the IP address is ???bound??? to the public and private
user identities. The concept was stolen both from the mobile industry, where the concept
of registration is used with cell phones accessing the network, and the Internet,
where dynamic IP addressing is commonly used.
Only one public user identity can be stored on the device. Other public user identities
are stored in the service provider??™s HSS and are shared with the device during registration.
This allows the device to then store the other associated public user identities in
memory (but not on the UICC).
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