The IMS: IP Multimedia Concepts And Services, Second Edition

3.7. Sharing a single user identity between multiple devices

Traditionally, in the CS every single user has her own Mobile Station International ISDN (MSISDN) number that is used to reach the user. It is not possible for a single user to use multiple terminals with the same MSISDN number simultaneously. Having two mobile stations with identical MSISDN numbers would cause significant conflicts in the network. Nowadays, users may have more than one item of UE with totally different capabilities: big/small screen, camera/no camera, full keyboard and so forth. Different UE may serve different purposes (e.g., one for gaming, another for ordinary voice and video sessions). From the user's point of view, the user should be reachable via the same identity regardless of the number of UEs that she is using simultaneously. The IMS makes this feature possible.

Release 6 IMS allows users to register the same public user identity from a number of items of UE. In addition, a user is able to indicate her preferences regarding a single UE at the registration phase. Different registrations can be differentiated by means of the private user identity and the used IP address. Figure 3.6 shows an example in which a user has two items of UE: one for video sessions and another for chat and gaming applications. When someone is calling the user – say, Joe – it is his S-CSCF that makes the decision as to which UE is going to be contacted in the first place. This decision can be done based on the preferences ...