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An IMS Subscription stores the subscription data for an IMS user. This data is assumed to indicate the association of public and private identities and which Public User Identities within a subscription are shared and which are not. An IMS subscription supports multiple IMS users.

Identity Types

The

• IP Multimedia Private Identity (IMPI) is a unique permanently allocated global identity assigned by the home network operator, and is used, for example, for Registration, Authorization, Administration, and Accounting purposes. Every IMS user shall have one IMPI.
   
• IP Multimedia Public Identity (IMPU) is used by any user for requesting communications to other users (e.g. this might be included on a business card). There can be multiple IMPUs per IMPI. The IMPU can also be shared with another phone, so that both are reachable using the same identity (e.g. a single phone-number for an entire family).

Public identities can be grouped together to form an implicit registration set. The public identities in this set get implicitly registered via single IMS registration of one of the Public User Identity's in that set. No single public identity shall be considered as a master to the other Public User Identities. A simple diagram of implicit registration of Public User Identities is depicted below.

Public User Identities may be shared among multiple Private User Identities within the same IMS subscription. Hence a Public User Identity may be simultaneously registered from multiple UEs that use different Private User Identities and different contact addresses.

The following relationships are possible between Public User Identities and Private User Identities in IMS deployments:

• Single Private Identity Bound to Multiple PUIDs in Implicit Set
• Single Private Identity Bound to Implicit Set Plus Additional PUID 
• Multiple Private Identities Bound to Single PUID
• Multiple Private Identities Bound to Implicit Set of Multiple PUIDs

Private Identity Interrelationships to Public User Identities in IMS Deployments

Single Private Identity Bound to Multiple PUIDs in Implicit Set

Single Private Identity Bound to Implicit Set Plus Additional PUID

Multiple Private Identities Bound to Single PUID

Multiple Private Identities Bound to Implicit Set of Multiple PUIDs