The SIP (header) P-ASSOCIATED-IDENTITY provides
this information to the device.
Figure 4.3 illustrates the relationship between the private user identity and the public
user identity. Notice that there can be multiple service profiles associated with one
private user identity. Likewise there can be multiple public user identities associated
with one private user identity.
A good example of how a subscriber can have multiple identities is when a subscriber has
multiple devices. Typically, subscribers have a cell phone, an e-mail account, a home telephone
number, a Blackberry, a personal e-mail account, and possibly another set of identities
used for hobbies or other interests. Each one of these uses a public user identity.
I may wish to have all of my business calls directed to my desk phone at work (using
a public user identity consisting of my work phone number). At the same time, I may
want to have family and friends directed to my personal cell phone. This is also identified
by a unique public user identity.
In the IMS, a subscriber will have the ability to direct calls to different devices depending
on who is calling, and when they are calling. This is accomplished through the
use of a Presence server, responsible for directing certain calls to certain devices according
to preset conditions (such as who is calling and time of day, for example).
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