The I-CSCF in the other network then has the responsibility of identifying the location
of the user being addressed (possibly through the Subscriber Locator Function).
The location means identifying the S-CSCF assigned to the user, as well as the HSS
where the subscription data is stored. The SLF can provide this information to the
I-CSCF. This becomes an important role within any IMS helping to secure access and
further protect the identities (routing addresses) of the S-CSCF and HSS resources.
This is somewhat different than the gateway function provided by the P-CSCF. The
P-CSCF acts as a gateway to non-IMS networks and the access point to reach the IMS,
while the I-CSCF acts as a gateway between two IMS networks.
Another important function of the I-CSCF is the assignment of the S-CSCF. The
S-CSCF is assigned according to capability or service provider policy. There are two
options available for implementation. One approach is to assign the S-CSCF according
to the services that need to be supported. For example, if a video conference is being
set up, the S-CSCF providing access to video resources is assigned. This approach may
be favorable to service providers looking to distribute media support throughout the
network, or in specific portions of the network.
14 Chapter 1
This model works well in data networks where services and platforms are located
according to the media to be supported.
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