It became a contest
to see which vendor could sell their products based on proprietary versions of multiple
protocol standards in an effort to lock operators into their proprietary network model
(which means buying all of their network elements from one vendor).
xiv Introduction
This is what has led to the evolution of IMS. The IMS architecture defines one common
protocol standard to be used network-wide for all sessions within the wireless
network. This eliminates the multiple protocol nightmare that exists in the VoIP world
now. Operators no longer need to worry about purchasing network elements that support
the same version of H.248 as their media gateway controllers (MGCs). But it goes
further than that.
The 3GPP has identified specific functions that must be supported within their networks
to ensure security and to support authentication and the sharing of subscriber
data with roaming partners. They have added some new functions to support this important
aspect of the network: call session control (or the Call Session Control Function,
or CSCF). The CSCF is the element in the IMS that brings session control back to the
core where it can be managed and scaled.
There are many vendors who will promote the idea that the CSCF is best supported
at the edge of the network, residing on the access platforms such as media gateway
controllers (MGCs), but this model has already been tried.
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