The access network therefore is any wireline, wireless, or VoIP network. At some
point there is an entry point where IMS procedures begin, as outlined in the following
sections. The roles of each of the entities defined in the IMS model are provided in the
sections that follow.
Call Session Control Function (CSCF)
The heart and soul of the IMS lies within this function. Call session control is the
primary function of the network core. The function of call session control is actually
distributed throughout the network to make the network more efficient and scalable,
rather than trying to support this function through one huge centralized platform.
At the same time, one probably does not want to distribute this function at every
switch. This then becomes unmanageable, and as the network evolves and grows, it
becomes unmanageable as well. Cost is what prevented applications from being implemented
at the switch level, and it is cost and the inability to manage and maintain very
large IP networks at the switch level.
There is no rule that the various call session control functions must be distributed.
Indeed there will be some implementations (especially in the early deployments) where
this will be the case. There will probably be many instances where all three CSCFs will
be implemented in the same network entity. But when the network begins to expand
and grow, it begins to make more sense to separate the various control functions into
separate, discrete entities.
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