In wireline networks it has
been only recently that operators have begun widespread expansion of their SS7 networks
to support Number Portability applications.
10 Chapter 1
So considering what we have seen in this industry historically, we can safely say that
IMS implementations have begun already, but it will be a slow implementation. Indeed
operators and vendors alike are estimating IMS will take many years to fully mature
and replace the existing SS7 infrastructure.
This means that the legacy networks and the IMS networks will have to interoperate
with one another. There are a number of functions that have been defined for interoperating
between the legacy network and the IMS network, which are explained in more
detail in Chapter 2.
Entities in an IMS Core Network
As stated already, the main focus of the IMS is session control. When one looks carefully
at the 3GPP standards for IMS, you will not see any switching functions as part
of the architecture. Switches, and network nodes used for access, are not part of the
IMS model. They are used simply to access the IMS, but the IMS does not care how a
subscriber enters the network.
To the IMS, the access network is transparent. There are gateway functions that
convert the legacy signaling (mostly SS7) to SIP prior to interfacing to the IMS. This
is important to understand, because as one begins to look at possible implementation
solutions, it can become confusing as to how the architecture is different than what we
have today in standard legacy switch-based networks.
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