However, they are lacking in the control and security area, which is why IMS
has become so important to so many large operators.
The VoIP domain, then, still must have a connection into the IMS, which is through
the BGCF/MGCF. If the operator owns both a VoIP network and an IMS network, then
it can pass calls/sessions between the two domains using the MGCF. However, if the
VoIP provider is not the same as the IMS provider, then the BGCF is used as an interconnect
part. As you will learn in later chapters, there are specific functions provided
by the BGCF that differentiate it from the MGCF.
Once the signaling reaches the BGCF, it must be routed to the P-CSCF within the
area (or whatever means the operator chooses for P-CSCF assignment). Remember
that all sessions must be routed through the P-CSCF first, which then routes to the
I-CSCF, which is then responsible for routing to the S-CSCF.
VoIP only supports voice and data, and therefore there must be other interfaces to
support other media types, for example, the delivery of messaging in a wireless network
and the support for video over IP (IPTV). These require other network elements and
a separate means for delivery. This is because, even though the network itself cannot
support the media types, the network elements are designed for voice only.
Support of these media types involves overlay networks, such as GPRS in the GSM
network.
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