In other words, if the operator is using someone
else??™s VoIP infrastructure for routing into the IMS domain, then the VoIP network will
access the IMS through the P-CSCF within the IMS network.
If the operator is using its own BGCF, then the signaling may be sent to a P-CSCF
within the same operator??™s network, or be sent to another operator??™s network acting
as an IMS domain. There are also some cases where operators may implement two
completely independent networks: one the legacy circuit-switched network and one
the IMS network. They will need the BGCF then for generating the Session Initiation
Protocol (SIP) signaling needed to communicate with the IMS, while still maintaining
a connection back in the circuit-switched domain.
Consider the BGCF as the call controller between both the circuit-switched network
and the IMS. The BGCF maintains the circuit-switched connection as well as the IP
connections, for as long as the session requires. When the session is complete, the BGCF
will then release the circuit-switched facilities by using the SS7 release procedures. It
does the same on the IP side using the SIP procedures. This means that the BGCF must
also be stateful, maintaining the status of every session under its control.
Once the BGCF creates the signaling using SIP, connection into the IMS is possible,
but there are two variations of SIP.
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