As shown in Figure 2.2,
the MG is under the control of the MGCF, which is the same as the BGCF, with the
BGCF providing a gateway function into each individual network.
There will usually be multiple media gateways connecting to one single MGCF, providing
a very economical means for supporting voice services in an IP-based domain.
The media gateway also supports customer premises equipment by providing an IP
facility to the customer network, and connecting via IP PBX or some other VoIP entity
at the customer premises.
The media gateway is really the switching fabric in VoIP networks. The switching
fabric is what interconnects one circuit (or port) to another for phone calls and other
Moving from Legacy to Convergence 45
session types. The media gateway performs the task of connecting the various circuits
to one another for an IP network, under the control of the MGCF.
Once the signaling reaches the IMS domain, session control is managed by the IMS,
providing the necessary call control signaling back into the circuit-switched domain.
For example, a voice call may be originated within the circuit domain, passed to the MG
and associated MGCF, and routed into the IMS. The bearer path remains within the IP
network, under the control of the domain it is residing in.
If the session is terminated within the IMS domain, the SIP will terminate the call
using the BYE method.
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