The MGCF provides the SIP signaling
needed by the IMS network to be able to set up a session within the IMS domain
and control that session. The BGCF in turn connects into the P-CSCF for the serving
IMS network, at which point the call is now under the control of the IMS network (at
least the portion of the call within the IMS domain).
The Media Gateway (MG) We have so far talked about the signaling traffic, but the
bearer traffic must be routed as well. It is important to understand that the IMS itself
is really about call control, and not bearer traffic. In other words, to control the transmission
of voice, data, messaging, video, and any other media we can think of through
an IP network, while maintaining security and preventing unauthorized access, there
needs to be a set of functions where this can be accomplished. It has already been
proven through decades of network experience that the best means for maintaining
session control in any network is through a central core function, which is what the
IMS was designed around.
The bearer traffic in this scenario is coming from a circuit switch which only connects
via non-IP facilities. This bearer traffic must then be converted to an IP facility. This
is accomplished by sending the bearer traffic to a media gateway (MG). The MG then
connects to the legacy facility on one side, and IP on the other.
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