The purpose of the VoIP gateways is to convert not only the bearer path to packet,
but also the signaling required for routing the bearer traffic through the IP domain.
VoIP uses a variety of signaling methods that are not always fully compatible with SS7,
so breakout media gateway controller functions (BGCF) may be needed to convert this
signaling. The purpose of the BGCF is to accept the SS7 signaling and use that signaling
to determine how the bearer traffic is to be routed through the IP domain.
The BGCF does not necessarily ???convert??? in purest terms; it would be more accurate
to say that the BGCF reads the SS7 signaling and uses the parameters within the
SS7 signaling to determine where the bearer traffic needs to be sent. It is not converted,
because most of the parameters are relevant to the circuit-switched facilities
and do not apply to the IP domain. There are no echo cancellers, for example, within the
Moving from Legacy to Convergence 43
IP domain, so the parameters identifying these requirements are not needed within
the IP signaling.
The BGCF will create new signaling for the bearer traffic within the IP domain,
based on the destination of the bearer traffic and the attributes of the media themselves,
but without regard to how it was received from the circuit-switched domain.
This signaling then gets sent to the IMS access point, dependent on the architecture of
the IP domain transiting the traffic.
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