MEGACO and SIP Interworking

Imagine the case where the
two MGs of Figure 8-13 happen to be controlled by separate MGCs. In that
case, a protocol needs to be used between the two MGCs. The obvious choice for that protocol is SIP. Once a local descriptor is available at the gateway
where the call is being originated, this can be passed in a SIP INVITE as
a SIP message body. Upon acceptance of the call at the far end, the corresponding
session description is carried back as a SIP message body within
the SIP 200 (OK) response and is passed to the originating side gateway.
Once the gateway on the originating side has acknowledged receipt of the
remote session description, then the MGC can send a SIP ACK to complete
the SIP call setup.