The signaling gateway does not concern
itself with these upper-layer protocols, leaving that responsibility to the BGCF/MGCF
instead. So the operator can connect its existing SS7 network into the signaling gateway
44 Chapter 2
using IP as the transport behind it and begin routing all their SS7 signaling over the
IP network.
So now that we have identified all of the pieces needed to connect from the circuitswitched
domain, we can talk about the flow of traffic between the networks. The call is
established within the circuit switch and routed through trunking facilities to a media
gateway connecting into the circuit-switched domain. The MG is controlled by a MGCF,
which means the signaling comes from the MGCF.
The MGCF then interfaces to the circuit switch for signaling via SS7. The MGCF
may support circuit-switched facilities for interfacing into SS7, or there may be a signaling
gateway function to convert the facility to IP (using SIGTRAN) prior to connecting
into the MGCF.
The SS7 message is then used to determine what the bearer traffic is, what is required
at the destination to access the bearer traffic, and the final destination for the
session. The MGCF then creates a SIP message for routing into the IMS. The bearer
traffic continues through media gateways to the final destination.
The MGCF interfaces into the IMS via a BGCF (the same function, but the BGCF
acts as a signaling gateway into another network).
Pages:
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121