In other words, if a service provider has both an IMS and a legacy
network, the MGCF interfaces between the two network domains, but not to other
service provider networks.
Breakout Gateway Control Function (BGCF) The BGCF works like the MGCF but is
a gateway to other providers??™ networks (see Figure 1.8). The MGCF interfaces to the
Figure 1.7 Location of the MGCF
P-CSCF I-CSCF S-CSCF
HSS AS SLF
MGCF
Architecture of the IMS 21
SS7 network within its own domain, but when it needs to connect to another service
provider network, it interfaces through the BGCF.
The BGCF provides a gateway function between networks. For security reasons, it
should only interface with other BGCFs in other networks. The BGCF will interface
directly with the S-CSCF within its own network, so that when a request or response
is to be forwarded to another network, the S-CSCF can route the message directly to
the BGCF for routing to the other network.
The S-CSCF routes to the BGCF according to the TEL URI within the request or response.
Routing tables within the S-CSCF identify the proper BGCF according to TEL
URI. When routing a session into the PSTN, the S-CSCF has no knowledge of routing
within the PSTN. It relies on the service of the BGCF to route into the PSTN.
Media Resource Function (MRF) There are two functions within the MRF; the media resource
function controller (MRFC) and the media resource function processor (MRFP).
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