The MGCF acts as a SIP user agent in the IMS
environment, and as an SSP in the SS7 environment.
The MGCF communicates with other switches in the same network using SS7 on the
legacy side, but when the MGCF must communicate with switches in another network,
it does so through the Breakout Gateway Control Function (BGCF). Think of the BGCF
as the gateway into a network, providing a firewall function from one network to another.
The MGCF then manages communications within one network domain interfacing
between the SS7 and the SIP domains in one network domain. This is illustrated
in Figure 5.2.
Figure 5.2 simplifies the relationship between the BGCF and the S-CSCF between
networks by eliminating all of the other network elements. Chapter 1 provides additional
information about the role of the BGCF and the MGCF.
When the S-CSCF receives an INVITE for a subscriber outside of the IMS, the SCSCF
forwards the INVITE to the BGCF. It is then up to the BGCF to determine how
to route the INVITE into the PSTN. If the INVITE is addressed to a subscriber within
the same network, then the BGCF will route the message to the MGCF for connecting
120 Chapter 5
into the SS7 domain. However, if the subscriber is in another network, the BGCF will
then forward the message to the BGCF within that network. The S-CSCF uses the TEL
URI to determine which BGCF should receive the INVITE.
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