While the AS recognizes the REGISTER method, it does not process
the message other than by recording the status; the S-CSCF is the SIP registrar
within the IMS domain.
Media Gateway Control Function (MGCF) This function is mentioned here only because
it does provide access into the IP/SIP domain, although it is not really considered part
of the IMS (see Figure 1.7). The MGCF connects into the Public Switched Telephone
Network (PSTN) domain, providing a gateway function between the SS7 world and the
IP/SIP world.
When a call originates in the SS7 side and terminates in the SIP domain, the SS7
messaging (ISUP) is sent to the MGCF, which is responsible for mapping the SS7 message
(or BICC) into a SIP request prior to forwarding on to the P-CSCF.
Remember that the P-CSCF does not support SS7; it is only a SIP device, therefore
there must be a function that provides the conversion between networks. This is one
means of providing this, while at the same time controlling the bearer path.
Figure 1.6 The Application Server (AS) within the IMS
P-CSCF I-CSCF S-CSCF
HSS AS
20 Chapter 1
The bearer path consists of a trunk that terminates into a media gateway, under the
control of the MGCF. The trunking is controlled per the ISUP messaging and passed
via a media gateway to the IP network under the control of SIP. This would be common
in networks with well-established VoIP networks.
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