This is then forwarded back into the circuit-switched network
via the BGCF, which creates an SS7 REL message back to the switch.
The easiest way to relate to this is to remember that signaling is used to communicate
from one network element to another regarding the facilities that are needed to
form a connection end-to-end. If the facility is IP, then MGCF is needed outside of the
IMS, and CSCF is used within the IMS. We will discuss more about the CSCF functions
in later chapters when we discuss the inner workings of the IMS network itself. For
now, know that the CSCF provides session control within the IMS domain.
The MG therefore becomes an important element when converting voice (or other
non-IP media) into packetized media for routing through a packet network. But what
if you are using a SIP phone within an IP network? The SIP phone acts as the SIP user
agent, and while the MGCF is still used for call control within the VoIP domain, this
can later be eliminated and replaced with the CSCF within the IMS.
The long-term goal is to have all SIP-enabled devices, eliminating the need for media
gateways and other devices to packetize various non-IP media. Once a phone or other
device is capable of packetizing the media and creating SIP signaling, the CSCF is all
that is needed for session control.
The Media Resource Function (MRF) In the circuit-switched domain, service tones and
recordings are all provided via the switch and other external elements (such as voice
response units).
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