3GPP TS 24.228 -- IMS Signalling flows for session initiation: Non Hiding
MO#2 / S-S#3 / CS-T (page 3 of 3)
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Mobile Origination, located in home network
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PSTN Termination performed by Home Network of Originator
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CS Network Termination
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SS7: COT/IAM:
Based on the continuity support of the outgoing channel selected, MGCF sends a IAM or COT message
to the CS Networks. In case the outgoing channel supports continuity indication, MGCF has already
sent out the IAM message, and at this point sends out a COT message.
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SS7: ACM/CPG:
The CS Networks establishes the path to the destination. In the present case the CS Networks responds
with an ACM message containing a "subscriber free" indication, implying that the called party is being
alerted.
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If the CS Network is alerting the destination user, MGCF indicates this to the calling party by a 180
Ringing provisional response.
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As the indication of called party being alerted ("subscriber free" indication) may not be available in
ACM, the 180 Ringing is only sent when the indication is available. An ACM without the "subscriber
free" indication will not trigger any SIP message.
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