Signalling in EPC / LTE
Figure 2-23 Service request (CSFB call rejected by the user)
Service abort procedure This procedure can be invoked by the VLR to abort a mobile terminating CS fallback call during call establishment. The procedure applies to UEs that are simultaneously attached for EPS services and non-EPS services, but not to UEs attached for EPS services and SMS only. MSC
MME Paging / CS service notification
server
PAGING-REQUEST SERVICE-ABORT-REQUEST
REL
Extended Service Request Service Reject (Cause = CS service temporarily not available)
Figure 2-24 Service abort procedure If the VLR decides to abort a MT CSFB call for which it has sent an SGsAP-PAGING-REQUEST message to the MME, and the VLR has not received an SCCP connection establishment containing the Initial L3 message from the UE via the A or Iu interface, the VLR shall send the SGsAPSERVICE-ABORT-REQUEST message to the MME. When the MME receives the SGsAP-SERVICE-ABORT-REQUEST message from the VLR, the MME shall set the Call Cancelled Flag to "true". If the MME receives an NAS EMM EXTENDED SERVICE REQUEST message from the UE with Service type set to "MT CSFB" and CSFB response set to "CSFB accepted by the UE" and the Call Cancelled Flag is set to "true", the MME shall set the Call Cancelled Flag to "false" and will reject the CS fallback call [5, 3GPP 24.301]. When the MME receives the SGsAP-SERVICE-ABORT-REQUEST message after the UE has accepted the CS fallback call, the MME shall discard the SGsAP-SERVICE-ABORT-REQUEST message. 58