Sunday, 15 November 2015

eNodeB-Interpretation of the LTE SRVCC Basic Service Workflow

Currently, the VoLTE relevant functions will be enabled in some areas soon. Apart from some basic services such as voice calls, the LTE network employed SRVCC to solve the problems of voice control and voice continuity during a switch to the CS domain. This case intends to briefly interpret the basic SRVCC service workflow.

 




I. Overally speaking,
1. The following items are supported:
a. SRVCC;
b. SRVCC carriage capability (indicating the MME) in the “Attach Request message” at the NAS layer and in the IE of the "MS Network Capability" in the “Tracking Area Updates” message during the UE ATTACH process or the TAU process.
c. MME’s storage of the UE’s SRVCC capability indication for dealing with the SRVCC process.
UEs equipped with the SRVCC function are supported. UEs need to be equipped with the SRVCC function during the Attach process and TAU process. Additionally, the SRVCC function should also be supported when the UE communicates with the MME through "MS Network Capability" IE of the “Attach Request message” and “Tracking Area Updates” messages on the NAS layer. The MME also needs to store the SRVCC function of the UE for SRVCC progress processing.
 
2. The following items are supported:
a. SRVCC;
b. The UE request service equipped with the SRVCC function is supported. During the MME request process, "SRVCC Operation Possible" IE is carried in the “Initial Context Setup Request” message of S1-AP so as to notify E-UTRAH. Both the UE and the MME need to support the SRVCC capability.
3. The UE reports the measurement report.
The eNodeB determines whether to trigger the SRVCC process according to the UE measurement report.
II. Detailed Analysis:
1 . The eNodeB receives the measurement report of the UE.
2. The eNodeB judges according to the measurement report. If the VoIP service (QCI=1) is already established by the UE and that the target 2G/3G GERAN/UTRAN cell does not support the VoIP capability, the SRVCC process can be triggered. To do this, send the handover request carrying the “whether to perform the handover of both the PS domain and the CS domain” to the MME.
3. The MME communicates with the MSC Server through the Sv port requesting the PS to CS handover of the VoIP service.
4. The MSC Server exchanges signaling with the MSC so as to finish the handover resource preparation of the CS domain.
5. The MSC Server communicates with the IMS domain so as to finish the conversation transfer process of the IMS service.
6. The MSC Server sends to the MME the PS to CS handover response message carrying the CS HO command for the UE to be handed over to GERAN/UTRAN.
7. The MME synchronizes the PS to CS handover and the PS to PS handover.
8. The MME demands that the eNodeB finish the handover preparation through the handover commands.
9. The eNodeB demands that the UE be handed over from E-UTRAN to the target GERAN/UTRAN .
10. The UE accesses the target cell. The VoIP service is switched from the PS domain to the CS domain.
III. Note:
1. During the SRVCC processing process, The MME is able to trigger PS HO, deactivate (the GBR service) or suspend (the NGBR service) for the established non-voice services according to the network type, UE capability, and service type.
2. If the UE returns to the LTE network when the CS domain voice service ends, the UE will notify the MME through the TAU process. The MME checks whether services are suspended so as to resume the suspended services of the UE.

Budi Prasetyo

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