Cisco Cisco ASR 5000
ISR with Circuit Switched Fallback
Call Flows ▀
SGSN Administration Guide, StarOS Release 18 ▄
Call Flows
To support CS fallback, existing procedures are modified and some additional CS fallback specific procedures added to
the EPS. Additions are done to the "Attach" and "TA update" procedures which activate an interface called the SGs.
This interface is between the MME and MSC. It is used by the MSC to send paging messages for CS calls to the UE on
the LTE system.
the EPS. Additions are done to the "Attach" and "TA update" procedures which activate an interface called the SGs.
This interface is between the MME and MSC. It is used by the MSC to send paging messages for CS calls to the UE on
the LTE system.
Example of a CS fallback call
Figure 48. CS Fallback Call
Table 21. Steps in a CS fallback call
Step Description
1.
The MSC receives an incoming voice call and sends a CS page to the MME over a SGs interface.
2.
The MME uses the TMSI (or IMSI) received from the MSC to find the S-TMSI (which is used as the paging address on the
LTE radio interface).
LTE radio interface).
3.
The MME forwards the paging request to the eNodeB in the TAs where the UE is registered. The eNodeBs perform the
paging procedures in all the cells in the indicated TAs.
paging procedures in all the cells in the indicated TAs.
4.
The paging message includes a special CS indicator that informs the UE that the incoming paging is for a terminating CS
call.
call.