LTE: Which network node converts PS call to CS call in the LTE network ?

Question

If UE doesn't have any PS session and any CSFB call initiated then UE redirected to WCDMA or GSM and after ending CS session UE moved to LTE network by performing Tracking area update.

But in case PS session is on going and CS call have to be initiated then Handover to WCDMA will be performed. But when CS call finished and PS session is also still ongoing then which procedure will be performed to move again LTE Network? If its Handover then how measurement reporting will be done.

Answer 1

Hi,

For CS Fallback, MME will convert PS to CS switching.

The flow goes like following :
MO cal :

UE sends ESR to mme, mme gives the switchover command to ENB, ENB make UE to other technology fallback(handover). Then the call continous in other tech (3G or 2G).After call finishes it is upper layer job to reselect the ENB.

Terminated call :

MSC send the paging request to MME via SG interface. MME forwards to UE, Then UE will start ESR to MME, Then same flow contineous as per above flow.

Answer 2

Hi,

Serving Gateway will initiate the trigger to CS call.