Hello Experts.
What could cause a UE not to fallback to 3G during a VoLTE call?
I remember there was a parameter in Huawei MME blocking SGs paging (so CS Fallback) if there is QCI1 bearer active.
So from your experience QCI = 1 prevents CS Fallback.
Not directly QCI-1.
It is configurable on MME.
What is the scenario in your case?
Why do you expect a CS Fall Back during a VoLTE call?
It’s an E/// 4G Network with E// VoLTE.
When a VoLTE call is established, the UE never fallback to 3G during the call.
So SRVCC is not triggered.
If I drop the call, then I can go down to 3G, but not during the call.
So, this is totally different from your case. 
ANd I don’t have an answer for this…
If you fall back when you make call, it’s not VoLTE anymore.
SRVCC happens when call is ongoing on VoLTE and then due to coverage, will go 3G.
CSFB and SRVCC aren’t related together.
Of course, thanks.
But i don’t understand why SRVCC is not triggered…
Not triggered or fails?
Not triggered at all.
The call quality is going bad… and the handset stays in 4G.
QCI=5, ARP=5
Do you mean SRVCC to 3G is not happening?
Configured thresholds are OK?
What thresholds?
Do you mean in eNB?
b2 thresholds with qci1.
-100 dB.
You need to check A1, A2, B2 thresholds for QCI1.
The thresholds are fine, because on the same radio we have 2 PLMN and the other one has the same configuration and it is working fine.
Check if utranFreqRelation is correctly configured.
Also check LAC TAC mapping is proper or not.
I don’t think LAC/TAC mapping is necessarily required in SRVCC…
Check whether 3G NBRs are properly defined.
We had exprienced such issues around 2018.
Reason were SGSN sides.
Need to activate a few switches on SGSN sides, but dont remember switch details.
An example picture for your information:
For example for the RIM check these counters or messages and see if these are pegging any data or not.
In this case, the SGSN has to decode the incoming GTP message correctly and forward the RIM message to the destination RNC/BSS.
- SGSN would decode the received GTP RAN info relay message and construct a RANAP or BSSGP RIM message.
- Appropriate actions would be taken to forward the RIM message to the destination RNC/BSS.