Mobile in 4G not doing CSFB to 3G

Hi Expert,
Need your support on one issue.
Mobile is connected to 4G but it is not getting CSFB to 3G in Drive Test.
When only lock to 3G it is going to no service mode.
We are working on main where signal strenth is going.
This issue raise only one sector.
What are the posible reason?

It can be possible blocking can be possible at 3G end.
Is it blind redirection or measurement based CSFB?

First check the 3G stats of that sector. Is it carrying CS traffic or not?

Yes, it is carrying 3G Traffic.

Extended service message.
Coming or not.
Means reject or extended service request.


It’s failing from 3G part.
Check check 3G call in drive too.


Then issue with 3G end blocking.

Right, LTE has no issues. It’s 3G end.

Check 3G LAC TAC Mapping and external definitions.

Channel elements blocking.

No Blocking at 3G end.

Check only 3G call there.
Which 3g cell is trying in LTE CSFB and only 3g call.
3G has some congestion.

When we force connect to 3G it is coming in no service mode.

RRC congestion.

No congestion…

There is no 3G working there.
Check 3G idle.
3G idle is latching.
And then try make call.
If it going emergency, then CGI issue.
No call made then in that 3G cell.
Go 3G idle and make calls.

As per him cell is carrying traffic?

It can carry HO taffic too…