5G-SA to 5G NSA (LTE anchor) transition

eta004 · February 17, 2021, 2:49pm

In this L3 snap the box is the one where 5G-SA to 5G NSA (LTE anchor here) transition happened.
Can anyone guide me what all I need to check to understand this transition?

RFSpecialist · February 17, 2021, 2:51pm

Can you check RRC setup in 5G SA?
It is a very long message (like RRC Connection reconfig in NSA).
There should be indication to go to LTE+NSA.
Any transision from SA to NSA and to SA is ruled be network, not by UE.

Can you get from 5G SA SIB1 the values of those timers?
Based on them the UE goes to another RAT in poor RF when RRC fails.

So based on the 3 parameters above in poor RF the UE goes to LTE+NSA and will quit 5G SA ( after few RRC attempts failure).
Those 3 parameters/IE are not present in all 5G SA networks, can be missing too.

eta004 · February 17, 2021, 2:55pm

One thing I observed SA to LTE happened after TAU.

I think key measurements there are poor SSB RSRP:

This is what triggers transition from SA to NSA.
Call event is just a higher layer NAS message that was not yet submitted to lower layers.

I dont see these IE in SIB1.