Detach type in network initiated detach

Question

My question is when will MME initiate detach request with detach type as IMSI detach. I know that IMSI detach will happen when MME wants the subscriber to move out of non-eps service and move the association with the MSC to GS-NULL state .

In the network to UE direction:
Bits
3 2 1
0 0 1 re-attach required
0 1 0 re-attach not required
0 1 1 IMSI detach
1 1 0 reserved
1 1 1 reserved

I searched through all the spec but couldn't find a spec ref when will MME initiate detach request with detach type as IMSI detach.

Answer 1

Agree, standards seems to not clearly indicate when does MMEi decides to send detach request with IMSI detach. some reference below in case it helps ..

3GPPi 24.301 reads followings >>
When receiving the DETACH REQUEST message and the Detach type IE indicates "IMSI detach", the UE shall not deactivate the EPSi bearer context(s) including the default EPS bearer context. The UE shall set the MMi update status to U2 NOT UPDATED. A UE may send a DETACH ACCEPT message to the network, and shall re-attach to non-EPS services by performing the combined tracking area updating procedure according to subclause 5.5.3.3, sending a TRACKING AREA UPDATE REQUEST message with EPS update type IE indicating "combined TA/LA updating with IMSI attach".

According to 29.118, If the MME decides to perform a network initiated detach procedure and the state of the SGs association to a VLR for that MS is not SGs-NULL, the MME shall send an SGsAP-EPS-DETACH-INDICATION message to the VLR indicating "MME initiated IMSI detach from EPS services".