Why core network contains IMSI in MS identity IE of combined attach accept, why not new TMSI?

Question

In TS 24.301 5.5.1.3.4.2, if combined attach is success(i.e Combined attach successful for both EPS services and non-EPS), why core network contains IMSI in MS identity IE of combined attach accept, and why not just allocate a new TMSI?
In what situation will core network use IMSI in MS identity?

Answer 1

What I understood from the mentioned section of specification, it is not mandatory in case of combined attach procedure, attach accept wouldn't contain TMSI. There are two cases as following:
1. Attach accept can have either IMSI or TMSI
2. Attach accept neither have IMSI nor TMSI.

If core network (MME) can't allocate GUTI then in that case MME may send IMSI in attach accept. However it is not mentioned in the specification exactly when MME would not be able to allocate GUTI for the received attach request.
I am expecting that someone will share more information in this topic.

Comments

Thank you for your reply. Can you tell me which spec mention that if CN can not send GUTI, it will contain  IMSI in MS identity IE. What I think is that MS identity IE is not relative to 4G. It is relative to 3G, so it is not contain in normal EPS only attach accept.