Why only IMSI Attach In LTE?

Question

Please explain below scenario possibility in LTE.

UE has requested a combined attached procedure but network accepted as IMSI only (Is this behavior possible in LTE?)

If Yes, then what UE will try: Re-attach to 3G or 2G or stay in LTE? Do we have any specific scenario for only IMSI attach .

Answer 1

I think it is impossible that network accepted as IMSI only.
In TS 24.301 5.5.1.3.4, it said:

Depending on the value of the EPS attach result IE received in the ATTACH ACCEPT message, the following different cases can be distinguished:
1) The EPS attach result IE value indicates "combined EPS/IMSI attach": attach for EPS and non-EPS services, or for EPS services and "SMS only" have been successful.
2) The EPS attach result IE value indicates "EPS only": attach for EPS services has been successful but attach for non-EPS services or "SMS only" has failed.

There is just two cases. Furthermore,
in TS 24.301 8.2.1 "EPS attach result" IE is mandatory in attach accept and
in TS 24.301 9.9.3.10, there are only two cases of EPS attach result IE:
0 0 1 EPS only
0 1 0 combined EPS/IMSI attach

You can download TS24.301 here:
https://portal.3gpp.org/desktopmodules/Specifications/SpecificationDetails.aspx?specificationId=1072

Answer 2

Posted on behalf of Bart https://www.facebook.com/groups/LTE.Professionals/

Again.. It's me who doesnt know or the person is not really sure what he is asking about..

The Combined Attach as stated here http://www.lteandbeyond.com/2014/01/combined-attach-in-lte.html is the Attach to EPS and non-EPS services.

Where IMSI Attach is the Attach where the IMSI is used to authenticate the subscriber towards the network.
As described here http://www.lteandbeyond.com/2013/03/what-is-guti-and-imsi-attach.html

In other words I believe you could do combined Attach using GUTI.