When UE attachs for both EPS and non-EPS services then it is called combined attach procedure. Or in simple words when UE performs registration for LTE network and Non-LTE network (e.g, WCDMA or GSM) simultaneously it is called combined attach.
Now question is why do we need simultaneous registration -
It is mainly for CS-Fallback. Unless a system operator/UE use Voice Over IMS, we use legacy network (e.g, WCDMA, GSM, C2K) for voice call. In this case, UE has to go through the registration for the legacy network. The idea of Combined Attach is to perform the attach process for LTE and Legacy Network simultaneously.
For a combined attach procedure, the UE should indicate "combined EPS/IMSI attach" in the EPS attach type IE. Upon receiving it network may initiate EMM common procedures, e.g. the identification, authentication and security mode control procedures, depending on the received information such as IMSI, GUTI and KSI.
Depending on the value of the EPS attach result IE received in the ATTACH ACCEPT message, the following different cases can be distinguished:
- 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.
- 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.