Difference between normal attach and combined attach procedure ?

Question

This query come out to give answer one of the query. Is really UE should perform combined attach procedure( EPS and non-EPS) for handover between LTE to other 3GPP technology like UMTS/GERAN ?

Answer 1

LTE Case:-
Normal Attach procedure is used to register with only EPS network (e.g LTE Only) where as combined attach procedure is used to registered with EPS and Non-EPS networks simultaneously (e.g for both LTE and WCDMA or GSM).

UMTS Case:--
Normal Attach procedure is used to register with only CS Network only where as combined attach procedure is used to registered with CS and PC networks simultaneously.

why we are using this kind of procedure in UMTS is in order to reduce the additional signalling with network.For LTE this is mainly for CS-Fallback purpose.

Answer 2

Normal Attach: UE Register only for EPS Services. Attach Type Ie in Attach Request will be EPSAttach.
Combined Attach: UE register for both EPS and Non-EPS Services. The Attach Type will be EPS/IMSI Attach. This is a prerequisite to do a CSFB.