Is combined attach (EPS and non-EPS) procedure necessary to support handover between LTE and UMTS/GERAN ?

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

In UMTS, we had a terminology called 'Combined Registration'. It means that UE simultaneously performs registration for CS network and PS network. LTE is bascially packet only (PS only).

Now coming to the question of Why do we need this kind of simultaneous registration for two different network ? 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.

Now coming to main question if handover is possible without combined attach, answer is yes for PS but for CS it is not.

Comments

I think combined attach procedure in required to get Packet Switch connectivity as much as possible ( I means if UE is going far to LTE and close to UMTS then MME will proceed for handover to UMTS).

Here I have one use case. UE sends measurement report to Enodeb when it is connected to LTE network and in combined attach procedure UE also perform RAU and TAU procedure.

 Let us assume there is one Tracking Area and it has 2 Routing Areas . Assume one routing area has 1 nodeb and otherone has 2nd Nodeb. Enodeb tells only Target Nodeb to MME is handover Required message. It might be first Nodeb is served by SGSN1 and second Nodeb served by SGGN2 . In that case MME will initiate  handover signalling to the SGSN which is current belong to the Nodeb in Handover Required message.

So MME has to select relevant SGSN based of RAU sent by UE.
Now question is that without combined attach procedure how MME will track the current location of UE with respect to UMTS network or GERAN.

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Probably you are right, I am not completely confident.