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What is combined attach and how it is triggered?

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Can someone explain this step by step?

posted Jul 16, 2014 by anonymous

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1 Answer

+1 vote

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.

Initiation Procedure

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.
answer Jul 16, 2014 by Pardeep Kohli
Similar Questions
+3 votes

I want to know combined attach procedure and how it is different between UMTS and LTE?

0 votes

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?

+2 votes

During a combined attache, a UE can receive an ATTACH-REJECT if for the specific PLMN the EPS service are not allowed.
In this case, the EMM cause value received in the ATTACH REJECT message should be 14.
The 124.301 ETSI standard tells that in this situation, if the UE operates in CS/PS mode 1, it may select GERAN or UTRAN
radio access technology instead of LTE for the same PLMN.
The standards doesn't tell anything about UE's operating in CS/PS mode 2: must they choose another PLMN?

0 votes

From UE point of view, I want to know when UE triggers integrity protected Attach request and when it sends as plain message ?

+1 vote

When doing IOT test, I encounter an issue. One sony mobile phone initiate detach request just 2 seconds after it attach to EPC successfully.

The attach request is a combined EPS/IMSI attach and the attach accept result is EPS only with EMM cause 'CS domain not available'.

Some one suggest it maybe due to the mismatch of the parameter PTI (procedure translation identifier) in the attach request and attach accept. I checked it and find PTI are the same in the two message.

I also tried another old type sony mobile phone, this old phone does not have such strange behavior.

After compare the attach request and attach accept messages of these two mobile phone, I did not find anything wired.

So does any one knows what is the reason behind?

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