LTE: Detach Request (Network Initiated)

·            The detach procedure is either initiated by the UE or by the network. The network
           initiates the detach procedure:

        -          To detach the UE for EPS services or non-EPS services or both
 -          To disconnect the UE from the last PDN to which it is connected
 -          To inform the UE to re-attach to the network and re-establish all PDN connections

·            If the MME performs a local detach (i.e. no explicit signalling), it will inform the UE with an EMM message (e.g. SERVICE REJECT or TRACKING AREA UPDATE REJECT) with EMM cause #10 "implicitly detached" only when the UE initiates any EMM procedure

·            If the detach procedure for EPS services is performed, then all the active EPS bearer context(s) are deactivated locally without peer-to-peer signalling between the UE and the MME

·            The MME initiates the detach procedure by sending a DETACH REQUEST message to the UE

·            The Detach type IE included in this message indicates the type of detach. 4-bits are used for this purpose. 3-bits (LSBs) indicate whether ‘re-attach required’ or ‘re-attach not required’ or ‘IMSI detach’. Bit-4 is always set to zero for network initiated detach

·            If the detach type IE indicates that "re-attach required", the UE shall deactivate all the active EPS bearer context(s) locally and then send a DETACH ACCEPT message to the network. Furthermore, the UE shall, initiate attach or combined attach procedure. The UE should also re-establish any previously established PDN connections (user interaction is necessary in some cases when the UE cannot re-activate the EPS bearer(s) automatically)

·            If the detach type indicates "IMSI detach", then the UE shall not deactivate any of the EPS bearer context(s). The UE shall send a DETACH ACCEPT message to the network and re-attach to non-EPS services by sending TRACKING AREA UPDATE REQUEST message with EPS update type IE indicating "combined TA/LA updating with IMSI attach"

·            The network may include an EMM cause IE to specify the reason for the detach. If the detach type indicates "IMSI detach" or "re-attach required", then the UE shall ignore the EMM cause IE if received

·            For UE initiated Detach procedure, check my earlier post here

Reference 3GPP TS 24.301

Example: Network initiated DETACH REQUEST message

14 comments:

  1. What happens when UE receive EMM CAUSE IMPLICITLY DETACHED with DETACH Type ‘re-attach not required’ ?

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    1. In that case Ue will not try to connect with network..

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  2. Hi, Can you explain about when the Network initialize DETACH REQUEST, how to trigger that event ? thanks!

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    1. One can't trigger this in a live NW scenario. But if you are using a network simulator, it is possible to send detach request from NW

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    2. thank for you answer, I'm testing a live MME, In case S1 intra ENB Handover, after UE moved to target ENB (Handover success) then UE not send TAU (Tracking Area Update) to live MME. next, MME will send Detach Request to UE with type is re_attach_required, EMM_cause = implicitly_detached.
      I just meet that event only one time and save to log, so I think it's possible to trigger live NW scenario, but I don't know how to trigger that event again ...!
      Do you have some ideal about that way ? maybe I forgot something.

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  3. Don't use S1 intra ENB intral handover, let try S1 intra MME handover, and don't send TUA to Target MME ...! you can see the the message you want ! good luck.

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  4. May I know in which cases NW may send ‘re-attach required’ in Detach Request with EMM Cause 'None'?

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  5. From 24301, If the detach type indicates "IMSI detach" or "re-attach required", then the UE shall ignore the EMM cause IE if received. The UE anyway going to ignore it so I assume that the NW wouldn't set it in general

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  6. what happens if a NW sends a detach request indicating "re-attach required" when the UE is attached for emergency bearer services on FPLMN?

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    1. Tricky. Couldn't find any clause in the spec. I think that the NW doesn't required to do that.

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  7. I would like to know what happens to the UE if I detach it from the network without "reattach required", will try to find another EPS close by to attach to or another network or will it try to reattach to the same one

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