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