In the EMM-DEREGISTERED state, the EMM context in MME holds no valid location or routing information for the UE. The UE is not reachable by a MME, as the UE location is not known.
In the EMM-DEREGISTERED state, some UE context can still be stored in the UE and MME, e.g. to avoid running an AKA procedure during every Attach procedure.
During the successful Inter-RAT TAU/RAU/handover procedure and ISR activated is not indicated to the UE, the old S4 SGSN/old MME changes the EMM state of the UE to GPRS-IDLE/PMM-DETACHED/EMM-DEREGISTERED.
The UE enters the EMM-REGISTERED state by a successful registration with an Attach procedure to either E-UTRAN or GERAN/UTRAN. The MME enters the EMM-REGISTERED state by a successful Tracking Area Update procedure for a UE selecting an E-UTRAN cell from GERAN/UTRAN or by an Attach procedure via E-UTRAN. In the EMM-REGISTERED state, the UE can receive services that require registration in the EPS.
The UE location is known in the MME to at least an accuracy of the tracking area list allocated to that UE (excluding some abnormal cases).
In the EMM-REGISTERED state, the UE shall:
always have at least one active PDN connection (unless the UE supports "Attach without PDN connectivity");
setup the EPS security context.
After performing the Detach procedure, the state is changed to EMM-DEREGISTERED in the UE and in the MME. Upon receiving the TAU Reject and Attach Reject messages the actions of the UE and MME depend upon the 'cause value' in the reject message, but, in many cases the state is changed to EMM-DEREGISTERED in the UE and in the MME.
If all the bearers belonging to a UE that does not support "Attach without PDN connectivity"
are released (e.g., after handover from E-UTRAN to non-3GPP access), the MME shall change the MM state of that UE to EMM-DEREGISTERED. If the UE that does not support "Attach without PDN connectivity"
camps on E-UTRAN and the UE detects that all of its bearers are released, the UE shall change the MM state to EMM-DEREGISTERED. If all the bearers (PDP contexts) belonging to a UE are released, while the UE camps on GERAN/UTRAN, the UE shall deactivate ISR by setting its TIN to "P-TMSI"
as specified in TS 23.060
. This ensures that the UE performs Tracking Area Update when it re-selects E-UTRAN. If the UE switches off its E-UTRAN interface when performing handover to non-3GPP access, the UE shall automatically change its MM state to EMM-DEREGISTERED.
The MME may perform an implicit detach any time after the Implicit Detach timer expires. The state is changed to EMM-DEREGISTERED in the MME after performing the implicit detach.