5.31.7.2.1 Overview

The UE and the network may negotiate over non-access stratum signalling the use of extended idle mode DRX for reducing its power consumption, while being available for mobile terminating data and/or network originated procedures within a certain delay dependent on the DRX cycle value. Extended DRX in CM-IDLE is supported for E-UTRA and NR connected to 5GC. Extended DRX in CM-CONNECTED with RRC_INACTIVE mode is supported for WB-E-UTRA, LTE-M and NR connected to 5GC. RRC_INACTIVE is not supported by NB-IoT connected to 5GC.

The negotiation of the eDRX parameters for NR, WB-E-UTRA and LTE-M is supported over any RAT.

Applications that want to use extended idle mode DRX need to consider specific handling of mobile terminating services or data transfers and in particular they need to consider the delay tolerance of mobile terminated data. A network side application may send mobile terminated data, an SMS, or a device trigger and needs to be aware that extended idle mode DRX may be in place. A UE should request for extended idle mode DRX only when all expected mobile terminating communication is tolerant to delay.

NOTE 1: The extended idle mode DRX cycle length requested by UE takes into account requirements of applications running on the UE. Subscription based determination of eDRX cycle length can be used in those rare scenarios when applications on UE cannot be modified to request appropriate extended idle mode DRX cycle length. The network accepting extended DRX while providing an extended idle mode DRX cycle length value longer than the one requested by the UE, can adversely impact reachability requirements of applications running on the UE.

UE and NW negotiate the use of extended idle mode DRX as follows:

If the UE decides to request for extended idle mode DRX, the UE includes an extended idle mode DRX parameters information element in the Registration Request message. The UE may also include the UE specific DRX parameters information element for regular idle mode DRX according to clause 5.4.5. The extended DRX parameters information element includes the extended idle mode DRX cycle length.

The AMF decides whether to accept or reject the UE request for enabling extended idle mode DRX. If the AMF accepts the extended idle mode DRX, the AMF based on operator policies and, if available, the extended idle mode DRX cycle length value in the subscription data from the UDM, may also provide different values of the extended idle mode DRX parameters than what was requested by the UE. The AMF taking into account the RAT specific Subscribed Paging Time Window, the UE's current RAT and local policy also assigns a Paging Time Window length to be used and provides this value to the UE during Registration Update procedures together with the extended idle mode DRX cycle length in the extended DRX parameter information element. If the AMF accepts the use of extended idle mode DRX, the UE shall apply extended idle mode DRX based on the received extended idle mode DRX length, the UE's current RAT (NR, NB-IoT, WB-E-UTRA or LTE-M) and RAT specific Paging Time Window length. If the UE does not receive the extended DRX parameters information element in the relevant accept message because the AMF rejected its request or because the request was received by AMF not supporting extended idle mode DRX, the UE shall apply its regular discontinuous reception as defined in clause 5.4.5. For NR, Paging Time Window applies for extended DRX lengths greater than 10.24s as defined in TS 38.304 [50]. For WB-E-UTRA, Paging Time Window applies for extended DRX lengths of 10.24s and greater as defined in TS 36.304 [52].

When the UE is accessing NR, if the AMF provides an extended idle mode DRX cycle length value of 10.24s and the registration area of the UE contains only NR cells, the AMF does not include a Paging Time Window. If the AMF provides an extended idle mode DRX cycle length value of 10.24s and the registration area of the UE contains E-UTRA cells and NR cells if the UE supports both E-UTRA and NR, the AMF includes a Paging Time Window.

For WB-E-UTRA and LTE-M the eNB broadcasts an indicator for support of extended idle mode DRX in 5GC in addition to the existing indicator for support of extended idle mode DRX in EPC as defined in TS 36.331 [51]. For NR the gNB broadcasts an indicator for support of extended idle mode DRX as defined in TS 38.331 [28]. This indicator is used by the UE in CM-IDLE state.

NOTE 2: A broadcast indicator for support of extended idle mode DRX is not needed for NB-IoT as it is always supported in NB-IoT.

The specific negotiation procedure handling is described in TS 23.502 [3].

NOTE 3: If the Periodic Registration Update timer assigned to the UE is not longer than the extended idle mode DRX cycle the power savings are not maximised.

For RAT types that support extended DRX for CM-CONNECTED with RRC_INACTIVE state, the AMF passes the UE's accepted idle mode eDRX values to NG-RAN. If the UE supports eDRX in RRC_INACTIVE, based on its UE radio capabilities, NG-RAN configures the UE with an eDRX cycle in RRC_INACTIVE as specified in TS 38.300 [27] up to the value for the UE's idle mode eDRX cycle as provided by the AMF in "RRC Inactive Assistance Information" as defined in clause 5.3.3.2.5.

If an eDRX cycle is applied in RRC_INACTIVE, the RAN can buffer DL packets up to the duration of the eDRX cycle chosen by NG-RAN if the eDRX cycle does not last more than 10.24 seconds. If the CN based MT communication handling support indication is received in RRC Inactive Assistance Information, the NG-RAN may select an eDRX cycle that lasts more than 10.24s. In this case, based on implementation the NG-RAN may send an indication in N2 message that the UE is transitioning to RRC_INACTIVE state and the NG-RAN determined eDRX values (i.e. the eDRX cycle length and the Paging Time Window length) for RRC_INACTIVE to the AMF. The CN takes the indication in the N2 message into account, then handles mobile terminated (MT) communication as specified in clause 5.31.7.2.4 and it can apply high latency communication as specified in clause 5.31.8. The AMF replies to NG-RAN that the indication in the N2 message has been taken into account and the MT signalling or data may be buffered by the Core Network based on high latency communication. If and when the NG-RAN chooses to send the indication is up to NG-RAN implementation. If the NG-RAN delays sending the indication and it receives a DL NAS message for the UE, the NG-RAN proceeds as described in clause 4.8.1.1a of TS 23.502 [3].

NOTE 4: If the indication that the UE is transitioning to RRC_INACTIVE state is not sent (or sent after UE has entered RRC_INACTIVE state) by the NG-RAN then until CN receives it the CN cannot apply the high latency communication functionality, other NFs will not be aware of the UE reachability, certain high latency communication related services provided to the AF via NEF would not be available, NAS message delivery might fail and downlink data in RAN might be lost.

NOTE 5: The CN based MT communication handling support indication in RRC Inactive Assistance Information is provided when all entities (e.g. AMF, SMF and UPF) involved in the CN support corresponding functionalities (including the support providing buffered downlink data size) based on deployment and configuration.

When the UE has PDU Session associated with emergency services, the UE and AMF follow regular discontinuous reception as defined in clause 5.4.5 and shall not use the extended idle mode DRX. Extended idle mode DRX parameters may be negotiated while the UE has PDU Session associated with emergency services. When the PDU Session associated with emergency services is released, the UE and AMF shall reuse the negotiated extended idle mode DRX parameters in the last Registration Update procedure.

NOTE 6: For a UE with an emergency PDU Session NG-RAN can keep the UE in RRC_CONNECTED state depending on configuration. NG-RAN handling of eDRX for RRC_INACTIVE based on implementation is described in TS 38.300 [27].

The UE shall include the extended DRX parameters information element in each Registration Request message if it still wants to use extended idle mode DRX. At AMF to AMF, AMF to MME and MME to AMF mobility, the extended idle mode DRX parameters are not sent from the old CN node to the new CN node as part of the MM context information.