5.31.8 High latency communication

Functions for High latency communication may be used to handle mobile terminated (MT) communication with UEs being unreachable while using power saving functions as specified in clause 5.31.7 or due to discontinuous coverage as described in clause 5.4.13. "High latency" refers to the initial response time before normal exchange of packets is established. That is, the time it takes before a UE has woken up from its power saving state and responded to an initial downlink packet or signal.

When a NR RedCap UE requests to use the power saving functions as specified in clause 5.31.7, then the AMF may, based on local policy, reroute the Registration Request to another AMF that supports High latency communication as specified in clause 6.3.5.

High latency communication is supported by extended buffering of downlink data in the UPF, SMF or NEF when a UE is using power saving functions in CM-IDLE state or in RRC_INACTIVE state, or when the UE is using a satellite access with discontinuous coverage and the UE is not reachable. For UPF anchored PDU sessions the SMF configures during AN release or when NG-RAN indicates via the AMF the UE is in extended DRX for RRC_INACTIVE, the UPF with user data Forwarding Action Rule and user data Buffering Action Rule according to TS 29.244 [65]. The rules include instructions whether UPF buffering applies or the user data shall be forwarded to the SMF for buffering in the SMF. For NEF anchored PDU sessions only extended buffering in the NEF is supported in this release of the specification. During the Network Triggered Service Request procedure or Mobile Terminated Data Transport procedures when using Control Plane CIoT 5GS Optimisation, the AMF provides an Estimated Maximum Wait Time to the SMF if the SMF indicates the support of extended buffering. The SMF determines the Extended Buffering Time based on the received Estimated Maximum Wait Time or local configuration. The handling is e.g. specified in the Network Triggered Service Request procedure, clauses 4.2.3.3, 4.2.6, 4.24.2 and 4.25.5 of TS 23.502 [3].

High latency communication is also supported through notification procedures. The following procedures are available based on different monitoring events:

• UE Reachability;
• Availability after DDN failure;
• Downlink Data Delivery Status.

An AF may request a one-time "UE Reachability" notification when it wants to send data to a UE which is using a power saving function (see event subscription procedure in clause 4.15.3.2 of TS 23.502 [3]). The SCS/AS/AF then waits with sending the data until it gets a notification that the UE is reachable (see notification procedures in TS 23.502 [3]).

An AF may request repeated "Availability after DDN failure" notifications where each UE reachability notification is triggered by a preceding DDN failure, i.e. the AF sends a downlink packet to request a UE reachability notification when the UE becomes reachable. That downlink packet is discarded by the UPF or SMF or NEF (see notification procedures in TS 23.502 [3]).

An AF may request repeated "Downlink Data Delivery Status" notifications when it wants indications that DL data has been buffered or when buffered DL data has been delivered to the UE.

If MICO mode or extended idle mode DRX is enabled, Idle Status Indication allows the AF to determine when the UE transitions into idle mode. When requesting to be informed of either "UE Reachability" or "Availability after DDN failure" notification, the AF may also request Idle Status Indication. If the UDM and the AMF support Idle Status Indication, then when the UE for which MICO mode or extended idle mode DRX is enabled transitions into idle mode, the AMF includes in the notification towards the NEF the time at which the UE transitioned into idle mode, the active time and the periodic registration update timer granted to the UE by the AMF, the eDRX cycle length and the Suggested number of downlink packets if a value was provided to the SMF.

An AF may provide parameters related to High latency communication for different methods to UDM, via NEF, as part of provisioning capability as specified in clause 5.20. The UDM can further deliver the parameters to other NFs (e.g. AMF or SMF) as specified in clause 4.15.6 of TS 23.502 [3].

If the AMF is aware that some signalling or data is pending in the network for an UE that is known as being unreachable for a long duration, e.g. for UE's having extended idle mode DRX, extended DRX for RRC_INACTIVE or MICO enabled, the AMF maintains the N2 connection for at least the Extended Connected Time and provides the Extended Connected Time value in a NG-AP message to the RAN. The Extended Connected Time value indicates the minimum time the RAN should keep the UE in RRC_CONNECTED state regardless of inactivity. At inter-RAN node handovers, if some signalling or data are still pending, the target AMF may send the Extended Connected Time value to the target RAN node.