This clause describes the functionalities for the support of eXtended Reality (XR) services that require high data rate and low latency communications. An overview of XR services is available in TR 38.835, while the service requirements are documented in TS 22.261.
XR-Awareness relies on QoS flows, PDU Sets, Data Bursts and traffic assistance information (see TS 23.501).
The following PDU Set QoS Parameters may be provided by the SMF to the gNB as part of the QoS profile of the QoS flow, and to enable PDU Set based QoS handling at least one of them shall be provided:
PDU Set Delay Budget (PSDB): as defined in TS 23.501, upper bound for the duration between the reception time of the first PDU (at the UPF for DL, at the UE for UL) and the time when all PDUs of a PDU Set have been successfully received (at the UE in DL, at the UPF in UL). A QoS Flow is associated with only one PSDB, and when available, it applies to both DL and UL and supersedes the PDB of the QoS flow.
PDU Set Error Rate (PSER): as defined in TS 23.501, upper bound for a rate of non-congestion related PDU Set losses between RAN and the UE. A QoS Flow is associated with only one PSER, and when available, it applies to both DL and UL and supersedes the PER of the QoS flow.
PDU Set Integrated Handling Information (PSIHI): indicates whether all PDUs of the PDU Set are needed for the usage of PDU Set by application layer, as defined in TS 23.501.
During the Xn-handover preparation procedure, the source gNB sends the stored PDU Set QoS Parameters as part of the QoS profile to the target NG-RAN node.
In addition, the UPF can identify PDUs that belong to PDU Sets, and may indicate to the gNB the following PDU Set Information in the GTP-U header:
PDU Set Sequence Number;
Indication of End PDU of the PDU Set;
PDU Sequence Number within a PDU Set;
PDU Set Size in bytes;
PDU Set Importance (PSI), which identifies the relative importance of a PDU Set compared to other PDU Sets within the same QoS Flow.
5GC may provide XR traffic assistance information to gNB through NG AP TSC Assistance Information (TSCAI) as specified in clause 5.37.8 of TS 23.501 (for both GBR and non-GBR QoS flows):
UL and/or DL Periodicity;
N6 Jitter Information (i.e. between UPF and Data Network) associated with the DL Periodicity.
This assistance information can be used by the gNB to configure DRX to enable better UE power saving.
In addition, 5GC may provide the following information through NG-U as specified in clause 188.8.131.52 of TS 23.501:
Indication of End of Data Burst in the GTP-U header of the last PDU in downlink.
This information can be used by the gNB to push the UE back to sleep when possible.
In the uplink, the UE needs to be able to identify PDU Sets and Data Bursts dynamically, including PSI. How this is done is left up to UE implementation but when possible for a QoS flow, this is indicated to the gNB via UE Assistance Information.
Most XR video frame rates (15, 30, 45, 60, 72, 90 and 120 fps) correspond to periodicities that are not an integer (66.66, 33.33, 22.22, 16.66, 13.88, 11.11 and 8.33 ms respectively). The gNB may configure a DRX cycle expressed in rational numbers so that the DRX cycle matches those periodicities, e.g. for the traffic with a frame rate of 60 fps, the network may configure the UE with a DRX cycle of 50/3 ms.
Configured grants may be configured without the need for the UE to monitor possible UL retransmissions, thus increasing the number of power saving opportunities for the UE.
When the PSIHI is set for a QoS flow, as soon as one PDU of a PDU set is known to be lost, the remaining PDUs of that PDU Set can be considered as no longer needed by the application and may be subject to discard operation at the transmitter to free up radio resources.
In uplink, the UE may be configured with PDU Set based discard operation for a specific DRB. When configured, the UE discards all packets in a PDU set when one PDU belonging to this PDU set is discarded due to discard timer expiry.
The gNB may perform downlink PDU Set discarding based on implementation by taking at least PSDB, PSI, PSIHI parameters into account.
In case of congestion, the gNB may use the PSI for PDU set discarding. For uplink, dedicated downlink signalling is used to request the UE to apply a shorter discard timer to low importance SDUs in PDCP.
During a handover from a gNB supporting PDU Set based handling to another gNB, the source gNB signals the PDU Set Information over Xn-U if the target node has signalled the support of PDU Set based handling in the Xn Handover Request Acknowledge message.
During a handover from a gNB not supporting PDU Set based handling to a gNB supporting PDU Set based handling, the target gNB may indicate the support of PDU Set based handling to the SMF during the Path Switch Request procedure (in case of Xn handover) or Handover Resource Allocation procedure (in case of NG handover), the SMF will act as specified in TS 23.501. If the indication is absent, the SMF infers that PDU Set based handling is not supported by the target NG-RAN node, then the SMF will act as specified in TS 23.501.
In order to support ECN marking for L4S at gNB as specified in TS 23.501, SMF provides ECN marking request per QoS flow level to the gNB as part of PDU Session Resource Management procedure. If the gNB supports ECN marking, it provides the status indication back to the SMF which is used by the SMF as specified in TS 23.501. During Xn Handover Preparation procedure, source gNB provides the ECN marking request to target gNB.
For ECN marking for L4S at UPF, SMF requests the gNB to report congestion information per QoS flow level as part of PDU Session Resource Management procedure. If the gNB supports ECN marking for L4S at UPF, it provides the status indication back to the SMF which is used by SMF as specified in TS 23.501. During Xn Handover Preparation procedure, source gNB provides the ECN marking UPF request to target gNB.
For congestion reporting from gNB to UPF, SMF requests the gNB to report congestion information per QoS flow level as part of PDU Session Resource Management procedure. If the NG-RAN supports congestion information reporting, it provides the status indication back to the SMF which is used by the SMF as specified in TS 23.501. During Xn Handover Preparation procedure, source gNB provides the congestion information request to target gNB.