Enhancement of URLLC support in the 5G Core network
The main functionalities introduced here are the support of redundant transmission, QoS monitoring, dynamic division of the Packet Delay Budget, and enhancements of the session continuity mechanism.
Redundant transmission for high-reliability communication
Some URLLC services request very high reliability that hardly can be supported in an economical way by a single transport path. To support such services, a redundant transmission mechanism is specified. User packets are duplicated and simultaneously transferred to the receiver via two disjoint user plane paths. The redundant packets are then eliminated at the receiver side. With this, service failure can be avoided even in case the packet transmission via one path occasionally fails or exceeds the delay requirement.
Different options are specified to support redundant transmission at different layers:
- Dual-connectivity-based end-to-end redundant user plane paths. Two redundant PDU Sessions with independent user plane paths are established between UE and DN . Packet replication and elimination are performed by the upper layer of UE and DN, which are not specified in 3GPP.
- Support of redundant transmission on N3/N9 interfaces. For a PDU Session used for URLLC services, two redundant N3/N9 tunnels with independent user plane paths are established between UPF and NG-RAN to transfer the duplicated user packets. Packet replication and elimination are performed by NG-RAN and UPF.
- Support of redundant transmission at transport layer. This approach assumes that the backhaul provides two disjoint transport paths between UPF and RAN. The redundancy functionality within NG-RAN and UPF makes use of the independent paths at transport layer, which is not specified in 3GPP.
QoS Monitoring
QoS Monitoring is defined in this release for the measurement of packet delay between UE and PSA UPF. The NG-RAN is required to provide the QoS Monitoring of UL/DL packet delay at the Uu interface. The QoS Monitoring of UL/DL packet delay between NG-RAN and PSA UPF can be performed at different levels of granularities, i.e. per QoS flow level, or per GTP-U path level.
Dynamic division of Packet Delay Budget
The Packet Delay Budget (PDB) of URLLC services is typically more stringent than for traditional services. To obtain a more accurate delay budget for NG-RAN, SA WG2 decided to allow a dynamic value for the core network PDB (CN PDB), so that the SMF or NG-RAN can dynamically calculate delay budget of NG-RAN based on the CN PDB.
Enhancements of session continuity
When a UE moves, the user plane path of low latency services need to be optimized to reduce the latency and to guarantee session continuity.
PSA relocation for Ethernet PDU Session is specified in this release. The target UPF will assist in the update of Ethernet forwarding tables of Ethernet switches in the DN, so that UL/DL traffics will switch to the target UPF once the UE moves.
For ULCL relocation, a forwarding tunnel between the old and new UL CLs is introduced to avoid packet loss during relocation.
AF-influenced traffic routing mechanism is further enhanced to allow flexible coordination between SMF and AF when user plane change events happen.
References
The redundant transmission and QoS monitoring mechanisms are specified in TS 23.501[1], Clause 5.33. Dynamic division of Packet Delay Budget is specified in TS 23.501 [1], Clause 5.7.3.4. Enhancements of session continuity mechanisms are mainly in TS 23.501 [1], Clause 5.6.7, and TS 23.502 [2], Clause 4.3.5 and Clause 4.3.6.
[1] 3GPP TS 23.501: "System Architecture for the 5G System; Stage 2".
[2] 3GPP TS 23.502: "Procedures for the 5G System; Stage 2".
[3] 3GPP TR 23.725: "Study on enhancement of Ultra-Reliable Low-Latency Communication (URLLC) support in the 5G Core network"