Ultra-Reliable and Low Latency Communication (or URLLC) was introduced in 5G Release 15 with limited capabilities. Release 16 improves on the capabilities, through the introduction of redundant transmission, QoS monitoring, dynamic division of the Packet Delay Budget, and enhancements to the session continuity mechanism.

URLLC Redundant transmissions

Often, where ultra high reliability is required, a single transport path will not provide the required results. In R16, a redundant transmission mechanism can be specified, where the packets are duplicated and sent over different user plan paths. Duplicated packets are then eliminated at the receiver end.

Different options are specified to support redundant transmission at different layers:

Dual-connectivity-based end-to-end redundant user plane paths.

Redundant PDU Sessions are established between the UE and the DN via different paths. Packet replication and de-duplication are done at higher layers, outside of the 3GPP standard.

// PDU Sessions
// Provide end-to-end user plane connectivity between a 
// UE (User Equipment) and a specified DN (Data Network) 
// through the UPF (User Plane Function). 

Redundant transmission inside N3/N9 tunnels.

Two redundant N3/N9 tunnels are formed, using seperate paths, and are established between the UPF and the NG-RAN to transfer duplicated packets. Packet replication and de-duplication are done at the UPF and NG-RAN

// The User Plane Function reference points 
// N3: Interface between the RAN (gNB) and the (initial) UPF. 
// N9: Interface between two UPF’s (i.e the Intermediate I-UPF 
// and the UPF Session Anchor) 
// N6: Interface between the Data Network (DN) and the UPF 
// N4: Interface between the Session Management Function (SMF) 
// and the UPF 

Redundant transmission at the transport layer.

This approach assumes a backhaul provides diverse paths between the UPF and the RAN. This is not specified in the 3GPP standard. 2x N3 tunnels are used for this level of redundancy.

URLLC QoS monitoring


In this release, we introduce QoS Monitoring to measure the packet delay between the User Equipment (UE) and the Packet-Switched Core Network (PSA) User Plane Function (UPF). Within the Next Generation Radio Access Network (NG-RAN), the monitoring of UL/DL packet delay at the Uu interface is required. Furthermore, the monitoring of UL/DL packet delay between the NG-RAN and PSA UPF can be conducted at various levels of granularity, such as per QoS flow or per GTP-U path.

// GTP-U (GPRS Tunneling Protocol-User Plane)
// A protocol used in mobile networks to encapsulate and 
// transport user data packets between different network 
// elements, such as the Radio Access Network (RAN) and the 
// core network. GTP-U is responsible for carrying user data 
// traffic, including voice, video, and data packets, over the 
// mobile network infrastructure. It helps ensure the reliable // and efficient transmission of data between various network 
// components.

Dynamic division of the Packet Delay Budget

URLLC services often have stricter requirements for packet delay compared to traditional services. To improve the accuracy of delay budget estimation in the Next Generation Radio Access Network (NG-RAN), a decision was made to enable a dynamic value for the core network Packet Delay Budget (CN PDB). This allows the Session Management Function (SMF) or NG-RAN to calculate the NG-RAN’s delay budget dynamically based on the CN PDB, ensuring more precise delay management within the network.

// Session Management Function (SMF)
// The SMF is responsible for managing and controlling the 
// session-related functions and interactions between the User 
// Equipment (UE) and the core network

Enhancements to the session continuity mechanism

The Release 16 introduces a PDU Session Anchor (PSA) relocation in Ethernet PDU Sessions. The target User Plane Function (UPF) assists in updating the Ethernet forwarding tables of Ethernet switches in the Data Network (DN), enabling UL/DL traffic to switch to the target UPF seamlessly upon UE movement. To prevent packet loss during Uplink Classifier (ULCL) relocation, a forwarding tunnel is introduced between the old and new ULCLs.

The AF-influenced traffic routing mechanism is further improved to facilitate flexible coordination between the Session Management Function (SMF) and Application Function (AF) when user plane change events occur. These enhancements aim to minimize latency, ensure smooth transitions, and maintain reliable connectivity for low latency services during UE mobility.

// AF-influenced traffic routing mechanism
// the AF-influenced traffic routing mechanism is designed to 
// enable flexible coordination between the Session Management // Function (SMF) and the Application Function (AF) when user 
// plane change events occur. 
// This mechanism allows the AF, which is responsible for 
// managing and controlling application-specific policies and 
// requirements, to influence the routing of user traffic.

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