Indirect network connection

Description

Indirect network connection is a mode of network connection defined in 3GPP Release 19 where a User Equipment (UE) connects to the 5G network through a relay UE. In this architecture, the relay UE acts as an intermediary, forwarding traffic between the remote UE and the network. This is particularly useful for extending network coverage to areas with poor direct coverage, such as indoors, underground, or at cell edges, and for supporting devices with limited power or antenna capabilities. The concept is part of the broader work on 5G proximity services (ProSe) and sidelink relay, which was initially introduced in earlier releases for LTE and evolved for 5G in Release 17 and beyond.

The indirect network connection architecture involves several key components. The remote UE is the device that seeks connectivity but cannot directly communicate with the gNB. The relay UE is a UE that has a direct connection to the 5G network (via Uu interface) and can also communicate with the remote UE over a PC5 sidelink interface. The relay UE forwards traffic between the remote UE and the network, effectively acting as a Layer 2 or Layer 3 relay. The 5G core network, including the Access and Mobility Management Function (AMF) and Session Management Function (SMF), manages the connection and session for the remote UE, often treating the relay UE as an access node. The relay UE may be a dedicated relay or a regular UE that offers relay capabilities. The connection can be established using various relay discovery and selection procedures defined in 3GPP specifications such as TS 23.304 (ProSe in 5G) and TS 22.104 (Service requirements for cyber-physical control applications).

How it works: The remote UE discovers a suitable relay UE using sidelink discovery procedures. Once selected, the remote UE establishes a PC5 connection with the relay UE. The relay UE then establishes or uses an existing PDU session with the 5G network to forward the remote UE's traffic. The network authenticates and authorizes the remote UE, possibly with the help of the relay UE. The relay UE may operate in a transparent manner, where the remote UE's traffic is encapsulated and forwarded, or in a non-transparent manner, where the relay UE may terminate some protocols. The 5G system supports both Layer 2 and Layer 3 relay architectures, with Layer 2 relay being more efficient for user plane forwarding. The indirect connection can be used for various services, including enhanced mobile broadband (eMBB), ultra-reliable low-latency communication (URLLC), and massive machine-type communication (mMTC), depending on the relay's capabilities and network configuration.

The purpose of indirect network connection is to enhance coverage, reliability, and energy efficiency. It allows UEs that are out of coverage or have limited transmission power to access the network via a relay, thereby improving service availability. It also enables new use cases such as public safety communications, where direct device-to-device communication is essential, and industrial IoT, where sensors may be deployed in hard-to-reach locations. The concept aligns with 3GPP's ongoing efforts to integrate sidelink and relay technologies into 5G-Advanced and future 6G networks.

In summary, indirect network connection is a Rel-19 feature that leverages relay UEs to extend 5G connectivity. It builds on earlier ProSe and sidelink relay work, providing a standardized way for remote UEs to access the network indirectly. This mode of connection is expected to play a key role in expanding the reach of 5G services and enabling new applications.

Purpose & Motivation

Indirect network connection was created to address the limitations of direct network coverage in 5G, particularly for UEs that are out of coverage, have low power, or are in challenging radio conditions. It solves the problem of providing connectivity to devices that cannot directly communicate with a gNB by using a relay UE as an intermediary. This is essential for extending coverage in indoor, underground, or remote areas, and for supporting public safety and industrial IoT use cases. The concept evolved from earlier LTE ProSe and sidelink relay work, which demonstrated the feasibility of UE-to-network relaying. In Release 19, 3GPP standardized indirect network connection to ensure interoperability and to integrate it with 5G core network procedures, enabling seamless service continuity and efficient resource utilization.