5G-R Reliability Solution Ensures Railway Digitalization

With the development and popularization of 5G technologies, they are being extensively applied in various fields. In September 2023, the approval of the 5G for railway (5G-R) test frequency has garnered wide attention, indicating that railway communication is also entering the 5G era.

The services carried by the 5G-R system include high-speed railway dispatch, command, operation control, security monitoring, emergency handling, and automatic driving. Ensuring the system’s reliability and security is paramount. ZTE has actively explored this area and proposed a comprehensive reliability solution (see Fig. 1). This solution incorporates a hardware redundancy and  backup architecture. Leveraging various network innovation solutions, the network reliability is assured at multiple levels, including the network level, node level, link level, board level, and component level, thereby facilitating the safe operation of railway trains.

Network-Level Reliability Solution

ZTE's 5G-R core network meets the requirements for railway application scenarios and reliability, and provides a redundancy backup mechanism at multiple layers, including network-level disaster recovery backup, intra-device redundancy design, inter-NF redundancy design, and link-layer redundancy design, comprehensively guaranteeing the robustness and reliability of the 5G-R network.

For network deployment, the core network is structured around railway bureaus. Two sets of core networks are deployed for each railway bureau to form a remote dual-DC disaster recovery and backup network. Redundant NFs are deployed in separate DCs. When one DC or NF is faulty, the peer NF automatically switches over to recover services upon detecting the fault.

At the core network device level, ZTE’s 5G-R core network employs an NFV cloud architecture design. Various types of VMs or containers that compose the NF use the active/standby or load sharing mechanisms and anti-affinity deployment. The core network is built on highly reliable cloud resource pools, and provides elastic scaling and redundancy of resources through the self-healing, regeneration, and migration capabilities of the virtualization platform.

At the NF layer of the core network, active/standby or load sharing mechanisms are in place, with NF faults rapidly detected through the dynamic registration and discovery mechanism of the network repository function (NRF). The core network provides an enhanced redundancy solution for access and mobility management function (AMF) sets and optimizes service recovery experience through a hot-standby redundancy mechanism between two AMF sets. The core network supports the NF bypass function. If multiple NFs in a remote dual-DC scenario are faulty simultaneously, basic services can be degraded and remain available.

At the link layer of the core network, each link of the core network employs dual-plane dual-network architecture. If any link fails, a backup link is available. When the N4 link between the control plane of the core network and the UPF encounters a fault, inertial operation ensures that services in a connected state are not affected.

Node-Level Reliability Solution

The node provides both base band unit (BBU)-level and remote radio unit (RRU)-level backup solutions.

The BBU operates in hot standby mode, with the active and standby BBUs connected in a star or chain topology. Both BBUs are connected to multi-level RRUs via optical fibers and are deployed remotely along the railway or at the same site. Two RRUs are deployed at the same site. The RRUs are split first, and then connected side by side to achieve 10 MHz network coverage. After the dual-BBU system is activated, the RRUs establish a connection with the active BBU, which provides services. If the active BBU fails, the RRU switches over to the standby BBU via the optical interface, establishing a connection with the standby BBU, which provides services, minimizing service impact time to just seconds. When a single point of failure occurs on the RRU, the BBU ensures that services are maintained with only a reduction in service quality level.

In hot standby dual-active mode, two RRUs are deployed at the same site. After the RRU system starts normally, the two RRUs are split and then connected side by side to establish a hot standby dual-active configuration. Each 8T8R RRU is split into two 4T4R logical cells, and the baseband unit combines one 4T4R logical cell from each RRU into an 8T8R logical cell. This improves spectrum efficiency and supports large-capacity services. When one RRU device fails, services continue smoothly with only a reduction in quality, effectively combining spectrum efficiency and reliability.

The RRU devices support fast switchover in different single-point fault scenarios. They support fast switchover of optical ports in free-competition mode after the RRU devices are started, fast switchover of optical ports in forced mode that is determined and initiated by the BBU side, and directional switchover of optical ports in active-active mode if an RRU is faulty.

Board-Level Reliability Solution

For a single BBU shelf, the solution includes redundancy for the transmission main control board, baseband processing board, and power board to deal with potential board faults.

The transmission main control board operates in hot standby mode. When the main control board is functioning normally, the containers providing services are deployed on both the active and standby boards in a 1:1 ratio. It performs real-time backup of the public data, including the database, the private data stored on the service module, base station clocks, and communication data. Data consistency is audited periodically during operation to ensure that the backup data is timely and accurate. The combination of hardware interrupt signals and a software fast detection mechanism shortens the fault detection time, enhances the system’s sensitivity to faults, and can identify faults at the second level and trigger a switchover.

When the system detects a fault and triggers the switchover of the main control board, the container of the standby board operates quickly. The service logic cell is neither deleted nor re-established, reducing service recovery time. Additionally, the baseband board and RRU device are not reset. The external clock, communication and data link are quickly switched over to the new active board, allowing services to be recovered within seconds.  

Link-Level Reliability Solution

Links include fronthaul links between the BBU and the RRU, as well as backhaul links between the BBU and the bearer devices. The BBU and RRU are connected via optical fibers. The optical fiber link-level reliability solution mainly relies on the RRU ring network, which requires two pairs of optical fibers. When an optical fiber CPRI link bearing services is faulty, fast detection through the millisecond-level optical interface triggers the forward optical interface switchover, ensuring that services are not interrupted.  

Transmission protection is implemented for the backhaul link between the BBU and the bearer devices. When a transmission link experiences an abnormality, the transmission protection mechanism is triggered through a millisecond-level fast detection mechanism, without triggering board switchover. This improves switchover performance while reducing the impact on services.

Component-Level Reliability Solution

Based on years of R&D experience in the industry, ZTE selects top-tier components and processing technologies and uses advanced chip technologies and high-quality materials in production, ensuring product stability while enhancing flame retardant and anti-corrosion properties. For the 5G-R scenario, a specialized reliability solution is developed and manufactured using a dedicated line in an automated factory. All-around intelligent testing and verification are performed to ensure product reliability and adaptability in various harsh installation and deployment scenarios.

With the widespread application of 5G technologies, the 5G industry chain has become increasingly mature, laying a solid foundation for upgrading railway private networks. Accelerating the technical research, promotion, and application of the 5G-R system in the railway industry will further enhance China's railway informatization and intelligence, advance China's railway innovation capability, and contribute to the global development of high-speed railways. ZTE will continue to innovate and work with China State Railway Group to safeguard railway operation security.