SPN Empowers Changsha Rail Transit to Build a Premium Intelligent Metro

In June 2022, Changsha Metro Line 6 commenced initial operation, featuring ZTE's SPN equipment for its dedicated transport system adopts, marking the debut of the first private SPN transport network for rail transit in China.

The metro’s communication network is essential for guaranteeing the smooth  operation of the metro system. It encompasses complex communication services, including traditional services such as train control, scheduling communication, video imaging, detection and monitoring, as well as emerging services such as massive sensor data transmission and unmanned driving. These services vary in bandwidth and latency requirements, with train-related services demanding higher levels of security and reliability.

The conventional MSTP network, with a maximum bandwidth of 10 Gbps on the line side, cannot meet the rapidly growing bandwidth demand of metros and the convergence requirements of various service types. To address the specific technical requirements of Changsha Metro Line 6 for transport network deployment, ZTE has introduced an end-to-end SPN transport solution. Unlike traditional transport solutions, the SPN solution integrates TDM and packet switching, supports super-large bandwidth, and incorporates hard slicing and soft slicing to provide communication guarantees for both dedicated communications and the intelligent operation of metros.

Hierarchical Large and Small Granularity Hard Slicing Technology

The introduction of fine granularity unit (FGU) technology in Changsha Metro Line 6 marked its commercial debut in the existing rail transit networks. By utilizing its hard isolation and low latency features similar to SDH, production services with a bandwidth of approximately 100 Mbps—such as wireless communication systems and communication power monitoring—are carried over independent FGU channels to ensure physical isolation under the hard pipeline and meet the low latency transmission requirements of these services. For services with a bandwidth of 1000 Mbps or above—such as the video surveillance system, passenger information system, office automation system, and automatic ticketing system, FlexE large-granularity slices with a bandwidth of n×5G are employed. This enables efficient and rational allocation of bandwidth resources. In the metro service deployment, each line-side 100GE is divided into six large-granularity n×5G bandwidth channels, with one large-granularity channel further divided into four small-granularity channels to accommodate the requirements of different service channels.

Nested Slicing Based on Hard Slicing and Soft Slicing

The integration of hard slicing and soft slicing ensures the isolation of different types of metro services while improving the utilization of hard slicing bandwidth. As depicted in Fig. 1, 10 Gbps hard slice channels are allocated to metro video surveillance services within a total bandwidth of 100GE, ensuring complete isolation from other service channels. A PW/tunnel-based VPN (a basic soft slicing mode) is used to carry the video surveillance service from each station. At each station, a switching module facilitates convergence between the local PW/tunnel and those of other stations. These converged services are carried over the hard slice channel. Different service models such as virtual private wire service (VPWS) and virtual private LAN service (VPLS) can be deployed, maximizing bandwidth utilization of hard slice channels through statistical multiplexing technologies such as VPN.

Compared to the Ethernet over SDH (EOS) convergence function of traditional MSTP, which typically offers a backplane bandwidth of about 2.5 Gbps, SPN equipment can provide n×5G bandwidth to hard slicing services, with the maximum bandwidth reaching the line bandwidth. In addition, the convergence ratio of the virtual concatenation group (VCG) on the backplane of EOS is generally no greater than 48, posing constraints on networking applications. However, the soft slicing of SPN equipment, based on VPN technology, supports a significant number of PWs and tunnels, extending to the K level, fully meeting the application scenarios involving  multiple service types and a large number of sites and services.

Super-Large Bandwidth

A 100GE ring is deployed on the line side of the network to provide metro subsystems with redundant, reconfigurable, and flexible channels for information transmission and switching. Utilizing large-bandwidth elastic packet pipes, metro security check information can be collected in real time and processed centrally, facilitating simultaneous checking of both human bodies and objects. This approach improves the efficiency and accuracy of inspection and backtracking processes. Moreover, the SPN-based FlexE link bundling function binds two physical channels of 100GE interface to create a logical channel with large bandwidth. This enables high-rate services to be transmitted through low-rate physical ports and facilitates the smooth upgrade of line-side bandwidth to 200GE.

Looking ahead, a unified smart platform for metro development and operation will be built. The transport system will carry services such as smart O&M, smart travel, smart station, smart factory, smart driving, smart scheduling, and smart operation. The availability of 200GE ultra-large bandwidth on the line side will provide sufficient channel capacity for the development of this smart platform.

Supporting E1 CBR

In metros, the E1 service usually serves as an important production service, such as the dispatching telephone service, which has high requirements for service security, reliability, and delay indicators. SPN can carry E1 services at a constant bit rate (CBR). Client services are encapsulated into 10 Mbps small-granularity hard channels in the form of 66B blocks, without packet processing. This solves the soft isolation problem of the original PWE3 mode, ensuring not only service bandwidth and bit error performance but also providing delay performance similar to SDH. Through end-to-end FGU channels, E1 CBR ensures the hard isolation of E1 services on the entire path, meeting the high security and reliability requirements of metro production services.

Complete Network Protection Modes

The SPN device offers network-level hierarchical protection capabilities, supporting carrier-class fast protection switching with service switchover time of less than 50 ms. Additionally, it supports controller-based rerouting to achieve permanent 1+1 protection  similar to that of ASON.

Furthermore, SPN equipment supports VPN FRR/IP FRR protection on the customer service sublayer to provide redundancy protection to PE nodes. It also supports link aggregation group (LAG) protection to enhance bandwidth and link reliability. On the network transmission sublayer, SPN supports linear protection, dual-homing protection, ring network protection, and SR-BE protection in any topology. On the slicing channel layer, it supports MTN path 1 +1 protection function, with service protection switching time reduced to several milliseconds. The rerouting and slicing channel protection capabilities of SPN provide superior functions and performance compared to SDH and MSTP technologies, ensuring the high security and reliability of rail transit services.

Ultra-High-Precision 1588 Time Synchronization

The SPN equipment supports synchronous Ethernet function for stable and reliable frequency synchronization, along with PTP function for ultra-high-precision time synchronization, providing necessary synchronization signals to the wireless subsystem of metros.

SPN’s high-precision time synchronization includes ultra-high-precision clock sources and ultra-high-precision time transfer technologies. Time servers track satellites, enhancing performance from 100 ns to 30 ns. The ultra-high-precision time transfer technology improves the transmission time synchronization precision of SPN through optimized interface timestamp processing, evolution of the 1588 time synchronization protocol, and enhanced single-fiber bidirectional link symmetry. SPN can deliver ultra-high time precision of ±5 ns to each site, ensuring precise synchronization for various new metro services in the future.

Intelligent Management and Control

ZTE’s new universal management engine (UME) system integrates network management, service control, and network analysis functions, serving as the core system that enables network resource pooling, network connection automation and optimization, and O&M automation. Through the UME system, the SPN network achieves centralized network management and control, unified network-wide policies, end-to-end network visualization, and intelligent O&M, thereby improving O&M efficiency.

Moreover, it implements unified management of resources across the entire network, offering real-time visualization and visual analysis of existing network resource information across multiple dimensions, including services, tunnel connections, capacity, and sites, enabling accurate identification of network resource bottlenecks. Based on SDN technology, the SPN network supports centralized path calculation, ensuring optimal route determination during service provisioning, and preventing rerouting failures caused by resource conflicts during service recovery.

Additionally, the SPN network supports one-click automatic and rapid deployment, an end-to-end service quality detection mechanism, intelligent communication fault diagnosis, and automatic optimization and adjustment of channel bandwidth, providing strong support for smart metro operation.

Since the launch of Changsha Metro Line 6, its communications system has been operating smoothly, with all service performance indicators meeting the strict requirements of the intelligent rail transit system for transport networks. This serves as an important reference for future large-scale commercial deployment. ZTE remains committed to offering comprehensive technical support for the operation of the Changsha Metro communications transmission network, and strives to establish a premium rail transit network.