China Unicom fiber optic cable fell down

A physical cut in China Unicom's core optical cables caused partial 4G and 5G base station outages, affecting network management and some voice services, but data forwarding remained largely unaffected.

Incident Overview

The disruption occurred when two main core optical cables were physically cut, triggering a dual-link failure in the local transmission network. This was due to single-path sharing of a single physical node, where two OTN loops lacked physical separation, making the network vulnerable to a single cable cut. The incident led to equipment disconnections and partial outages at multiple base stations.

Impact on Services4G and 5G base stations reported link-down alarms, causing temporary loss of management connectivity. Subscriber services experienced abnormal voice calls, and both registration success rates and call completion rates declined. Data forwarding services were not affected, meaning internet traffic continued to flow despite the management disruptions.

The incident coincided with a surge in IoT device updates, which contributed to network congestion and amplified the impact.

Network Response and Restoration

China Unicom conducted a VPN service cutover on a core network device, reconfiguring VPNs on new LoopBack interfaces. During this process, some static routes were invalidated, temporarily affecting base station monitoring. After reconfiguration, all services were fully restored.

Broader Context

China Unicom is actively involved in international submarine cable projects, such as the Asia Link Cable (ALC) and SEA-H2X, which enhance connectivity between China, Southeast Asia, and other regions. These cables feature intelligent optical network scheduling and dual-landing architectures to ensure high reliability and rapid automatic link switching. While the recent incident was local and terrestrial, it highlights the importance of physical cable protection and network redundancy in maintaining uninterrupted service.

Key Takeaways

Physical cable cuts can cause dual-link failures if network loops share a single physical path. Even with partial outages, data services can remain operational if proper routing and redundancy exist. Proactive network design, including diverse routing and intelligent optical scheduling, is critical to minimize service disruptions in both terrestrial and submarine networks.