Mission critical synchronization in railways: ensuring safety

This week’s nationwide disruption to the UK rail system highlighted just how critical the underlying telecoms capability is to ensuring the safe and reliable operation of this key infrastructure.

In the world of railways, synchronization isn’t just about keeping the trains on time; it’s also about ensuring the safety of every passenger and staff member. This article will explore how mission-critical synchronization is an essential pillar in modern railway systems.

Understanding GSM-R and its synchronization requirements

Global System for Mobile Communications – Railway (GSM-R) is a specialized communication system used for railway operations. It provides secure and reliable voice and data communication between equipment, people, and railway control centers. One key aspect of GSM-R is its reliance on precise time and frequency synchronization to maintain communication integrity and efficiency.

GSM-R networks require synchronization accurate to +/- 50ppb, which is typically achieved using an E1 signal provided by a Primary Reference Clock (PRC), and SyncE or NTP over IP/MPLS. Any E1 “slips” can cause a loss of synchronization with a direct correlation in loss of up/down link of the Base Station.

Accurate synchronization helps ensure that signals and control messages are received and processed in real-time, which is essential for preventing accidents. This involves synchronizing base stations and network elements to a common time reference, typically provided by GPS or timing sources mentioned above. Any deviation in synchronization can lead to communication delays or failures, compromising railway safety.

The role of synchronization in GSM-R

Synchronization has a pivotal role in ensuring the seamless and safe operation of modern railway systems. Accurate time and frequency synchronization are essential for coordinating train movements, managing signalling systems, and enhancing overall operational efficiency. Without proper synchronization, the risk of collisions and other safety hazards increases exponentially.

Synchronization ensures that all subsystems, from trackside equipment to onboard controls, operate in harmony. This not only facilitates timely train departures and arrivals but also ensures that safety protocols and regulations are consistently upheld.

ETCS and ERTMS: the backbone of European railway safety

The European Train Control System (ETCS) calculates a safe maximum speed for each train and grants movement authority, permission to proceed to the next railway fixed block section. It includes cab signalling for the driver and on-board systems that take control if permissible speed is exceeded or movement authority is denied.

ETCS level 2 utilises GSM-R to communicate permissions, signalling and control of trains to the cab. GSM-R is standardized, which allows for the interoperability of cross-border train travel while maintaining safety. ETCS relies wholly on GSM-R to provide movement authority direct to the cab, removing the requirement for trackside hardware. Train positioning, direction of travel and speed are communicated to the Radio Block Center (RBC) via GSM-R.

Any movement authority is transmitted to the train continuously via GSM-R increasing the importance of a well synchronised and stable GSM-R network. Loss of connectivity means the ETCS cannot communicate with the trains to provide movement authority resulting in trains stopping on the track which could result in a catastrophic incident.

Both ETCS and GSM-R are subsystems within The European Rail Traffic Management System (ERTMS) which integrates various railway systems to optimize traffic management and safety.

ETCS, GSM-R and ERTMS rely heavily on precise time and frequency synchronization. Accurate synchronization ensures that signalling information is transmitted and processed in real-time, allowing for immediate responses to potential safety threats. This level of precision is critical for maintaining the high safety standards required in European railways.

The future of railway synchronization

At present, train-to-ground communication systems use GSM-R. However, GSM-R does have its limitations: fewer carrier services, narrow bandwidth and low throughput, making it difficult to meet future requirements such as intelligent and automated high-speed railways. Use of GSM-R will likely evolve to long-term evolution for railway (LTE-R), which is the next generation of high-speed railway train-to-ground communication systems. Compared with GSM-R, LTE-R significantly reduces network complexity, increases bandwidth and provides more carrier services.

LTE-R will be required to carry the traffic safety control commands discussed. Time Synchronization will remain key to ensuring the safe operation of next generation railway networks. At present, GSM-R often uses NTP to complete train-to-ground time synchronization. However, NTP has some disadvantages, such as a low synchronization accuracy, large delays, and poor stability of the transmission process, which makes it likely unable to meet the high-precision time synchronization requirements of the LTE-R systems of the future.

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