# Towards Intelligent, Programmable, and Open Railway Networks
## 2 ETCS Levels and Architecture  
The deployment of ETCS application levels depends on the existing railway infrastructure. ETCS Level 1 provides train cab signaling where movement authorities can be provided through switchable, fixed Eurobalises and Euroloops. The Eurobalises installed on the trackside send data about the train route to the onboard equipment, and based on the data, the maximum train speed and breaking curves are calculated. In addition, the Euroloops (loop infill) or radio-infill solutions are used for transmission of distant signal data to trains over a long distance. ETCS Level 2, the radio block center (RBC), sends movement authorities to the onboard equipment using the GSM-R, so no line-side signals are required. Eurobalises, acting as passive positioning beacons, help the train to calculate its position, which is refined by additional sensors. In ETCS Level 3, the trains continuously monitor their own position and provide train position data to the RBC. Level 3 is a fully radio-based system without track-based detection equipment. Based on the received train positioning data, the RBC calculates the smallest safe train distance at any time. The role of the train integrity functions is crucial as the trackside equipment disappears. ETCS Level 3 is under standardization. The ERTMS/ETCS system architecture includes onboard ETCS equipment and track-side ETCS equipment [19; 20]. The onboard equipment consists of a central logic unit, rail path sensors, control devices, cab displays, and a module for radio communications. The trackside equipment is a fixed part of the ETCS installation, and depending on the ETCS level, it can be composed of Eurobalises, Euroloops, a lineside electronic unit (LEU), radio infill unit (RIU), key management center (KMC), interlocking system, and RBC. AnEurobalise is a system for the transmission of safety-relevant information between the train and the trackside equipment. It consists of beacons situated along the tracks that transmit when a train antenna is above them, an onboard balise transmission module and antenna unit, and a trackside signaling system. Fixed data balises provide special information, such as speed restrictions and gradients. Transparent data balises are connected to the LEU, which transmits dynamic data to the train, e.g., signal indications. An Euroloop is a leaky feeder system that transmits the signal aspects to the trains. An RIU is a radio-based communication system that allows to send the message of the next signal in the travel direction in advance to the train before passing the relevant information point. The interlocking system is responsible for the supervision and safety control of routes, switch points, signals, and track locks. It prevents conflicting train movements through arrangements of tracks, such as crossings and junctions. The KMC is responsible for the management (installation, update, and deletion) of the cryptographic keys that are used to secure radio communications between the ETCS entities. The main task of the RBC is to monitor and guide the trains in its area. It uses the train and trackside information to generate movement authorities and to send them to the trains by radio. The implementation of the RBC function depends on the existing railway infrastructure and the operator's requirements. The functions, architecture, and user interfaces of the RBC are not standardized, are structured differently by various manufacturers, and are centered around safety-relevant functionality. Typical RBC functions include generating movement authorities, monitoring trains' movements in the RBC area, announcing radio gaps, evaluating potential hazardous situations, triggering emergency stops, the first assignment of a train route, joining and splitting of trains, adaptive speed control, stop evaluation, sending the national values to trains, determining safe reactions in case of dangerous situations on a train, etc. The operation of the RBC essentially enables dispatcher control and maintenance access.