The Needs for Operational Applications
The operational applications relying on wireless communications can be divided into three main categories:
- • The communication and signaling systems for train control and command;
- • The monitoring systems to limit the risk of injury to persons and damage to property and ensure safe and reliable operations;
- • The video applications, such as CCTV and TV platform.
The Communication and Signaling Systems
The Different Strategies for Signaling Applications
A large number of techniques and strategies for signaling applications are deployed around the world. They all have the same basic objective: to keep a safe distance between trains. This safe distance can be maintained by measuring the current train position, its relative velocity to other trains and the other trains locations and directions of movement in the same area. All these data are continuously transmitted to other trains via wireless links. These continuous informations about trains’ close area allow to reduce inter-train intervals and thus increase traffic capacity without infrastructure investments.
Several such signaling systems are deployed depending on country: the Communication Based Train Control (CBTC), the Advanced Train Control System (ATCS), the Command, Control and Communication System (C3S), the Incremental Train Control System (ITCS), the Positive Train Control (PTC), the Positive Train Separation (PTS) or the European Train Control System (ETCS). The standard IEEE 1474 establishes the performance and functional requirements for a CBTC system .
The main operational functions, often linked to each others, can be identified below:
- • The Automatic Train Protection (ATP): ATP is a general function, that consists of a railway technical installation to ensure safe operation in the event of human failure. Different systems are deployed around the world relying on inductive systems, cab signaling or radio-based systems. The communications between the wayside and the train is then assured by inductive loop and radio frequency transmissions;
- • The Automatic Train Control (ATC): ATC is a general class of ATP that involves a speed control mechanism in response to external inputs. It is considered to be the safety-critical part;
- • The Automatic Train Operation (ATO): ATO is an operational safety enhancement device used to help automate operations of trains;
- • The Automatic Train Supervision (ATS): ATS refers to a system within an ATC system which monitors the system status and provides the appropriate controls to direct the operation of trains in order to maintain intended traffic patterns and minimize the effect of train delays on the operating schedule.
A CBTC or an ETCS system is described as an ATC system using high resolution train location determination; continuous, high-capacity, bidirectional train- to-wayside data communications; and train-borne and wayside processors capable of implementing ATP functions, and optional ATO and ATS functions . CBTC applications include the train localization, the train-to-ground transmission and all the communications between the computers in the train and the ground computers .
Safety-critical applications generally rely on low throughput (10-100 kbps) but require a high availability (at least 99.999 % of the time) and high levels of robustness and reliability (typically a packet error rates of 10-3 for 200 ko length packets) . Furthermore, KPI have to be checked regardless of the mobility conditions: handover times below 10 ms, time of communication establishment, etc. All these KPI are described in the IEEE 1474 standard .