The Class A devices are the components of the Railway Signalling (CCS, Control-Command and Signalling) directly conceived and designed in the ERTMS era (after 2000) to be able to connect the various parts of the subsystems that compose it: trackside (ACC/ACCM, SCCM, RBC, PL, Radio Block Center), on-board (EVC: European Vital Computer) and teleradiocommunications (TLC/GSM-R) of the ERTMS ERTMS itself. All this is the same for all European countries.
The Class B devices are the components of the CCS railway signalling designed before the ERTMS era (pre-2000) by each individual nation. Today, in order to be connected with ERTMS-ACC/ACCM, these devices must have several interfaces with ERTMS (GAT: link from ACC/ACCM) itself and with EVC (STM: link with Balise No European and with other national devices), as established by ERTMS specifications, to ensure homogeneity among European nations.
In principle, it has not forbidden the use of Class B devices for ERTMS, but has provided for appropriate interfaces with the ground (GAT) and the train (STM). This is so that trains travelling throughout Europe have a homogeneous “running environment”, without having to change locomotives and drivers at the borders of each nation (technical and operational interoperability).
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