Why ETCS over GPRS?
ETCS Level 2 is a digital radio-based signal and train protection system. All trains automatically report their exact position and direction of travel to the RBC (Radio Block Centre) at regular intervals, trhough the GSM-R network.
GSM-R is a circuit switched connection, though which a continuous channel is set between the base station and the moving unit. This feature ensures a high Quality of Service, but not a a very efficient usage of the connection, as the amount of data exchanged is rather low. This is the reason why GSM-R has been considered in the last years as the bottleneck for the efficiency of the whole ERTMS system.
A milestone in the evolution of GSM-R in packet networks is the introduction of general packet radio services or GPRS to complement current services with packet data.
In packet switched communication, more than one user shares the same connection and the information transmitted is split up into packages sent each one after another, and only when there is a need for it by one of the user.
The basic principle of the ETCS over GPRS for railway application is building a data-oriented system which re-uses as much as possible of the existing voiceoriented system, GSM-R, and keeping its compatibility.
Once GPRS is activated, a virtual continuous connection between the network participants (e.g. train and RBC) is established. During a session, a user is assigned to one pair of uplink and downlink frequency channels. This is combined with time domain statistical multiplexing, i.e. packet mode communication, which provides the common use of one frequency for several users. Only if information (e.g. ETCS telegrams) must be transmitted, the radio link will be reserved by a user .
Currently UNISIG consortium’s working to standardize ETCS over GPRS as the communication technology chosen for ERTMS application and next years all railway suppliers will realize ETCS lines equipped with GPRS-R network infrastructures.
A succinct but comprehensive summary of what’s going on with that has been provided at the last ERTMS Conference 2014.
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