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The DSRC system mainly consists, in terms of radio, of two units: the Road Side Unit (RSU) and the
On Board Unit (OBU). The RSU is placed at the tollgate, being responsible for starting and ending
the communication with the OBU, in order to carry out the transaction. Of the two units, it can be
said that the RSU is fully active, since it is always connected and is the only one which transmits a
carrier, so as to provide communication. This carrier is also used by the OBU so as to enable it to
communicate, by reflection, with the RSU. The OBU is the equipment placed inside the vehicle in
order to permit its identification. This module only has to generate a sub‐carrier, which modulates
the carrier received from the RSU, reflecting it back to the RSU. The OBU is passive, given that it
does not generate a carrier to communicate with the RSU. Furthermore, the OBU is only active when
it detects the RSU carrier, its consumption thus being minimised.
The use of the MDR standard makes it possible for the communication between the OBU and the
RSU to be established through microwaves or infrareds. The following figure shows a schematic
representation of the module installed on board the motor vehicle and its links to the various
technologies available for operating the system.

Figure 63 – OBU and different technologies

Source: European Patent Application EP 1944736 A1
These technological projects form a part of wider initiatives on matters such as road safety (for
example the European Commission's eSafety initiative) and road tolling.
The goal of intelligent transportation systems (ITS) is to improve the effectiveness, efficiency, and
safety of the transportation system. Effective deployment of ITS technologies depends in part on
the knowledge of which technologies will most effectively address the issues of congestion and

304 ITU‐T's Technical Reports and Specifications