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JPL's Wireless Communication Reference WebsiteChapter: Network Concepts and Standards
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Figure 10: DSRC protocol implementation (AFC and DRG)
To make the investment of an infrastructure built up by vehicle-roadside communication systems most beneficial, an optimal use of the system capacity can be achieved by offering several services at the same beacon site. Furthermore the user acceptance of newly introduced mandatory road pricing systems can be raised by combining them with additional services, which provide the driver with information about the current traffic situation and help him to find the optimal and safest route to his destination (such as Dynamic Route Guidance/Emergency Warning). An important requirement of road authorities and motorway operators is, that the reliability of a high-priority transaction should be not affected by the additional services. Therefore, in order to minimize the interference between services, an head-of-the-line priority strategy has been introduced.
As an example for a combination of different services at one beacon
two services are introduced, whose combination seems to be very
likely in a motor-way environment.
Automatic Fee Collection
With the Automatic Fee Collection application, a specific amount of
money has to be collected from all vehicles in return for the usage of
the road. Therefore it is a mandatory application with a high
priority. In an interactive
dialogue, which is initiated by the beacon, information
related to the authentification procedure, vehicle classification,
amount of money to be paid, etc. is exchanged (PresentationRequest,
PresentationResponse, TransactionRequest). Typically,
the money is debited directly from a ICC (Integrated Circuit card)
in order to protect the driver's anonymity.
After the ICC operation, the debiting process is
acknowledged by the vehicle in another short exchange of data
(TransactionResponse, TransactionAcknowledge).
The figure 10 shows the implementation of an
Automatic Fee Collection application process using the DSRC protocol.
Due to the usage
of security mechanisms required to protect the financial transactions,
the ICC operation time is typically much longer, than the actual
communication time on the link (typical values are around 40 ms for the
ICC operation and 12 ms for the data exchange).
Dynamic Route Guidance
The Dynamic Route Guidance (DRG) application distributes information
from regional traffic centers about the current traffic situation
in a broadcast message (INFO message) by the beacon in order to allow
the driver or an on-board equipment to find the
optimal route to his destination.
To determine the traffic situation
the regional traffic center relies on information about the travel
times of vehicles, which are therefore transmitted to the beacon
(TravelTime report).
Combination of AFC and DRG
In the scenario, that is considered here, it is assumed, that the operator
has assigned a higher priority to the AFC service. After having
received the ID of a new vehicle, the beacon will first start
the AFC application. For the DRG
service only the remaining system capacity will be available. An
on-going DRG service will have to be interrupted immediately in case
another vehicle needs to start the AFC application. The typical
implementation would have the following characteristics (see figure
10):
