Broadcasting Traffic Information over Bursty Channels

Telematics services, particularly road traffic information, are normally of interest to many drivers in a geographically large area. Therefore, digital broadcast networks, such as Digital Audio Broadcasting (DAB) are well suited for distributing such traffic information services. In this paper, DAB channel measurements have been performed under typical driving conditions in a hilly terrain with quite challenging propagation conditions. The results show that errors occur in bursts of varying length and frequency, interrupting long intervals of completely error free reception. Based on these measurements, different data transmission methods on ISO-OSI Layer III have been evaluated by simulations and optimized with respect to minimizing overhead and maximizing the probability of successful reception of data. The goal was finding a suitable tradeoff between the two contradicting objectives. Simulation results showed that only relatively small data objects, up to 500 Bytes, can be broadcast en bloc. For longer data objects, segmentation is essential. We therefore recommend the use of the Multimedia Object Transfer (MOT) protocol, which provides such a segmentation functionality. The additional overhead caused by the MOT protocol layer is by far compensated by its gains, i.e. increased reliability and reduced latency.