Time-stable geocast for ad hoc networks and its application with virtual warning signs

Virtual warning signs allow to increase road safety. Like real traffic or warning signs, they are attached to a certain geographical position or area. When a vehicle enters this area, the virtual warning sign is displayed for the driver. In contrast to real traffic signs they allow faster adaptation to the current road and traffic situation like icy roads or accident warnings. For the realization of virtual warning signs, geocasting is a promising approach. Existing geocast solutions lack the time-stability required for realizing virtual warning signs, which we call abiding geocast. An abiding geocast solution delivers a message either periodically or on demand, everytime a new vehicle enters the warning sign area. This article introduces the virtual warning sign idea and discusses the design space, three reasonable solutions and simulation results for abiding geocast in an ad hoc network. The first approach is an infrastructure-based server solution to store the messages. The second is an infrastructure-less approach that maybe more suitable for an ad hoc network. A node inside the geocast destination region is elected to act temporarily as a server for geocast messages. The last approach works infrastructure-less, too, by complementing the exchange of neighbor information necessary for many geographic unicast routing protocols with geocast information. In simulations we compare the delivery ratio and the resulting network load of the three approaches.