Smart Broadcast algorithm for inter–vehicular communications Elena Fasolo, Roberto Furiato, Andrea Zanella University of Padova – Department of Information Engineering Via Gradenigo 6/B, 35131 Padova, Italy {andrea.zanella, fasoloel, gim14}@dei.unipd.it Abstract— Broadcast is a fundamental service for self– organizing wireless networks but its importance increases in the context of inter–vehicle communication (IVC) sys- tems where it can be used to propagate urgent informa- tion, as, for instance, hazards or alarms, and to develop safety automatic services. In this case, broadcast protocols have to guarantee high reliability and low propagation de- lay of the information all over the network. In this paper we define a distributed, position–aware broadcast proto- col (Smart Broadcast) for highway inter–vehicular networks, where each node, receiving a broadcast message, forwards it after a random backoff delay that depends on the node distance from the source. Then, by simulations, we evalu- ate the broadcast protocol performance and compare it with other algorithms. Finally we conclude with some theoretical observations. Index Terms—Ad hoc, broadcasting, Broadcast Protocol, Minimum Connected Dominating Set, inter–vehicular net- works 1. I NTRODUCTION One of the most interesting research trends of the last period regards inter–vehicle communication (IVC) sys- tems, also known as Car Networks. These systems are intended for a broad range of applications, including pri- mary services such as emergency notification in cases of accidents, but also more advanced applications as cooper- ative driving assistance, car–to–car audio/video commu- nications, nomadic Internet access, and so on. IVC represents, now, a challenging scenario for ad hoc based communications. On the one hand, the presence of car batteries looses the constraints for energy–aware communications; on the other hand, services related to the car–mobility require new design paradigms at most layers of the OSI model. Hence, the peculiarities of the IVC scenario ought to be exploited in order to gain advan- tages in the design of physical, MAC and routing layer solutions. In particular, such peculiarities include the availability of timing and localization information pro- vided by the Global Positioning System (GPS). Recently, some European/International projects have started research activities on this topic. At the physi- cal layer, the main trends are on using the UTRA–TDD and the IEEE 802.11 standards, though the latter one is getting more and more attention. The research effort has been mainly addressed to the medium access mech- anisms, routing protocols, network management strate- gies, and applications provisioning. However, little at- tention has been devoted to the design and analysis of efficient and reliable broadcast propagation mechanisms, which are of primary importance in the IVC scenario. Broadcasting, indeed, is widely used in self–organizing wireless networks, for management, control and data ex- change purposes. In particular, an efficient and reliable broadcast mechanism can be a fundamental service in IVC scenarios for propagating alert messages to the up- coming vehicles [1]. In this study we define and analyze a practical broad- cast mechanism for inter–vehicular networks, named Smart Broadcast, which performs rather closely to the theoretical bounds without requiring perfect knowledge of the network topology. The algorithm is evaluated by means of extensive simulation campaign, proving to be able to guarantee high reliability, low propagation latency and redundancy reduction. . This paper is organized as follows. In Section 2 we overview related works and some broadcast protocols. In Section 3 we describe, in detail, the Smart Broadcast Pro- tocol and in Section 4 some simulation results are shown. Finally we conclude with some observations and future works. 2. RELATED WORK The simplest broadcasting mechanism is the flooding, where the broadcast message is re-broadcasted by each node that gets the message. Clearly, this technique is simple but, in a CSMA/CA scenario, it may lead to high collision probability and high data redundancy (Broadcast Storm Problem [2]), thus resulting rather inefficient in terms of radio resource usage, promptness of the message delivery and reliability. On the contrary, in a TDMA fashion there is not the collision problem so that the broadcasting is simply performed by flooding although this approach lacks in flexibility and time delay performance. The optimum broadcast algorithm is based on the MCDS (Minimum Connected Dominating Set), which is defined as the minimum cardinality set of connected IWS 2005 / WPMC’05 - Aalborg, Denmark Copyright  2005 WPMC 1 18-22 September 2005