STEID: A Protocol for Emergency Information Dissemination in Vehicular Networks * Josiane Nzouonta and Cristian Borcea Department of Computer Science New Jersey Institute of Technology {jn62, borcea}@njit.edu Abstract— This paper presents STEID, a spatio-temporal emergency information dissemination protocol for vehicular net- works. Its main goal is to quickly disseminate traffic alerts about accidents or congestions to every car that passes through an emergency zone during the lifetime of the emergency. To achieve this goal, we propose a hybrid network architecture consisting of WiFi clusters connected through proxy servers and cellular links. STEID executes on top of this architecture and ensures the emergency message delivery to all the intended vehicles in a short time interval. Simulations conducted with hundreds of vehicles moving at speeds between 45mph and 70mph demonstrate this fact. We also compare STEID to a solution using only cellular communication with a central server for information dissemination. The results indicate that the average delay for STEID is between two and four times less than the one for cellular-only approach for regular highway speeds, while the traffic load imposed on the cellular network decreases by more than 65%. I. I NTRODUCTION In recent years, most new vehicles come equipped with GPS receivers and navigation systems. Car makers such as Ford, GM, and BMW have already announced efforts to include significant computing power inside their cars [2], [13]. We envision that in the near future the number of vehicles equipped with computing technologies and wireless network interfaces will increase dramatically. These vehicles will be capable to run network protocols that will exchange messages for safer and more fluid traffic on the roads. Avoiding accidents and traffic jams are two main immediate benefits of vehicular networks. For instance, most drivers would like to receive real-time alerts about accidents hap- pening at a short distance in front of their vehicles at night, in the fog, or heavy rain. This type of accidents could lead to collision chains involving tens of vehicles. Although not life-threatening, traffic jams are huge nuisances. The ability to receive an alert about a potential traffic jam would allow drivers to take alternate routes, saving both time and fuel. The problem addressed in this paper is: How to ensure that a traffic emergency message is disseminated with high probability and low delay to all the potentially affected vehi- cles? If we assume that participating vehicles (not all moving vehicles) are equipped with an embedded computer, a GPS receiver, navigation software, and one or multiple wireless network interfaces, the problem can be reduced to designing a * This work is supported in part by the NSF grant CNS-0520033. reliable and efficient safety information dissemination protocol in vehicular networks. To understand the requirements for such a protocol, let us consider the scenario illustrated in Figure 1. When an accident or other event (e.g., road work) susceptible to lead to traffic congestion happens, a traffic alert message is generated. Such a message is either triggered automatically by a car involved in an accident (e.g., a sensor for inflated airbags could generate an event propagated to the embedded computer through the on-board diagnostic system interface), by a person (driver, road worker, or police officer) or by a system capable of detecting/predicting traffic jams. Such an alert is characterized by: • the location of the emergency, defined as a pair of road and relative position of the emergency on that road. • the emergency zone or alert region, defined as all the roads leading to the emergency location. • the lifetime of the emergency, defined as the time needed to return to regular traffic conditions after an emergency. These characteristics are determined by applications running on vehicles. Let τ max be the maximum safest delay in deliver- ing the message to a vehicle. This value could be the average time it would take a vehicle to move from the entry of the alert zone to the nearest exit (to avoid traffic congestion). If the dissemination protocol ensures that every vehicle already in the zone or every new vehicle entering the zone during the emergency lifetime receives the alert within τ max seconds, we say that the protocol provides both spatial and temporal reliability as defined below. Spatial Reliability: A protocol provides spatial reliability if the alert is delivered to all participating vehicles within the emergency zone. Temporal Reliability: A protocol provides temporal reliabil- ity if a participating vehicle does not spend more than τ max time within the emergency zone before receiving the alert. For instance, spatial reliability means that all vehicles present in the alert region in Figure 1.a must receive the alert. Figure 1.b helps us to understand temporal reliability; vehicles G, I , and H, which entered the zone after the accident happened, must receive the alert before reaching the nearest exit. This paper presents the Spatio-Temporal Information Dis- semination protocol (STEID), a protocol that addresses both the spatial reliability and the temporal reliability requirements. STEID ensures timely delivery of alert messages to all the