# A Bandwidth-Efficient Dissemination Scheme of Non-Safety Information in Urban VANETs +
## 2 Related Work
### Our Dissemination Scheme for Roadways  
We presented in [11] a dissemination scheme that is valid for roadways. Its broadcast suppression algorithm relies on a distance-based contention. Whenever a vehicle hears a new message, it waits a short time, \(W\), for an almost simultaneous duplicate of the same message. The distance to the closest vehicle from which it received the same message is stored as \(d_{min}\). Then, it assigns itself a forwarding priority based on this distance. This priority is given by the amount of time it waits before forwarding, \(t_{w}\), described by Equation (1):  
\[t_{w}=T_{max}\times(1-d_{min}/r) \tag{1}\]  
Here, \(r\) is the communication range and \(T_{max}\) is the maximum per-hop delay. The latter is a very important parameter, because it affects the latency of the scheme and it helps to prevent several vehicles from trying to forward almost at the same time. In addition, it has to be small enough to consider the scenario as almost static during the contention. The same value should work properly in very different traffic situations.  
This equation lets the vehicle that is furthest from the last relay to forward first. This way, the number of hops (and hence, the number of retransmissions) is minimized.  
A vehicle may hear a duplicate of the same message before the wait times out. We assume that messages include a time to live (TTL) field, so any duplicate with a lower value than what the vehicle observed in the first reception indicates that the message is old. Hence, the vehicle aborts the contention. If the duplicate comes in the opposite direction of the first message, that means that another vehicle's timer expired sooner and got to forward first. In such a case, this vehicle does not have to forward the message. If it comes from a vehicle located over the previous relay, it is ignored because it belongs to the dissemination in the opposite direction. If no other duplicates are received, this vehicle is the one who forwards the message.  
In addition, this scheme has a store-carry-forward mechanism to handle isolated groups of vehicles. The scheme does not rely on beacons from other vehicles, and this, in turn, is the cause of its main characteristics: First, the way for a relay to determine if another vehicle has taken its role after its retransmission is to hear a new duplicate with a lower TTL. If it does not hear any new  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & **Goal** & \begin{tabular}{c} **Relay** \\ **Selection** \\ \end{tabular} & **ACKed** & \begin{tabular}{c} **Junction** \\ **Detection** \\ \end{tabular} & \begin{tabular}{c} **Store-Carry-** \\ **Forward** \\ \end{tabular} & \begin{tabular}{c} **Needs** \\ **Infrastructure** \\ \end{tabular} & \begin{tabular}{c} **Needs** \\ **Beacons** \\ \end{tabular} \\ \hline UMB, 2007 & fast & distance-based & hand-shake & digital map & no & yes & no \\ \hline UV-CAST, 2011 & \begin{tabular}{c} efficient, resilient \\ to disconnections \\ \end{tabular} & distance-based & in beacons & digital map & yes & no & yes \\ \hline TAF, 2012 & efficient & distance-based & no & \begin{tabular}{c} reception \\ angles \\ \end{tabular} & no & no & no \\ \hline ABSM, 2012 & \begin{tabular}{c} reliable, \\ efficient \\ \end{tabular} & \
\begin{tabular}{c} clustering \\ (CDS construction) \\ \end{tabular} & in beacons & - & yes & no & yes \\ \hline eMDR, 2012 & fast &
\begin{tabular}{c} location-based + \\ distance thresholds \\ \end{tabular} & no & digital map & no & no & yes \\ \hline \hline \end{tabular}
\end{table}
Table 1: Summary of the main works for urban dissemination. UMB, Urban Multihop Broadcast; UV-CAST, Urban Vehicular Broadcast; TAF, Two Angles Forwarding; ABSM, Acknowledged Broadcast from Static to Highly Mobile; CDS, connected dominating sets; eMDR, Enhanced Message Dissemination Based on Roadmaps.  
duplicate in a time interval of \(W+T_{max}\) after it forwarded the message, it concludes that there is not any new relay, and so, it must store the message and try to find one later on.