Broadcast Scheme (D-DAPS)
2.5 Broadcasting in VANETs
2.5.2 Position-Based Broadcasting
Position based methods privilege transmission to vehicles that cover a new additional area, or new vehicles.
39
The proposed schemes under this classification use GPS devices to obtain a vehicle position to determine relay vehicles which have the best coverage. Some schemes use the vehicle signal strength instead of GPS when it is unavailable- to decide if the vehicle can act as a best coverage vehicle. Below is a description of the major schemes under this classification.
2.5.2.1 The Last One Broadcast Scheme (TLOB)
The TLOB scheme [76] is designed to broadcast a safety message effectively with minimum end-to-end delay and overhead. It assumes that each vehicle is equipped with a GPS, so every vehicle can easily obtain the geographical location of other vehicles within its communication range. When an accident occurs, the victimized vehicle starts broadcasting a warning message to all vehicles close to the accident. Upon receiving the warning message, each receiver vehicle does not rebroadcast it immediately, but runs the TLOB scheme to find the last vehicle which is able to send the warning message to new vehicles. Each vehicle knows the distance value between itself, the victimised vehicle and its surrounding neighbours, so the receiver vehicle can easily check if there is a neighbour vehicle further than it.
If the receiver vehicle is the last vehicle, it rebroadcasts the warning message immediately, while other vehicles wait for a time period to take the next rebroadcast decision. After the timer expires, the vehicle checks whether it has received the message from any surrounding neighbours. If no message was received the vehicle rebroadcasts it, otherwise the message is dropped.
2.5.2.2 Least Common Neighbours Scheme (LCNS)
The LCN scheme [77] is a selective flooding scheme for forwarding emergency messages by initializing a timer for each vehicle based on its common neighbours. This
40
scheme aims to privilege rebroadcasting for only one candidate among the vehicles within the same radio communication.
It assumes that if the distance between the sender vehicle and any receiver vehicle is very small, the number of common neighbours increases. On the contrary, if the distance between them is very far the number of common neighbours decreases. Based on this rule, the vehicle that has the least common neighbours is furthest from the sender, and so the best candidate to rebroadcast the emergency messages. The best candidate also sets its timer value according to the number of common neighbours. In some cases, many candidates rebroadcast simultaneously since they have the same common neighbours. To solve this problem, the LCN scheme uses a random function to adjust the timer value to the vehicles that have the same number of common neighbours.
2.5.2.3 Street Broadcast Reduction Scheme (SBRS)
The SBR scheme [78] is a novel scheme proposed not only to mitigate the broadcast storm problem in VANETs, but also to speed up delivery of the warning message notification and increase its reachability. In the SBR scheme, vehicles operate in two modes: warning mode, and normal mode.
Warning mode vehicles are responsible for broadcasting warning messages among the vehicles to inform them of their status. Such warning messages broadcast periodically every Tw seconds and have the highest priority at the MAC layer. On the other hand, the
Normal vehicles mode helps to disseminate the warning message to other vehicles in the street periodically every Tb seconds. They are also responsible for sending beacons
including information such as their positions and speeds, etc. The beacons have lower priority than the warning messages and are not rebroadcast by other vehicles. The SBR scheme allows the receiver vehicle to rebroadcast, only if the received message is a warning message, received for the first time, and the distance between it and the
41
receiver is larger than the distance threshold or both vehicles are in different streets. Otherwise, the message is discarded, as it is either a duplicated message, a beacon message, it does not cover additional coverage area, or it is in the same street.
2.5.2.4 Location-Based Flooding Scheme (L-BFS)
Emergency Warning Message (EWM) methods-based location and counter broadcast schemes have been proposed in [79], to suppress the broadcast storm problem, and achieve better network connectivity and message reachability. A random back-off delay timer function that has a Gaussian distribution is adjusted based on the vehicle position with regard to the previous forwarder. The lower mean value of the probability distribution function of the delay timer means that the vehicle is very close to the previous forwarder, and it has a high priority to forward the received packet. Max_count threshold is used to control the rebroadcast decision and minimize redundant forwarding. Each vehicle upon receiving a packet initializes a Counter and counts the number of duplicated packets received within a delay period. If the counter value is less than the Max_count retransmission by the current vehicle should be cancelled.
2.5.2.5 Position-Based Adaptive Broadcast Scheme (P-BABS)
This scheme [80] is designed for the purpose of delivering an emergency message by the appropriate candidate with minimum end-to-end delay. The PABS scheme uses position, direction, and velocity information of sender and receiver vehicles to identify one candidate to relay the emergency message to other vehicles in the street. The PABS scheme states that vehicles are not relevant to the place of accident, and vehicles that do not cover a new additional area should not act as a candidate vehicle. Hence, each vehicle calculates the time delay for each received message based on the spacing distance and velocities of the sender and the receiver vehicles, in order to give a shorter time delay for the further vehicle.42