Robust broadcast scheme regardless of vehicle distribution in vehicular ad hoc networks
© Choi et al.; licensee Springer. 2014
Received: 31 January 2014
Accepted: 11 August 2014
Published: 18 August 2014
In vehicular ad hoc networks (VANETs), the efficient and reliable dissemination of emergency messages in a highly mobile environment under a dense or sparse network is a significant challenge. This paper proposes a robust broadcast scheme for VANETs, called the virtual slotted p-persistence scheme, which operates efficiently regardless of the vehicle density and distribution. Via the exchange of hello messages, each vehicle maintains position information on neighbor vehicles in a neighbor table. When a vehicle receives an emergency message, it determines its vehicle group, called a virtual slot, based on the position information in its neighbor table. The proposed scheme guarantees that the vehicles in the farthest group from the broadcaster probabilistically rebroadcast the message first. Simulation results demonstrate that the proposed scheme outperforms the slotted p-persistence scheme in terms of the end-to-end delay, collision ratio, and network overhead, regardless of the vehicle density and distribution.
Vehicular ad hoc networks (VANETs) have recently emerged as a promising field of research for increasing road safety by enabling drivers and/or vehicles to communicate with each other . Most current applications targeting VANETs rely heavily on broadcast transmissions to disseminate safety-related information, such as look-ahead emergency warnings and information about unsafe driving conditions. Yet, broadcasting messages blindly can lead to frequent transmission contentions and collisions among neighbor vehicles. This problem is known as a broadcast storm .
This paper focuses on the broadcasting of emergency messages for safety applications. When designing an efficient and reliable broadcast protocol for VANETs, the broadcast storm problem must be considered. One solution to alleviate the broadcast storm in a VANET is to have the farthest vehicle from the broadcaster perform the rebroadcasting, and various broadcast storm mitigating schemes have already been proposed in [2–12]. The weighted p-persistence scheme  and the slotted p-persistence scheme  are representative examples of these broadcast approaches. The weighted p-persistence scheme assigns a higher probability to vehicles that are located farther away from the broadcaster and the slotted p-persistence scheme divides the transmission range into a pre-determined number of slots and assigns the pre-determined probability at each slot. However, these schemes have some problems. In a dense network, multiple vehicles may rebroadcast the message simultaneously, resulting in collisions. Also, in a sparse network, the waiting time can cause a long delay before the message is rebroadcasted. These mean that the performance of existing broadcast schemes is highly dependent on the vehicle density and distribution.
Accordingly, this paper presents a robust broadcast protocol for VANETs, called the virtual slotted p-persistence scheme, which operates efficiently regardless of the vehicle density and distribution. The remainder of this paper is organized as follows. Section 2 describes related works. Section 3 explains our proposed virtual slotted p-persistence scheme. Section 4 describes the simulation environment and compares the performance of the virtual slotted p-persistence scheme and the slotted p-persistence scheme. Finally, Section 5 provides some conclusions.
2 Related works
Many algorithms have already been proposed to cope with the broadcast storm problem [2–12]. In , the broadcasting schemes are categorized into two types: sender-oriented schemes and receiver-oriented schemes. In the case of sender-oriented schemes [3–5], the sender uses neighbor position information to select the farthest vehicle as the next forwarder. The advantage of sender-oriented schemes is that only a single vehicle rebroadcasts the message. Meanwhile, receiver-based schemes [6–12] use contention to automatically select the next forwarder(s) in a distributed fashion. All the one-hop receivers of an emergency message enter a contention phase after receiving the message. After a waiting time, which is calculated using the distance from the broadcaster, the message is rebroadcast. One of representative receiver-oriented approaches is a probabilistic scheme, where vehicles rebroadcast a received message using a predetermined probability. For example, Wisitpongphan et al. proposed the weighted p-persistence scheme and slotted p-persistence scheme as probabilistic broadcast suppression techniques .
Several methods have already been proposed to enhance the slotted p-persistence scheme based on estimating the vehicle density using hello messages. In , a dynamic broadcast scheme is proposed to control the rebroadcast probability p according to the vehicle density. Meantime,  presents a scheme to adjust the number of slots dynamically according to the vehicle density. However, in real highway scenarios, the vehicle density constantly varies and the vehicles cannot be evenly distributed in slots. Therefore, neither approach can prevent unnecessary waiting delay before rebroadcasting due to empty slots, i.e. slots with no vehicles.
Accordingly, this paper proposes a robust broadcast scheme for VANETs, called the virtual slotted p-persistence scheme, which operates efficiently regardless of the vehicle density and distribution. The proposed virtual slotted p-persistence scheme uses hello messages to periodically exchange the basic information between any two vehicles. Using the information in the received hello message, each vehicle maintains its own neighbor table. When a vehicle receives an emergency message, it determines its vehicle group, called a virtual slot, based on the position information in its neighbor table. The vehicles in the farthest group from the broadcaster then probabilistically rebroadcast the message first.
3 Virtual slotted p-persistence scheme
This section introduces the basic operation of the virtual slotted p-persistence scheme that includes how to build a neighbor table, how to group vehicles, and the rebroadcasting procedure. The proposed scheme assumes that every vehicle has a GPS and knows its geographical position.
3.1 Building neighbor table using hello messages
3.2 Grouping vehicles into virtual slot and rebroadcast probability
When a vehicle receives an emergency message, it determines its vehicle group based on the position information and moving direction of the neighbor vehicles in its neighbor table. In the proposed scheme, a vehicle group is called a virtual slot, as it virtually corresponds to a slot in the slotted p-persistence scheme.
where τ is a predetermined slot time.
Equation 3 means that the rebroadcast probability is set at for each virtual slot, except for the last virtual slot, where it is set at .
3.3 Rebroadcasting procedure
4 Performance evaluation
The performance of the proposed virtual slotted p-persistence scheme was compared with that of the slotted p-persistence scheme in terms of the end-to-end delay, collision ratio, and network overhead through a simulation using ns-2 .
4.1 Simulation environments
In the simulation, the distributed coordination function (DCF) of IEEE 802.11 was used as the medium access control (MAC) protocol, which was modeled as a shared-media radio with a 1-Mbps nominal bit rate and 500-m transmission range. To evaluate how the two schemes behaved under different vehicle densities, four traffic conditions were used: 10, 25, 50, and 100 vehicles/km/lane on a 5-km road section. The emergency message was broadcast by the source vehicle every second. The slot time τ was set at 2.5 ms and the hello interval was set at 1 s. The velocity of each vehicle was randomly selected among 80, 100, and 120 km/h.
Length of road section
10, 25, 50, 100 vehicles/km/lane
80, 100, 120 km/h
Slot time (τ)
0.5, 1 (for slotted p-persistence)
The number of vehicles per slot (N s )
1, 3, 5
4.2 Performance metrics
For the performance comparison, the following metrics were used.
End-to-end delay: defined as the delay between the time the broadcast message originated at the source vehicle and the time it reached all the vehicles in the road section.
Collision ratio: defined as the ratio of the number of broadcast messages lost by collision to the total number of broadcast messages.
Network overhead: defined as the total number of broadcast messages incurred during a single broadcast.
4.3 Simulation results
To identify the impact of the number of vehicles per slot (N s ) in the virtual slotted p-persistence scheme, we also have done the simulation when N s was set at 1, 3, and 5, respectively. As shown in Figures 8 to 10, the end-to-end delay, the collision ratio, and the network overhead of the virtual slotted p-persistence scheme increased a little bit as N s increased, respectively. This is because the probability that more than two vehicles in a slot rebroadcast simultaneously increases as N s increases, resulting in more collisions. Also, the collisions will increase the end-to-end delay and network overhead.
In the slotted p-persistence scheme, the number of vehicles in a slot varies according to the vehicle distribution, yet the rebroadcast probability p is pre-determined regardless of the number of vehicles in a slot. Therefore, a small probability p can result in a long end-to-end delay in a sparse network, while a high probability p can cause more collisions in a dense network. Also, in a sparse network, the slotted p-persistence scheme can cause a long waiting time before rebroadcasting when there are no vehicles in the previous slots. However, in the virtual slotted p-persistence scheme, the number of vehicles in a virtual slot can be controlled by N s , regardless of the vehicle distribution, and the rebroadcast probability p is determined optimally according to the number of vehicles in a virtual slot.
This paper proposed a virtual slotted p-persistence scheme in which the number of vehicles in a slot can be controlled regardless of the vehicle density and distribution. Thus, in the case of a sparse network, the proposed scheme can avoid unnecessary waiting before rebroadcasting by eliminating the empty slots that occur with the slotted p-persistence scheme. Plus, in a sparse or dense network, the proposed scheme can reduce collisions by controlling both the number of vehicles in a slot and the rebroadcast probability p according to the number of vehicles. Simulation results demonstrated that the proposed scheme outperformed the slotted p-persistence scheme in terms of the end-to-end delay, collision ratio, and network overhead, regardless of the vehicle density. In the future work, we will consider the proposed scheme to apply for urban scenario.
This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (NRF-2013R1A1A4A01012534), and the IT R&D program of MSIP/IITP [10041145, Self-Organized Software platform (SoSp) for Welfare Devices].
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