A Safety Message Broadcast Strategy in Hybrid Vehicular Network Environment

A Safety Message Broadcast Strategy in Hybrid Vehicular Network Environment Abstract As the Vehicle to Vehicle or Vehicle to Infrastructure communication in Vehicular Ad Hoc Network (VANET) is usually based on opportunity network, the transmission delay will be high with the low vehicle density or far transmission distance. As the result, the Quality of Service requirements of safety message dissemination cannot be satisfied. The paper proposed a safety message broadcast strategy based on VANET-cellular architecture, and it does not rely too much on traffic density or additional deployment of Road Side Unit. Meanwhile, the only relay node is selected from the candidate relay node subset via the acknowledgement mechanism of short control packet. The strategy can solve the problem of competitive channel and redundant data in VANET. 1. INTRODUCTION Vehicular Ad Hoc Networks (VANETs) combined with technology of global positioning system, sensor network and wireless communication, collecting real-time traffic information and timely broadcast to provide solutions for the safe running of vehicles, traffic management and data communication. The data services in VANET can be divided into traffic safety services (such as collision warning, road congestion, roads slip and traffic accident information) and non-safety services (such as Internet access and multimedia entertainment). Non-safety service focus on satisfying users’ spiritual needs and have lower priority, hence the requirement for transmission delay is not high. While traffic safety services play more active role in ensuring traffic safety and improving traffic efficiency. Therefore, for the purpose of a safe and smooth traffic, a fast and reliable broadcast routing algorithm needs to be designed for this kind of safety service message [1–2]. The reasons are discussed as below. First, as the communication distance of the vehicle node in VANET is relatively short, and the network keeps in the form of self-organizing. Moreover, as the topology changes rapidly, the links between nodes can only maintain a short period of time, thus reducing the connectivity of VANET. Especially in remote areas with low vehicle density or highway, the connectivity of VANET is generally poor. In urban environment with high vehicle density, due to the existence of large number of buildings, wireless signals concentrate and have strong interference, resulting in much shorter communication distance between than the theoretical communication range. Second, due to the wide deployment of traffic lights, the vehicles on the road often present in the form of cluster, which means that the vehicle nodes cannot form a stable and completely connected network to guarantee the connectivity of the vehicular network. Third, in VANET, safety messages and other messages share the communication channel. Without the limitation of relay nodes, collisions will occur frequently when nodes in the range of one hop contending for the communication channel, thus success rate for channel accessing is relatively low. When the vehicle density is high, the load of network information increases dramatically, causing the broadcast storm. The existence of these factors greatly increases the transmission delay and packet loss rate. As the Vehicle to Vehicle (V2V) or Vehicle to Infrastructure (V2I) communication is based on opportunity network, the communication opportunities would appear until the vehicle nodes are in the communication range. As the result, the existing communication strategies in VANET cannot meet the need of safety message broadcast. To address these problems, many studies have presented different VANET safety message broadcast strategies, most of which focused on communication methods based on IEEE802.11p. As IEEE802.11p can provide very high date rate but a very short transmission distance around 200 m, when the safety message is disseminated by multi-hop over a large area, there comes two major problems: broadcast storm and disconnected network. Restricted broadcasting and clustering are usually used to solve the broadcast storm problem [3–12]. Chang et al. [3] proposed the adaptive message forwarding method to avoid broadcast storm and guarantee delay. They constructed an analytical model to analyze several important network parameters, such as packet forwarding delay, packet connectivity probability number of forwarded messages. Komathy et al. [4] proposed an algorithm to select a set of forwarding nodes, and the sender estimates the locations of its neighbors with a non-geometric broadcast approach and selects a set of forwarding candidate nodes located at the boundary of sender’s nodes based on communication area overlap values. Lee et al. [5] proposed broadcast storm suppression scheme called N Hops Weighted p-Persistence Broadcasting mechanism. However, the above methods, which solve the broadcast storm by restricting rebroadcast, reduce the data expansion speed meanwhile. Najafzadeh et al. [6] proposed a new broadcasting approach that dynamically adjusted waiting time of a vehicle according to the number of neighbor vehicles and the distance to the source. Slavik et al. [7] proposed a method to solve this problem by creating a decision threshold function that was simultaneously adaptive to the number of neighbors, the node clustering factor and the Rician fading parameter. Chang et al. [8] proposed an adaptive message forwarding to avoid broadcast storm and guarantee the delay in active safe driving VANET. They also established an analytical model to analyze several important parameters: packet forwarding delay, packet connectivity probability, number of forwarded messages, etc. Rehman et al. [9] proposed a next nodes selecting method instead of broadcast. The method is based on the estimation of link qualities. The discussed methods are more suitable for rapidly changing environment of VANET. However, they increase the network resource cost of vehicle exchanging information. They are usually based on the information such as vehicle density, vehicle position and vehicle velocity. The broadcast algorithm based on clustering is adopted in the literature [10–12]. The vehicle nodes are divided into independent clusters to reduce the number of forwarding nodes. It can reduce the redundant data and increase the speed of data packets dissemination. However, the topology of VANET is constantly changing, and it will cause frequent information exchange between vehicles. In addition, the convergence speed of this kind of clustering algorithm is also a considerable issue. Store-carry-forward is designed to solve the connectivity problem in network. When the vehicle density is low, a vehicle cannot find a neighbor vehicle to forward the safety message at any time. Then, the number of vehicles is not sufficient to disseminate the safety message to other vehicle in the allotted time. Tan et al. [13] proposed a Multi-hop broadcast communication protocol based on phase information of crossroad. The intersection area is divided by traffic phase information, and relay nodes are selected sequentially in the effective area of each section. The broadcast based on infrastructure is also the hot spot of current research. A lot of studies have proved the infrastructure can significantly improve the data delivery ratio. Abdrabou et al. [14] proposed a road side unit placement mechanism to obtain the maximum distance between Road Side Units (RSUs), which stochastically limits the packet delivery delay of worst case to a certain bound. The main objective of Maheswari P U’s study is to find the reliable path from the source vehicle to the destination vehicle to transmit the traffic information [15]. Wang et al. [16] proposed a cooperative store-carry-forward scheme to reduce the transmission outage time of vehicles in the dark areas. The scheme utilizes bidirectional vehicle streams and selects two vehicles in both directions to serve as relays successively for the target vehicle via inter-RSU cooperation. These studies use RSU as relay node to forward data, and it can improve the network connectivity. However, in the real environment, the data delivery ratio is not high in Time-To-Live (TTL). The reason is that there are differences between the movement of vehicle in VANET, and some vehicles are usually difficult to meet others. The placement of RSU can improve the data delivery ratio, but it is still not enough to guarantee the safety message dissemination. Based on the above analysis, a safety message broadcasting strategy for VANET-Cellular hybrid network architecture is proposed in this paper to guarantee that safety message broadcasting does not rely too much on vehicle density and does not need additional deployment of RSU. Meanwhile, in order to reduce the collision probability of channel competition, considering multiple factors, such as vehicle moving direction, link stability, channel quality, the signal intensity and geographic locations, relay node set is prioritized, and the only one relay node is selected in the candidate relay node subset by the acknowledgement mechanism of short control packet, which can quickly transmitting the safety message to the vehicles in target area. Mathematical analysis and simulation results show that the method can quickly and reliably transfer safety information, reduce the broadcast message redundancy, enhance the reliability of the safety and real-time data transmission, and improve the coverage ratio of broadcast message, and as well as the utilization ratio of network resource. All the important abbreviations and variables are shown in Table 1. Table 1. Abbreviations and variables in the paper. Name Explanation Name Explanation V2V Vehicle to Vehicle T The back-off time V2I Vehicle to Infrastructure Ts The benchmark unit time slot VANET Vehicular Ad hoc Networks TRTR The time required to send a RTR packet TTL Time-To-Live TATR The time required to send a ATR packet VNN VANET Normal Node TDATA The time required to send a DATA packet VGN VANET Gateway Node PRTR The packet size of RTR packet RTR Request To Relay PATR The packet size of ATR packet ATR Answer To Relay PDATA The packet size of safety message packet d The distance between the nodes Ta Safety message upload time LET The link stability between the nodes SVGN The number of VGN in the transmission range PER The error packet rate Name Explanation Name Explanation V2V Vehicle to Vehicle T The back-off time V2I Vehicle to Infrastructure Ts The benchmark unit time slot VANET Vehicular Ad hoc Networks TRTR The time required to send a RTR packet TTL Time-To-Live TATR The time required to send a ATR packet VNN VANET Normal Node TDATA The time required to send a DATA packet VGN VANET Gateway Node PRTR The packet size of RTR packet RTR Request To Relay PATR The packet size of ATR packet ATR Answer To Relay PDATA The packet size of safety message packet d The distance between the nodes Ta Safety message upload time LET The link stability between the nodes SVGN The number of VGN in the transmission range PER The error packet rate Table 1. Abbreviations and variables in the paper. Name Explanation Name Explanation V2V Vehicle to Vehicle T The back-off time V2I Vehicle to Infrastructure Ts The benchmark unit time slot VANET Vehicular Ad hoc Networks TRTR The time required to send a RTR packet TTL Time-To-Live TATR The time required to send a ATR packet VNN VANET Normal Node TDATA The time required to send a DATA packet VGN VANET Gateway Node PRTR The packet size of RTR packet RTR Request To Relay PATR The packet size of ATR packet ATR Answer To Relay PDATA The packet size of safety message packet d The distance between the nodes Ta Safety message upload time LET The link stability between the nodes SVGN The number of VGN in the transmission range PER The error packet rate Name Explanation Name Explanation V2V Vehicle to Vehicle T The back-off time V2I Vehicle to Infrastructure Ts The benchmark unit time slot VANET Vehicular Ad hoc Networks TRTR The time required to send a RTR packet TTL Time-To-Live TATR The time required to send a ATR packet VNN VANET Normal Node TDATA The time required to send a DATA packet VGN VANET Gateway Node PRTR The packet size of RTR packet RTR Request To Relay PATR The packet size of ATR packet ATR Answer To Relay PDATA The packet size of safety message packet d The distance between the nodes Ta Safety message upload time LET The link stability between the nodes SVGN The number of VGN in the transmission range PER The error packet rate 2. SAFETY MESSAGE BROADCAST STRATEGY 2.1. Description of the strategy The VANET-Cellular architecture is shown in Fig. 1. There are two types of nodes in wireless network: VNN (VANET Normal Node) and VGN (VANET Gateway Node). VNNs are the mobile vehicles that only have IEEE802.11p communication interface, while VGNs are the mobile vehicles that not only have IEEE802.11p communication interface, but also have cellular mobile network interface. VNN can transmit the safety message to mobile management center by VGN through cellular network, then mobile management center sends the safety message to the corresponding area through cellular network. This approach will improve the efficiency of data distribution and coverage area. At the same time, as the volume of safety information data is small, the cost of communication will not be high. Figure 1. View largeDownload slide VANET-cellular hybrid network. Figure 1. View largeDownload slide VANET-cellular hybrid network. As the vehicle movement trajectory in the actual traffic is restricted by the actual road structure, the safety-related information dissemination to the subsequent vehicle in the same direction lane is more important. Based on above consideration, the broadcast strategies of safety message can be divided into two categories. The Flow chart of strategy is shown in Fig. 2. Figure 2. View largeDownload slide Flow chart of strategy. Figure 2. View largeDownload slide Flow chart of strategy. (1) Safety message broadcast strategy (same direction lane). This strategy selects the optimal relay node to forward safety message by the proposed algorithm. The mobile nodes continue forwarding safety message by multi-hop until its lifetime expires. The VGN which has received the safety message upload it to mobile management center by cellular network (only once). Purpose: the safety message broadcast strategy adopts VANET and cellular to transmit safety message in same direction lane, in order to improve the transmission speed and extend the transmission scope of the safety message. (2) Safety message broadcast strategy (reversed direction lane). This strategy selects the optimal relay node to forward safety message by the proposed algorithm. The mobile nodes stop forwarding safety message by multi-hop when any VGN receives the safety message and sends it to the mobile management center. Purpose: in the reversed direction lane, the safety message broadcast strategy adopts cellular network to upload the safety message to the mobile management center by VGN as soon as possible. 2.2. Optimal relay node selection algorithm 2.2.1. Relay node selection strategy In VANET, safety message and other message share the information channel. If the number of relay nodes is unconstrained, frequent collisions will occur when nodes in range of one hop contending for the communication channel, thus channel access success rate is relatively low. When the vehicle density is high, the load of network information increases dramatically, causing the broadcast storm. Therefore, an Optimal Relay Node Selection Algorithm (ORNSA) is proposed in this paper. The candidate node set is divided into several subsets according to the node state information. A handshake protocol and its authentication mechanism are designed as well, in order to determine the optimal relay node in the subset of candidate node set. The steps for Improved RTS/CTS protocol are as follows: The current node broadcast DATA packet, and sets the retransmission timer. When the node within the communication range receives the DATA packet, if the packet is new, back-off timer of RTR (Request To Relay) is set. The node with highest priority is the first to send RTR packets, and packet retransmission timer is set automatically. If the current node receives RTR for the first time, Answer To Relay (ATR) is sent, and the packet retransmission timer is set automatically. Other nodes within the communication range receive ATR packets, and stop sending RTR packets. After the optimal relay nodes receive the ATR packet sent by the current node, if ID matches successfully, it becomes the new current node and the processing goes back to Step (1), starting to broadcast the DATA package. With the help of the information interactions in ORNSA, the only optimal relay node is selected, and two retransmission timers achieve the packet retransmission mechanism. It is shown in Fig. 3 Figure 3. View largeDownload slide Optimal relay node selection. Figure 3. View largeDownload slide Optimal relay node selection. 2.2.2. The prioritization of relay node set As the safety message and other message share the same channel in VANET, collisions occur frequently when nodes within the range of one hop contending for the communication channel, resulting in the low success rate of channel accessing. Therefore, in order to reduce the collision probability of channel competition and inhibit the redundancy of data transmission, candidate node should be prioritized, and back-off timer can be set according to the priority. The calculation of node priority is a typical optimization problem [17–19]. Considering the limited computing power and energy of mobile nodes, there are only three factors in the proposed calculation. Assuming (xi,yi) is the current location of the broadcast node, when a node located at (xj,yj) is ready to reply a RTR packet, it will calculate the node priority according to: (i) the distance between the node and the current broadcast node d; (ii) the link stability between the node and the current broadcast node LET; (iii) the error packet rate PER¯ calculated based on the signal-to-noise ratio. L=⌈σ⋅dR(a1⋅LET+a2⋅PER¯a1+a2)⌉σ=2,3,4... (1) (1) The link stability. The link stability can reflect the lifetime of communication links between two mobile nodes, and it is determined by distance, velocity and communication range. The formula of link stability between the two vehicles is as follow: LET=a(lnd(1+rv)lnR) (2) d=(xi−xj)2+(yi−yj)2 (3) rv=(vi⋅sinα−vj⋅sinβ)+(vi⋅cosα−vj⋅cosβ), (4) where the locations of two nodes are (xi,yi) and (xj,yj). α and β are slope between road and X-axis, respectively. a is a constant and a∈(0,1). R is the maximum communication range and d is the distance between nodes, R>1, d>1, LET∈(0,1).The closer the value of LET reaches 1, the higher the link stability is. (2) The channel quality. The channel quality is reflected by Packet Error Rate (PER). The packet error rate over fading channel can be approximated by [20]: PER¯=∑j=1nπj⋅(1−(1−λ(γj))N). (5) 2.2.3. Back-off timer setting In order to restrain the number of relay nodes, when a node receives a RTR packet, if the packet is new, the node starts to contend to be a relay node, and the node priority is calculated by formula (1). Thus, the node with higher priority has higher authority to forward. The higher forwarding authority the node is, the shorter the back-off time T before forwarding RTR is. The back-off time can be calculated as follows: T=(λ−L)⋅Ts, (6) where Ts is the benchmark unit time slot. In order to reduce the waiting time, and make the candidate relay nodes send RTR as soon as possible to improve the real-time performance of the algorithm, theoretically, the value of Ts should be as small as possible. However, actually if the value of Ts is too small, nodes with similar priorities will participate in contending for the forwarding permission. In conclusion, the computational formula for Ts is as follows: Ts=Max(ts⋅w2,da+TRTR), (7) where Ts is the unit time slot of communication protocol, w is the minimum contention window, TRTR is the time required to send a RTR packet and da is the time for accessing the communication channel. 3. THEORETICAL ANALYSIS 3.1. The effectiveness analysis of the relay node selection strategy (1) The forwarding time of the one hop for the safety message in VANET The forwarding time of the one hop for the safety message in VANET is defined as the time from vehicle node receiving safety message to sending it to the relay node. The time is an important index to measure the real-time performance of the algorithm, and the smaller the value is, the faster the safety message spreads in VANET. Since there is only one forwarding node in ORNSA, it is unnecessary to consider the collision probability. The time from relay node receiving safety message to forwarding it is as follows: thop=T+3da+TRTR+TATR+TDATA, (8) where T is the back-off time, da is the time for accessing communication channel. TRTR,TATR,TDATA is the time to send RTR, ATR and safety message, and it is affected by the network load, raising with the increase of the network load. (2) Effective data size ratio The effective data size ratio is defined as the proportion between effective data and total transmitted data. We assume that the distance between vehicle nodes on the road obey the exponential distribution with the parameter λ. The algorithm proposed in this paper only select a node to forward the packet, so there is no need to consider the collision. In the process of safety message broadcasting, the maximum transmission distance according to ORNSA is the transmission range R, and there are λ⋅R vehicle nodes on the road with the length R, moreover, the candidate relay nodes send RTR packet and only one optimal relay node can forward safety message packet. Thus the effective data ratio is as follows: eo≈PD(λ⋅Rσ⋅(PR+hR)+(PA+hA)+(PD+hD)). (9) In the traditional Mflood algorithm, all nodes in the range are involved in forwarding the safety message packets, and then the effective data ratio is as follows: ef=PDλ⋅R⋅(PD+hD). (10) While the broadcast packet persist algorithm based on distance such as Slotted1-persist algorithm, the effective data ratio is as follows: es=PDλ⋅RN⋅(PD+hD), (11) where PD,PR,PA represent the packet size of safety message packet, RTR packet, ATR packet, respectively, while hD,hR,hA represent the packet header size of safety message packet, RTR packet, ATR packet, respectively. Obviously, the effective data size of Mflood algorithm and Slotted1-persist algorithm mainly depend on safety message packet PD and hD, while ORNSA algorithm proposed in this paper is related to RTR packet and ATR packet. As generally PD is far greater than PT and PA, ORNSA in this paper should have better effective data size. 3.2. Effectiveness analysis of VANET-cellular hybrid transmission strategy (1) Safety message upload time Safety message upload time is the time Ta for safety message uploaded from the source node to the server: Ta={tcn=0t+c∑i=1nTielse, (12) where tc is the time needed for safety message uploaded from VANET Gate Node (VGN) to the server by cellular, n is the number of hops needed for safety message transmitted from VANET to VGN, Ti is shown in formula (8), and obviously Ta is related to n. Therefore, the value of n is discussed in the following. No matter safety message transmits in the same or reverse direction, the difference between relay nodes selection methods only exist in the last hop, and the number of hops from the source nodes to VGN are the same. Safety message broadcast strategy (same direction lane) always chooses the optimal relay node to broadcast by VANET, and other nodes (VGN uploads safety message to the server) will not forward the safety message by VANET after receiving it. Safety message broadcast strategy (same direction lane) finds VGN within the communication range. If there exists VGN, one of them will be randomly chosen as the relay node, otherwise, relay nodes will be chosen by the algorithm proposed in this paper. Above all, we only choose one direction as the research object. Assuming that the number of new arriving vehicles obeys the Possion distribution with the parameter λ in unit time, the ratio between the number of VGN and all vehicles is p, the vehicle speed is v and the communication distance is R. Given that the distance between two vehicles is d, then the probability that any two vehicles can communicate is as follows: P(d<R)=1−eλR. (13) If in time t, the mathematical expectation of the number of new arriving VGN is E(nt)=λpt. If two vehicles can communicate, d≤R. Then, E(d)=v(eλpR/v−1)−λpRλp(eλpR/v−1). (14) The number of VGN in the transmission range of the vehicle node which carries the safety message is SVGN, then the mathematical expectation is: E(SVGN)=⌊RE(d)⌋=⌊R(λp(eλpR/v-1))v(eλpR/v-1)-λpR⌋. (15) Obviously, the higher value E(SVGN) is, the lower value n is. While E(SVGN) is closely related to the vehicle density and the proportion between VGN and all vehicles. In summary, combined with the formulas (8), (12) and (13), we can see that if vehicle density is high, safety message transmission time based on VANET may be smaller and more reliable near the source node. While if vehicle density is relatively low or far from the source node, VANAET-Cellular hybrid transmission strategy will be more effective. 4. SIMULATION EXPERIMENT Assume that the width of each lane on the actual highway is 3.5 m, the simulation scene adopts a straight road model of 1000 m × 20 m. The mobile scene adopts VanetMobisim simulation, and the initial position of the vehicle generates randomly, with the speed changing in the range of 3–20 m/s. The communication between vehicles adopts 802.11p, and the date rate is 1MB/s, vehicle nodes send one packet per second, the size of data packet is 256–1024bytes, and the control packet RTR and ATR is 14bytes. The simulation software adopts NS-2. In the experiment, one of the vehicles in the queue had an accident, and sent safety message as the source node. Considering the signal interference, buildings and other factors, we assume that the vehicle effective communication range is 100 m, and the max of packet error rate is 8%. Probability of successful upload is the probability that source node finds VGN and uploads the safety message to the server within five hops. Figure 4 shows that with the raising of proportion of VGN and number of vehicle/km, the probability of successful upload increases. From the figure we can see that the density of vehicles is approaching 20 per kilometer, the probability that source node uploads the safety message to server within five hops is ~90%. In the city environment, the density of vehicles can easily reach 20 per kilometer. Figure 4. View largeDownload slide Probability of successful upload. Figure 4. View largeDownload slide Probability of successful upload. From Fig. 4 we can see the density of vehicles is approaching 20 per kilometer, the probability that source node uploads the safety message to server within five hops is about 90%. In the city environment, the density of vehicles can easily reach 20 per kilometer. Figure 5 shows that the simulation results of relationship between transmission delay in one hop and vehicle density and safety message packet size, respectively. We can see that ORNSA is superior to Slotted algorithm in the transmission delay in one hop, as there are two reasons: (i) ORSNA has introduced RTR and ART package, and determine the optimal relay node with two interactions, reducing retransmission due to collision; (ii) as the packet size of RTR and ART is far smaller than the data packet, both the transmission time and media access control layer access delay are smaller. Figure 5. View largeDownload slide Transmission delay. Figure 5. View largeDownload slide Transmission delay. In order to observe the time that safety message is delivered P by the VGN to server, we test end-to-end delay of 3G network packet in the real city road environment. In the experiment, the RSS of cellular network is guaranteed to be more than −70 dBm, and packet size is 515 bytes, then the simulation result is shown in Fig. 6. We can see that the transmission delay is around 100 ms. Figure 6. View largeDownload slide Transmission delay in 3G. Figure 6. View largeDownload slide Transmission delay in 3G. The successful delivery rate is the ratio between vehicle nodes successfully receiving the safety message and all vehicle nodes. The rate represents the reliability of the broadcast algorithm, and the higher the value is, the more reliable the broadcast route is, and the more applicable to the safety message transmission. Figure 7 shows that (i) The successful delivery rate is positively correlated with the vehicle density. When the vehicle density is low, the delivery rate of the three algorithms are relatively low. This is because when the vehicle density is low, the connectivity of VANET is low. When the vehicle density increases, the coverage performance of the three algorithms are improved, as the increase of vehicle density enhances the connectivity of VANET. In three algorithms, the delivery rate in Slotted1-persist algorithm is the lowest, due to the limitation of number of relay nodes and the lack of forward acknowledgement and retransmission mechanism in order to suppress the broadcast storm. The forward acknowledgement and retransmission mechanism in ORNSA can suppress the broadcast storm and ensure the reliability of the forwarding. The delivery success rate and the number of source nodes in the network are negatively related. The parameter is set as follow: the vehicle density is 100 per kilometers; the size of the date packet is 512 bytes. We can see from the figure that the proposed ORNSA shows better performance, with the increase of number of sources nodes, the load of network increases, the packet collision probability increases, and the successful delivery rate is decreases. Only one relay node is chosen in each hop, and the increase of network load is strictly control, then the successful delivery rate decreases slowly. Figure 7. View largeDownload slide Packet deliver success rate. Figure 7. View largeDownload slide Packet deliver success rate. 5. CONCLUSION As the the V2V or V2I communication in VANET is usually based on opportunity network, the transmission delay would be high with the low vehicle density or far transmission distance. It may not meet the Quality of Service (QoS) requirements of safety message dissemination. Moreover, with the development of mobile communication, more and more vehicles can connect to Internet by cellular network. In order to ensure Qos of the safety message dissemination, we proposed that the safety message broadcast strategy based on VANET-cellular architecture, and it does not rely too much on traffic density and does not need additional deployment of RSU. 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Comput. , 64 , 819 – 832 . 20 Khalili , R. and Salamatian , K. ( 2005 ) A New Analytic Approach to Evaluation of Packet Error Rate in Wireless Networks. Proc. Communication Networks and Services Research Conf., Halifa, NS, Canada, May 16–18, pp. 333–338. Institute of Electrical and Electronics Engineers, New York, USA. Author notes Handling editor: Jin-Hee Cho © The British Computer Society 2017. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png The Computer Journal Oxford University Press

A Safety Message Broadcast Strategy in Hybrid Vehicular Network Environment

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Abstract

Abstract As the Vehicle to Vehicle or Vehicle to Infrastructure communication in Vehicular Ad Hoc Network (VANET) is usually based on opportunity network, the transmission delay will be high with the low vehicle density or far transmission distance. As the result, the Quality of Service requirements of safety message dissemination cannot be satisfied. The paper proposed a safety message broadcast strategy based on VANET-cellular architecture, and it does not rely too much on traffic density or additional deployment of Road Side Unit. Meanwhile, the only relay node is selected from the candidate relay node subset via the acknowledgement mechanism of short control packet. The strategy can solve the problem of competitive channel and redundant data in VANET. 1. INTRODUCTION Vehicular Ad Hoc Networks (VANETs) combined with technology of global positioning system, sensor network and wireless communication, collecting real-time traffic information and timely broadcast to provide solutions for the safe running of vehicles, traffic management and data communication. The data services in VANET can be divided into traffic safety services (such as collision warning, road congestion, roads slip and traffic accident information) and non-safety services (such as Internet access and multimedia entertainment). Non-safety service focus on satisfying users’ spiritual needs and have lower priority, hence the requirement for transmission delay is not high. While traffic safety services play more active role in ensuring traffic safety and improving traffic efficiency. Therefore, for the purpose of a safe and smooth traffic, a fast and reliable broadcast routing algorithm needs to be designed for this kind of safety service message [1–2]. The reasons are discussed as below. First, as the communication distance of the vehicle node in VANET is relatively short, and the network keeps in the form of self-organizing. Moreover, as the topology changes rapidly, the links between nodes can only maintain a short period of time, thus reducing the connectivity of VANET. Especially in remote areas with low vehicle density or highway, the connectivity of VANET is generally poor. In urban environment with high vehicle density, due to the existence of large number of buildings, wireless signals concentrate and have strong interference, resulting in much shorter communication distance between than the theoretical communication range. Second, due to the wide deployment of traffic lights, the vehicles on the road often present in the form of cluster, which means that the vehicle nodes cannot form a stable and completely connected network to guarantee the connectivity of the vehicular network. Third, in VANET, safety messages and other messages share the communication channel. Without the limitation of relay nodes, collisions will occur frequently when nodes in the range of one hop contending for the communication channel, thus success rate for channel accessing is relatively low. When the vehicle density is high, the load of network information increases dramatically, causing the broadcast storm. The existence of these factors greatly increases the transmission delay and packet loss rate. As the Vehicle to Vehicle (V2V) or Vehicle to Infrastructure (V2I) communication is based on opportunity network, the communication opportunities would appear until the vehicle nodes are in the communication range. As the result, the existing communication strategies in VANET cannot meet the need of safety message broadcast. To address these problems, many studies have presented different VANET safety message broadcast strategies, most of which focused on communication methods based on IEEE802.11p. As IEEE802.11p can provide very high date rate but a very short transmission distance around 200 m, when the safety message is disseminated by multi-hop over a large area, there comes two major problems: broadcast storm and disconnected network. Restricted broadcasting and clustering are usually used to solve the broadcast storm problem [3–12]. Chang et al. [3] proposed the adaptive message forwarding method to avoid broadcast storm and guarantee delay. They constructed an analytical model to analyze several important network parameters, such as packet forwarding delay, packet connectivity probability number of forwarded messages. Komathy et al. [4] proposed an algorithm to select a set of forwarding nodes, and the sender estimates the locations of its neighbors with a non-geometric broadcast approach and selects a set of forwarding candidate nodes located at the boundary of sender’s nodes based on communication area overlap values. Lee et al. [5] proposed broadcast storm suppression scheme called N Hops Weighted p-Persistence Broadcasting mechanism. However, the above methods, which solve the broadcast storm by restricting rebroadcast, reduce the data expansion speed meanwhile. Najafzadeh et al. [6] proposed a new broadcasting approach that dynamically adjusted waiting time of a vehicle according to the number of neighbor vehicles and the distance to the source. Slavik et al. [7] proposed a method to solve this problem by creating a decision threshold function that was simultaneously adaptive to the number of neighbors, the node clustering factor and the Rician fading parameter. Chang et al. [8] proposed an adaptive message forwarding to avoid broadcast storm and guarantee the delay in active safe driving VANET. They also established an analytical model to analyze several important parameters: packet forwarding delay, packet connectivity probability, number of forwarded messages, etc. Rehman et al. [9] proposed a next nodes selecting method instead of broadcast. The method is based on the estimation of link qualities. The discussed methods are more suitable for rapidly changing environment of VANET. However, they increase the network resource cost of vehicle exchanging information. They are usually based on the information such as vehicle density, vehicle position and vehicle velocity. The broadcast algorithm based on clustering is adopted in the literature [10–12]. The vehicle nodes are divided into independent clusters to reduce the number of forwarding nodes. It can reduce the redundant data and increase the speed of data packets dissemination. However, the topology of VANET is constantly changing, and it will cause frequent information exchange between vehicles. In addition, the convergence speed of this kind of clustering algorithm is also a considerable issue. Store-carry-forward is designed to solve the connectivity problem in network. When the vehicle density is low, a vehicle cannot find a neighbor vehicle to forward the safety message at any time. Then, the number of vehicles is not sufficient to disseminate the safety message to other vehicle in the allotted time. Tan et al. [13] proposed a Multi-hop broadcast communication protocol based on phase information of crossroad. The intersection area is divided by traffic phase information, and relay nodes are selected sequentially in the effective area of each section. The broadcast based on infrastructure is also the hot spot of current research. A lot of studies have proved the infrastructure can significantly improve the data delivery ratio. Abdrabou et al. [14] proposed a road side unit placement mechanism to obtain the maximum distance between Road Side Units (RSUs), which stochastically limits the packet delivery delay of worst case to a certain bound. The main objective of Maheswari P U’s study is to find the reliable path from the source vehicle to the destination vehicle to transmit the traffic information [15]. Wang et al. [16] proposed a cooperative store-carry-forward scheme to reduce the transmission outage time of vehicles in the dark areas. The scheme utilizes bidirectional vehicle streams and selects two vehicles in both directions to serve as relays successively for the target vehicle via inter-RSU cooperation. These studies use RSU as relay node to forward data, and it can improve the network connectivity. However, in the real environment, the data delivery ratio is not high in Time-To-Live (TTL). The reason is that there are differences between the movement of vehicle in VANET, and some vehicles are usually difficult to meet others. The placement of RSU can improve the data delivery ratio, but it is still not enough to guarantee the safety message dissemination. Based on the above analysis, a safety message broadcasting strategy for VANET-Cellular hybrid network architecture is proposed in this paper to guarantee that safety message broadcasting does not rely too much on vehicle density and does not need additional deployment of RSU. Meanwhile, in order to reduce the collision probability of channel competition, considering multiple factors, such as vehicle moving direction, link stability, channel quality, the signal intensity and geographic locations, relay node set is prioritized, and the only one relay node is selected in the candidate relay node subset by the acknowledgement mechanism of short control packet, which can quickly transmitting the safety message to the vehicles in target area. Mathematical analysis and simulation results show that the method can quickly and reliably transfer safety information, reduce the broadcast message redundancy, enhance the reliability of the safety and real-time data transmission, and improve the coverage ratio of broadcast message, and as well as the utilization ratio of network resource. All the important abbreviations and variables are shown in Table 1. Table 1. Abbreviations and variables in the paper. Name Explanation Name Explanation V2V Vehicle to Vehicle T The back-off time V2I Vehicle to Infrastructure Ts The benchmark unit time slot VANET Vehicular Ad hoc Networks TRTR The time required to send a RTR packet TTL Time-To-Live TATR The time required to send a ATR packet VNN VANET Normal Node TDATA The time required to send a DATA packet VGN VANET Gateway Node PRTR The packet size of RTR packet RTR Request To Relay PATR The packet size of ATR packet ATR Answer To Relay PDATA The packet size of safety message packet d The distance between the nodes Ta Safety message upload time LET The link stability between the nodes SVGN The number of VGN in the transmission range PER The error packet rate Name Explanation Name Explanation V2V Vehicle to Vehicle T The back-off time V2I Vehicle to Infrastructure Ts The benchmark unit time slot VANET Vehicular Ad hoc Networks TRTR The time required to send a RTR packet TTL Time-To-Live TATR The time required to send a ATR packet VNN VANET Normal Node TDATA The time required to send a DATA packet VGN VANET Gateway Node PRTR The packet size of RTR packet RTR Request To Relay PATR The packet size of ATR packet ATR Answer To Relay PDATA The packet size of safety message packet d The distance between the nodes Ta Safety message upload time LET The link stability between the nodes SVGN The number of VGN in the transmission range PER The error packet rate Table 1. Abbreviations and variables in the paper. Name Explanation Name Explanation V2V Vehicle to Vehicle T The back-off time V2I Vehicle to Infrastructure Ts The benchmark unit time slot VANET Vehicular Ad hoc Networks TRTR The time required to send a RTR packet TTL Time-To-Live TATR The time required to send a ATR packet VNN VANET Normal Node TDATA The time required to send a DATA packet VGN VANET Gateway Node PRTR The packet size of RTR packet RTR Request To Relay PATR The packet size of ATR packet ATR Answer To Relay PDATA The packet size of safety message packet d The distance between the nodes Ta Safety message upload time LET The link stability between the nodes SVGN The number of VGN in the transmission range PER The error packet rate Name Explanation Name Explanation V2V Vehicle to Vehicle T The back-off time V2I Vehicle to Infrastructure Ts The benchmark unit time slot VANET Vehicular Ad hoc Networks TRTR The time required to send a RTR packet TTL Time-To-Live TATR The time required to send a ATR packet VNN VANET Normal Node TDATA The time required to send a DATA packet VGN VANET Gateway Node PRTR The packet size of RTR packet RTR Request To Relay PATR The packet size of ATR packet ATR Answer To Relay PDATA The packet size of safety message packet d The distance between the nodes Ta Safety message upload time LET The link stability between the nodes SVGN The number of VGN in the transmission range PER The error packet rate 2. SAFETY MESSAGE BROADCAST STRATEGY 2.1. Description of the strategy The VANET-Cellular architecture is shown in Fig. 1. There are two types of nodes in wireless network: VNN (VANET Normal Node) and VGN (VANET Gateway Node). VNNs are the mobile vehicles that only have IEEE802.11p communication interface, while VGNs are the mobile vehicles that not only have IEEE802.11p communication interface, but also have cellular mobile network interface. VNN can transmit the safety message to mobile management center by VGN through cellular network, then mobile management center sends the safety message to the corresponding area through cellular network. This approach will improve the efficiency of data distribution and coverage area. At the same time, as the volume of safety information data is small, the cost of communication will not be high. Figure 1. View largeDownload slide VANET-cellular hybrid network. Figure 1. View largeDownload slide VANET-cellular hybrid network. As the vehicle movement trajectory in the actual traffic is restricted by the actual road structure, the safety-related information dissemination to the subsequent vehicle in the same direction lane is more important. Based on above consideration, the broadcast strategies of safety message can be divided into two categories. The Flow chart of strategy is shown in Fig. 2. Figure 2. View largeDownload slide Flow chart of strategy. Figure 2. View largeDownload slide Flow chart of strategy. (1) Safety message broadcast strategy (same direction lane). This strategy selects the optimal relay node to forward safety message by the proposed algorithm. The mobile nodes continue forwarding safety message by multi-hop until its lifetime expires. The VGN which has received the safety message upload it to mobile management center by cellular network (only once). Purpose: the safety message broadcast strategy adopts VANET and cellular to transmit safety message in same direction lane, in order to improve the transmission speed and extend the transmission scope of the safety message. (2) Safety message broadcast strategy (reversed direction lane). This strategy selects the optimal relay node to forward safety message by the proposed algorithm. The mobile nodes stop forwarding safety message by multi-hop when any VGN receives the safety message and sends it to the mobile management center. Purpose: in the reversed direction lane, the safety message broadcast strategy adopts cellular network to upload the safety message to the mobile management center by VGN as soon as possible. 2.2. Optimal relay node selection algorithm 2.2.1. Relay node selection strategy In VANET, safety message and other message share the information channel. If the number of relay nodes is unconstrained, frequent collisions will occur when nodes in range of one hop contending for the communication channel, thus channel access success rate is relatively low. When the vehicle density is high, the load of network information increases dramatically, causing the broadcast storm. Therefore, an Optimal Relay Node Selection Algorithm (ORNSA) is proposed in this paper. The candidate node set is divided into several subsets according to the node state information. A handshake protocol and its authentication mechanism are designed as well, in order to determine the optimal relay node in the subset of candidate node set. The steps for Improved RTS/CTS protocol are as follows: The current node broadcast DATA packet, and sets the retransmission timer. When the node within the communication range receives the DATA packet, if the packet is new, back-off timer of RTR (Request To Relay) is set. The node with highest priority is the first to send RTR packets, and packet retransmission timer is set automatically. If the current node receives RTR for the first time, Answer To Relay (ATR) is sent, and the packet retransmission timer is set automatically. Other nodes within the communication range receive ATR packets, and stop sending RTR packets. After the optimal relay nodes receive the ATR packet sent by the current node, if ID matches successfully, it becomes the new current node and the processing goes back to Step (1), starting to broadcast the DATA package. With the help of the information interactions in ORNSA, the only optimal relay node is selected, and two retransmission timers achieve the packet retransmission mechanism. It is shown in Fig. 3 Figure 3. View largeDownload slide Optimal relay node selection. Figure 3. View largeDownload slide Optimal relay node selection. 2.2.2. The prioritization of relay node set As the safety message and other message share the same channel in VANET, collisions occur frequently when nodes within the range of one hop contending for the communication channel, resulting in the low success rate of channel accessing. Therefore, in order to reduce the collision probability of channel competition and inhibit the redundancy of data transmission, candidate node should be prioritized, and back-off timer can be set according to the priority. The calculation of node priority is a typical optimization problem [17–19]. Considering the limited computing power and energy of mobile nodes, there are only three factors in the proposed calculation. Assuming (xi,yi) is the current location of the broadcast node, when a node located at (xj,yj) is ready to reply a RTR packet, it will calculate the node priority according to: (i) the distance between the node and the current broadcast node d; (ii) the link stability between the node and the current broadcast node LET; (iii) the error packet rate PER¯ calculated based on the signal-to-noise ratio. L=⌈σ⋅dR(a1⋅LET+a2⋅PER¯a1+a2)⌉σ=2,3,4... (1) (1) The link stability. The link stability can reflect the lifetime of communication links between two mobile nodes, and it is determined by distance, velocity and communication range. The formula of link stability between the two vehicles is as follow: LET=a(lnd(1+rv)lnR) (2) d=(xi−xj)2+(yi−yj)2 (3) rv=(vi⋅sinα−vj⋅sinβ)+(vi⋅cosα−vj⋅cosβ), (4) where the locations of two nodes are (xi,yi) and (xj,yj). α and β are slope between road and X-axis, respectively. a is a constant and a∈(0,1). R is the maximum communication range and d is the distance between nodes, R>1, d>1, LET∈(0,1).The closer the value of LET reaches 1, the higher the link stability is. (2) The channel quality. The channel quality is reflected by Packet Error Rate (PER). The packet error rate over fading channel can be approximated by [20]: PER¯=∑j=1nπj⋅(1−(1−λ(γj))N). (5) 2.2.3. Back-off timer setting In order to restrain the number of relay nodes, when a node receives a RTR packet, if the packet is new, the node starts to contend to be a relay node, and the node priority is calculated by formula (1). Thus, the node with higher priority has higher authority to forward. The higher forwarding authority the node is, the shorter the back-off time T before forwarding RTR is. The back-off time can be calculated as follows: T=(λ−L)⋅Ts, (6) where Ts is the benchmark unit time slot. In order to reduce the waiting time, and make the candidate relay nodes send RTR as soon as possible to improve the real-time performance of the algorithm, theoretically, the value of Ts should be as small as possible. However, actually if the value of Ts is too small, nodes with similar priorities will participate in contending for the forwarding permission. In conclusion, the computational formula for Ts is as follows: Ts=Max(ts⋅w2,da+TRTR), (7) where Ts is the unit time slot of communication protocol, w is the minimum contention window, TRTR is the time required to send a RTR packet and da is the time for accessing the communication channel. 3. THEORETICAL ANALYSIS 3.1. The effectiveness analysis of the relay node selection strategy (1) The forwarding time of the one hop for the safety message in VANET The forwarding time of the one hop for the safety message in VANET is defined as the time from vehicle node receiving safety message to sending it to the relay node. The time is an important index to measure the real-time performance of the algorithm, and the smaller the value is, the faster the safety message spreads in VANET. Since there is only one forwarding node in ORNSA, it is unnecessary to consider the collision probability. The time from relay node receiving safety message to forwarding it is as follows: thop=T+3da+TRTR+TATR+TDATA, (8) where T is the back-off time, da is the time for accessing communication channel. TRTR,TATR,TDATA is the time to send RTR, ATR and safety message, and it is affected by the network load, raising with the increase of the network load. (2) Effective data size ratio The effective data size ratio is defined as the proportion between effective data and total transmitted data. We assume that the distance between vehicle nodes on the road obey the exponential distribution with the parameter λ. The algorithm proposed in this paper only select a node to forward the packet, so there is no need to consider the collision. In the process of safety message broadcasting, the maximum transmission distance according to ORNSA is the transmission range R, and there are λ⋅R vehicle nodes on the road with the length R, moreover, the candidate relay nodes send RTR packet and only one optimal relay node can forward safety message packet. Thus the effective data ratio is as follows: eo≈PD(λ⋅Rσ⋅(PR+hR)+(PA+hA)+(PD+hD)). (9) In the traditional Mflood algorithm, all nodes in the range are involved in forwarding the safety message packets, and then the effective data ratio is as follows: ef=PDλ⋅R⋅(PD+hD). (10) While the broadcast packet persist algorithm based on distance such as Slotted1-persist algorithm, the effective data ratio is as follows: es=PDλ⋅RN⋅(PD+hD), (11) where PD,PR,PA represent the packet size of safety message packet, RTR packet, ATR packet, respectively, while hD,hR,hA represent the packet header size of safety message packet, RTR packet, ATR packet, respectively. Obviously, the effective data size of Mflood algorithm and Slotted1-persist algorithm mainly depend on safety message packet PD and hD, while ORNSA algorithm proposed in this paper is related to RTR packet and ATR packet. As generally PD is far greater than PT and PA, ORNSA in this paper should have better effective data size. 3.2. Effectiveness analysis of VANET-cellular hybrid transmission strategy (1) Safety message upload time Safety message upload time is the time Ta for safety message uploaded from the source node to the server: Ta={tcn=0t+c∑i=1nTielse, (12) where tc is the time needed for safety message uploaded from VANET Gate Node (VGN) to the server by cellular, n is the number of hops needed for safety message transmitted from VANET to VGN, Ti is shown in formula (8), and obviously Ta is related to n. Therefore, the value of n is discussed in the following. No matter safety message transmits in the same or reverse direction, the difference between relay nodes selection methods only exist in the last hop, and the number of hops from the source nodes to VGN are the same. Safety message broadcast strategy (same direction lane) always chooses the optimal relay node to broadcast by VANET, and other nodes (VGN uploads safety message to the server) will not forward the safety message by VANET after receiving it. Safety message broadcast strategy (same direction lane) finds VGN within the communication range. If there exists VGN, one of them will be randomly chosen as the relay node, otherwise, relay nodes will be chosen by the algorithm proposed in this paper. Above all, we only choose one direction as the research object. Assuming that the number of new arriving vehicles obeys the Possion distribution with the parameter λ in unit time, the ratio between the number of VGN and all vehicles is p, the vehicle speed is v and the communication distance is R. Given that the distance between two vehicles is d, then the probability that any two vehicles can communicate is as follows: P(d<R)=1−eλR. (13) If in time t, the mathematical expectation of the number of new arriving VGN is E(nt)=λpt. If two vehicles can communicate, d≤R. Then, E(d)=v(eλpR/v−1)−λpRλp(eλpR/v−1). (14) The number of VGN in the transmission range of the vehicle node which carries the safety message is SVGN, then the mathematical expectation is: E(SVGN)=⌊RE(d)⌋=⌊R(λp(eλpR/v-1))v(eλpR/v-1)-λpR⌋. (15) Obviously, the higher value E(SVGN) is, the lower value n is. While E(SVGN) is closely related to the vehicle density and the proportion between VGN and all vehicles. In summary, combined with the formulas (8), (12) and (13), we can see that if vehicle density is high, safety message transmission time based on VANET may be smaller and more reliable near the source node. While if vehicle density is relatively low or far from the source node, VANAET-Cellular hybrid transmission strategy will be more effective. 4. SIMULATION EXPERIMENT Assume that the width of each lane on the actual highway is 3.5 m, the simulation scene adopts a straight road model of 1000 m × 20 m. The mobile scene adopts VanetMobisim simulation, and the initial position of the vehicle generates randomly, with the speed changing in the range of 3–20 m/s. The communication between vehicles adopts 802.11p, and the date rate is 1MB/s, vehicle nodes send one packet per second, the size of data packet is 256–1024bytes, and the control packet RTR and ATR is 14bytes. The simulation software adopts NS-2. In the experiment, one of the vehicles in the queue had an accident, and sent safety message as the source node. Considering the signal interference, buildings and other factors, we assume that the vehicle effective communication range is 100 m, and the max of packet error rate is 8%. Probability of successful upload is the probability that source node finds VGN and uploads the safety message to the server within five hops. Figure 4 shows that with the raising of proportion of VGN and number of vehicle/km, the probability of successful upload increases. From the figure we can see that the density of vehicles is approaching 20 per kilometer, the probability that source node uploads the safety message to server within five hops is ~90%. In the city environment, the density of vehicles can easily reach 20 per kilometer. Figure 4. View largeDownload slide Probability of successful upload. Figure 4. View largeDownload slide Probability of successful upload. From Fig. 4 we can see the density of vehicles is approaching 20 per kilometer, the probability that source node uploads the safety message to server within five hops is about 90%. In the city environment, the density of vehicles can easily reach 20 per kilometer. Figure 5 shows that the simulation results of relationship between transmission delay in one hop and vehicle density and safety message packet size, respectively. We can see that ORNSA is superior to Slotted algorithm in the transmission delay in one hop, as there are two reasons: (i) ORSNA has introduced RTR and ART package, and determine the optimal relay node with two interactions, reducing retransmission due to collision; (ii) as the packet size of RTR and ART is far smaller than the data packet, both the transmission time and media access control layer access delay are smaller. Figure 5. View largeDownload slide Transmission delay. Figure 5. View largeDownload slide Transmission delay. In order to observe the time that safety message is delivered P by the VGN to server, we test end-to-end delay of 3G network packet in the real city road environment. In the experiment, the RSS of cellular network is guaranteed to be more than −70 dBm, and packet size is 515 bytes, then the simulation result is shown in Fig. 6. We can see that the transmission delay is around 100 ms. Figure 6. View largeDownload slide Transmission delay in 3G. Figure 6. View largeDownload slide Transmission delay in 3G. The successful delivery rate is the ratio between vehicle nodes successfully receiving the safety message and all vehicle nodes. The rate represents the reliability of the broadcast algorithm, and the higher the value is, the more reliable the broadcast route is, and the more applicable to the safety message transmission. Figure 7 shows that (i) The successful delivery rate is positively correlated with the vehicle density. When the vehicle density is low, the delivery rate of the three algorithms are relatively low. This is because when the vehicle density is low, the connectivity of VANET is low. When the vehicle density increases, the coverage performance of the three algorithms are improved, as the increase of vehicle density enhances the connectivity of VANET. In three algorithms, the delivery rate in Slotted1-persist algorithm is the lowest, due to the limitation of number of relay nodes and the lack of forward acknowledgement and retransmission mechanism in order to suppress the broadcast storm. The forward acknowledgement and retransmission mechanism in ORNSA can suppress the broadcast storm and ensure the reliability of the forwarding. The delivery success rate and the number of source nodes in the network are negatively related. The parameter is set as follow: the vehicle density is 100 per kilometers; the size of the date packet is 512 bytes. We can see from the figure that the proposed ORNSA shows better performance, with the increase of number of sources nodes, the load of network increases, the packet collision probability increases, and the successful delivery rate is decreases. Only one relay node is chosen in each hop, and the increase of network load is strictly control, then the successful delivery rate decreases slowly. Figure 7. View largeDownload slide Packet deliver success rate. Figure 7. View largeDownload slide Packet deliver success rate. 5. CONCLUSION As the the V2V or V2I communication in VANET is usually based on opportunity network, the transmission delay would be high with the low vehicle density or far transmission distance. It may not meet the Quality of Service (QoS) requirements of safety message dissemination. Moreover, with the development of mobile communication, more and more vehicles can connect to Internet by cellular network. In order to ensure Qos of the safety message dissemination, we proposed that the safety message broadcast strategy based on VANET-cellular architecture, and it does not rely too much on traffic density and does not need additional deployment of RSU. 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The Computer JournalOxford University Press

Published: Aug 10, 2017

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