An Improved VANET Routing Protocol for Early-Warning

An Improved VANET Routing Protocol for Early-Warning Messages Transmission of Freeway Vehicle
Jun Bi, Qiuping Xu, Kexin Zhan
An Improved VANET Routing Protocol for Early-Warning Messages
Transmission of Freeway Vehicle
1
Jun Bi, 2Qiuping Xu, 3 Kexin Zhan
School of Traffic and Transportation, Beijing Jiaotong University, Beijing, China,
*1
MOE Key Laboratory for Urban Transportation Complex Systems Theory and
Technology,Beijing Jiaotong University ,Beijing,China
[email protected],[email protected],[email protected]
*1,2,3
Abstract
In order to reduce the incidence of traffic accidents such as rear-end collision, and guarantee the
security of the vehicles behind the accident spot in the freeway, especially in bad weather conditions,
an improved VANET (Vehicular Ad-Hoc Network) routing protocol, EIM-AODV (Emergency warning
and Inquire Message based on AODV) is presented. This paper combined traffic simulation with
network analog to simulate the performance of VANET in real traffic environment. The NS simulation
shows that the average delay of EIM-AODV is half of AODV when CBR (Constant Bit Rate) is 64kb/s
and it’s also significantly less than AODV when CBR is 128 kb/s. The results of simulation indicate
that, contrasting with AODV, EIM-AODV can reduce the network overhead, raise the packet delivery
radio and improve the delay jitter.
Keywords: Freeway Warning Information, Routing Protocol, Vehicle Communication, VANET
1. Introduction
In freeway transportation networks, traffic flow is so heavy and vehicles’ speed is so fast that if
there is a sudden vehicle stop or unexpected vehicle collision because of bad weather, road condition,
or personal factors, there will be serious traffic accidents, which will lead to more vehicles damage,
high death rate, and bad social effects. Therefore, the study of the freeway vehicle anti-collision system
has been paid more and more attention. For previous researches, technology of radar was mostly used
to measure the distance between vehicles, and the millimeter wave (MMW) radar took a dominant
position [1]. However, the MMW radar technology is mainly applied to the military field, which needs
complicated hardware techniques and high cost, so it is difficult to be extended to the civil field. How
to provide road status and potential accident alert for running vehicles is of great significance to reduce
vehicle rear-end collision and guarantee the driving safety, especially in rainy, snowy, foggy and other
harsh environments. Recently, using vehicular ad-hoc network to realize the real-time exchange of
vehicle warning information is a hot research direction [2].
Ad-Hoc [3] can quickly construct a mobile communication network without time and space
limitations, and every terminal node could freely move and has an equal status. Besides these inherent
advantages of Ad-Hoc, VANET [4-5] can also realize direct V2V or V2R communication even in the
high speed condition, therefore, in the existing road network, it can quickly and dynamically construct
a jump communication network between multiple vehicles, which is self-organized and distributed
control.
AODV (Ad-Hoc On-Demand Distance Vector Routing) [6-7] is a reactive routing protocol for AdHoc network, which adopts a basic request mechanism of routing finding and maintenance. It also has
some shortages, such as high network overhead and high packet delay, which may affect the efficiency
of submitting grouped data and the stability of network [8]. Therefore, how to reduce the cost becomes
a hot spot for the research field.
In order to reduce the routing overhead and delay, this paper presents an improved AODV protocol,
Emergency warning and Inquire Message based on AODV (EIM-AODV) which is based on the
topology change characteristics and communication characteristic of VANET. The average time-delay
and routing overhead are selected to evaluate the performance of EIM-AODV. The experiment result
shows that EIM-AODV can effectively cut down the routing overhead and reduce time-delay in highload transportation networks.
International Journal of Advancements in Computing Technology(IJACT)
Volume5,Number8,April 2013
doi:10.4156/ijact.vol5.issue8.20
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An Improved VANET Routing Protocol for Early-Warning Messages Transmission of Freeway Vehicle
Jun Bi, Qiuping Xu, Kexin Zhan
2. Emergency braking models of freeway vehicles
Germany Daimler - Benz company's research shows that, in the freeway just ahead of a second for
vehicles to get early warning information, rear-end collision accidents can be reduced 50% to 90%.
Now, we make a hypothesis that car-s0, car-A and car-B are driving normally on the freeway with
speed of 30 m/s (108 km/h) and deceleration of 5m/s2 when braking. Because of the horizon or
environmental influences, the braking response time is 1.5s (The response time is from the drivers’
braking decision when they watch the braking signal or receive the braking messages to the act that the
drivers step the brake). The distance between car-A and car-s0, car-A and car-B are both 40m (1.3s),
and the length of the vehicle is neglected.
If there is a traffic accident or emergency ahead, car-S0 is braked quickly to slow down. Finding the
brake signal of car-S0, car-A is braked immediately. The same principle is applied to car-B when car-A
is be braking. The braking process of these three cars is shown in Fig.1.
Figure 1. The process of vehicle braking
We assume that the brake time of car-S0 is T=0s, and the response time that from finding brake
signal of rear-car to brake is 1.5s, then the brake time for car-A and car-B is respectively for 1.5s and
3.0s.
As it is shown in Fig.1, when vehicles driving below the safe distance make brake decision only
through watching the brake signal of front cars, it will cause a massive pileup. If car-S0 finds a traffic
accident or emergency when running, at the same time of braking, it can send early-warning messages
using VENET to car-A and car-B which are behind of it. This method can remedy the shortages of
response-delay which is caused by the driver’s limited vision, especially in bad weather environment,
so that the rear-car can effectively make early response which can avoid a pileup.
3. VANET routing protocol available for freeway vehicle warning information
transmission
3.1. Vehicular Ad Hoc Network (VANET)
In recent years, with the rapid development of wireless network communication technology and
information technology, Vehicular Ad-Hoc Network (VANET) is the core of the Intelligent
Transportation System (ITS). VANET is a communication system which is specially designed to solve
the problem of communication between moving vehicles, realizing the real-time communication of the
vehicle to vehicle (V2V), vehicle to Infrastructure (V2I), and the combination of V2V and V2I. We can
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An Improved VANET Routing Protocol for Early-Warning Messages Transmission of Freeway Vehicle
Jun Bi, Qiuping Xu, Kexin Zhan
use these communication methods to improve road traffic safety and constitute a wireless mobile
network which is suitable for the traffic communication environment.
VANET can be used to realize these applications, such as the transmission of early warning
information between running vehicles, auxiliary driving, traffic information query, remote control of
vehicle through the network, communications between passengers and information service equipments.
At present, research institutions around the world have VANET as a key technology to provide early
warning information to solve the traffic congestion in the freeway.
3.2. Ad Hoc On-Demand Distance Vector Routing (AODV)
At present, the algorithm of VANET routing protocol has three categories: topology-based routing
protocols, location-based routing protocols and map-based routing protocols. AODV is the
representative of topology-based routing protocols, and it is one of the main standard VANET routing
protocols.
The establishment of AODV routing uses flood way, which may occur a serious redundant
forwarding, channel competition, transmission collisions, and other issues. First of all, wireless channel
is comprehensive spread, that a physical space may also be covered by a number of wireless signals of
mobile nodes, so it will produce a large number of redundant rebroadcast. Secondly, the space position
between mobile nodes rebroadcasting messages is relatively close, so it is likely to have serious
channel competition, especially in freeway condition where nodes are moving so fast that may lead to
the instability of transportation networks and lifetime shortening of routing links. In order to overcome
the shortcomings of AODV protocol, this paper presents an improved AODV protocol, EIM-AODV
which is suitable for freeway traffic warning information transfer, according to the characteristics of
vehicle motion model and the topological structural changes of VANET.
4. An improved VANET routing protocol based on AODV
4.1. The establishing principle about EIM-AODV
Emergency warning and Inquire Message based on AODV (EIM-AODV) is an improved
routing protocol based on AODV. It is also an on-demand distance vector routing protocol, with the
characteristics of on-demand routing protocol. Each network node only sends grouped routing in need
rather than periodically exchange the routing information to obtain the information of other nodes.
Meanwhile, EIM-AODV has the characteristics of distance vector routing protocol. That is to say, the
node routing tables only maintain the routings between its own node and others, without having to
master the entire network topology.
We can determine the transmission direction of the message by the service type of the vehicle internode communication. E.g., the query information of road conditions should be sent to vehicles before
the query node, and early warning information should be sent to vehicles behind it, as shown in Fig.2.
The message type should firstly been determined when the vehicle node sending data, and then the
position information of the source node should be obtained by GPS or other communication
equipments. The intermediate forwarding vehicle nodes obtain the location information of the source
node and the sort of the messages according to the Route Request Packet (RREQ).
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An Improved VANET Routing Protocol for Early-Warning Messages Transmission of Freeway Vehicle
Jun Bi, Qiuping Xu, Kexin Zhan
Figure 2. Message-conveying model
4.2. The set-up of IP packet for EIM-AODV
Early-warning and inquiry messages are the only types of information we consider in this paper. In
the implementation process of protocol, the source node should define the types of messages and
simultaneously inform other nodes of its position information when sending the routing enquiry.
Therefore it is necessary to optimize the IP header of the protocol by adding the position information,
driving direction and vehicular speed of the source node, order numbers of EIM, the types of the
messages and so on.
Table 1. IP packets with add information
0
4
Version
8
IHL
16
32bit
TOS
Identification
Total length
Flags
TTL
Protocol
Fragment offset
Header checksum
Source Address
Destination Address
EIM-No.*
Message Type*
Vehicular Position*
Driving Direction*
Vehicular Speed*
The new filed which identify with * is shown as the Table 1:
a) EIM- No. is the serial number of the EIM, which is composed of transmission time. If the car
receives the same EIM-No., it will abandon the message without handling and retransmission. Judging
whether received the same EIM can effectively reduce the waste of information resources and lower
the network overhead.
b) Message Type is the kind of EIM, which can be divided into early-warning and inquiry messages,
so as to distinguish the direction of the message.
c) Vehicular Position is the location information where the source vehicle sends EIM. Vehicles of
receiving information can obtain the specific position of the source note through GPS.
d) Driving Direction indicates the running direction when the source vehicle sending EIM, that
divided into 360 degrees, and set north as 0°.
e) Vehicular Speed is the driving speed when the source vehicle transmitting EIM.
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An Improved VANET Routing Protocol for Early-Warning Messages Transmission of Freeway Vehicle
Jun Bi, Qiuping Xu, Kexin Zhan
4.3. The process of EIM-AODV routing establishment
In freeway, the driving vehicle makes enquiries or early-warning messages according to the demand,
and sends EIM to the destination vehicle. At the moment, this vehicle is the source vehicle. If the
source node doesn’t have the path to the destination node in its own routing table, it will send the
RREQ to find the destination or a path can be reached. In the routes-establish process, the vehicle
nodes in different locations should make different strategies according to the type of EIM sent by the
source vehicle, as it is shown in Fig.3.
The process of EIM-AODV routing establishment is as follows:
Step1. The source node launches a RREQ at first, and the RREQ packets carry the following
information: Source address, EIM serial number, Destination address and Hop counter.
Step2. The intermediate nodes deal with the RREQ, resolve the IP packets, and obtain the sort of the
messages and the position information of the source node. After that, in order to determine that the
node belongs to the forward (Group F) or backward (Group B), we should compare the position
information of this node with that of the source node. Eventually, we can determine the transmit
direction of EIM, that early-warning messages should be sent to the rear vehicle and inquiry messages
to the vehicle ahead. For the warning messages, the destination node is only possible in the Group B,
only in this way it makes sense. If the nodes in Group F forward the warning message, it will only
cause packet delay and routing overhead. For the enquiry messages, the destination node should only
be in Group F, otherwise there is no significance.
Step3. We can determine whether the intermediate node received the RREQ or not, according to
Source address, EIM serial number and Destination address. If received, the RREQ will be abandoned.
Otherwise, it will be handled in the next process.
Step4. Compared with the destination address in RREQ, this node can be judged whether it is the
destination node. If it is, the RREQ can be finished and sent back to the source node. Otherwise, it will
be handled in the next process.
Step5. Record the corresponding routing information and establish the reverse routing. The
information includes the upstream node address, source address, destination address, broadcast ID,
EIM-No. and reverse route to response the record number. Meanwhile, the TTL can be recorded as
plus 1.
Step6. Continue to transmit the RREQ to neighbor nodes.
Figure 3. Forward strategy of EIM
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An Improved VANET Routing Protocol for Early-Warning Messages Transmission of Freeway Vehicle
Jun Bi, Qiuping Xu, Kexin Zhan
5. The establishment of emulation environment in freeway
To some extent, the performance of VANET depends on the selected vehicle-node movement
models. Therefore, in order to simulate the performance of EIM-AODV in a more realistic traffic
environment, we adopt the method of combining the traffic simulation and network simulation.
5.1Vehicle-node movement models establishment
Assume that there is a traffic accident or emergency in front, the car S quickly sends EIM messages
to the back cars, reminding them detour to avoid a chain collision.
EIM is transmitted backward one by one, whose transmission range is 1000m behind the source
vehicle S. According to the regulations for implementation of the Road Traffic Safety Law, the
maximum speed in the freeway shall not exceed per hour 120km, and the minimum speed shall not be
less than per hour 60km. We set the speed between 60km/h and 120km/h, and set the simulation time
for 600s. The space which the vehicles entering the transport area takes a random value between 40m
to 220m.The distribution of area vehicles is in accordance with Poisson distribution function, and the
size of EIM packet is 1024Byte. We select 15 nodes adopting the CBR (Constant Bit Rate), and use
VANET sending UDP packets. The transmission range of RFID is 0~250m. Emulation experiments
adopt the IEEE 802.11b protocol. The simulation parameters are shown in Table 2.
Table 2. Simulation parameters Settings
Parameter Name
Value
The simulation area
Speed for nodes
Size of EIM Packet
Node distribution state
1000m
60km/h ~120km/h
1024Byte
Poisson distribution function
Average distance between vehicles
40~220m
Simulation time
Nodes
Media Access Control
600s
15
IEEE802.11b
5.2 Analysis of simulation results
In the experiment, we compare the protocol capability of EIM-AODV with AODV using NS2 [9]
on the condition that the CBR is 64kb/s &128kb/s. We consider 5 performance indexes as follows:
1) Packet delivery ratio [10]: the packet delivery ratio shows the transmission efficiency of the
network with the given protocol. It is obtained as follows:
Packet delivery ratio =
No. of packets delivered
No. of packets sent
No. of packets delivered is the total numbers of delivered data packets received by each node. No. of
packets sent is the total numbers of data packets sent by each node.
2) Average end-to-end delay [10]: it is the average value of the time taken by the received data
packets to reach the destination from their origin. This includes all possible delays caused by queuing
delay at the interface, buffering during route discovery, retransmission delays at the MAC, propagation
and transfer times.
3) Delay jitter: delay refers to the time interval from the first bit of data packet into the router to the
last bit out from the router. Delay jitter is refers to the time delay variation.
4) Throughput: throughput refers to the amount of data which the router can handle per second in
case there is no frame loss.
5) Routing overhead ratio [10]: it refers a ratio of the control packets to the correctly delivered data
packets and is obtained as:
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An Improved VANET Routing Protocol for Early-Warning Messages Transmission of Freeway Vehicle
Jun Bi, Qiuping Xu, Kexin Zhan
Packet overhead ratio=
No. of control packets sent
No. of packets delivered
The control packets are route discovery packets and route maintenance packets.
Fig.4 indicates that the performance of EIM-AODV and AODV, when CBR is at the speed of
128kb/s. It is obvious that the delay of EIM-AODV is less than that of AODV.
Figure 4. Simulation results of delay contrast
As shown in Fig.5, the delay jitter of EIM-AODV and AODV are both violent. But most of the time
the delay jitter of EIM-AODV protocol is lower than AODV, and its peak value is only half of AODV.
Based on the sending direction and the source-node location, EIM-AODV can effectively control the
routing messages and reduce packet delay in the process of establishing routes, indicating that EIMAODV is more suitable for topology changes of Vehicle Networks.
Figure 5.Simulation results of Jitter contrast Figure 6. Simulation results of Throughput contrast
It can be seen from Fig.6, throughput first increases and then decreases. From 50s to 120s, the nodes
move so violent that the routes fail frequently. Meanwhile, EIM-AODV is based on the message type
to choose intermediate nodes, causing that certain messages fail to reach directly and nodes reestablish
route frequently, thus the transmission of data packets is blocked and the throughput may be lower than
that of AODV. However, most of the time throughput of EIM-AODV is significantly higher than that
of AODV. The EIM-AODV protocol resolves the IP packet to obtain the message sending direction,
which makes the intermediate-node no longer blindly retransmit RPEQ. As a result, the utilization rate
of network bandwidth can be improved.
The packet delivery ratio, routing overhead ratio and average end-to-end delay of EIM-AODV &
AODV are shown in Table III, when CBR is at the speed of 64kb/s and 128kb/s. The average delay of
EIM-AODV is half of AODV when V (CBR) is 64kb/s. However, if V (CBR) is 128 kb/s, the available
bandwidth reduces and the probability of the packet conflict rises due to the increase of the
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An Improved VANET Routing Protocol for Early-Warning Messages Transmission of Freeway Vehicle
Jun Bi, Qiuping Xu, Kexin Zhan
communication load, therefore the average delay increases. In the process of establishing routes, the
EIM-AODV protocol enables intermediate nodes to avoid forwarding a large number of useless route
request messages, thereby improving the utilization of network bandwidth. Therefore, it proves that
EIM-AODV is more effective than AODV in reducing network expenses, enhancing throughput and
improving the packet time-delay.
Table 3. Other performance simulation contrast
CBR(kb/s)
Protocol
Packet delivery ratio /%
Routing overhead
Average delay /ms
64
64
128
128
AODV
EIM-AODV
AODV
EIM-AODV
94.3
97.2
71.9
75.8
0.375
0.210
0.358
0.159
64.17
35.03
201.87
193.53
6. Conclusion
In this paper, we present an improved VANET protocol, EIM-AODV which can control the routing
messages broadcasting in specific traffic directions, by distinguishing the message types of VANET.
Through the NS emulation analysis, we get the result that, contrasting with AODV, EIM-AODV rs the
packet time-delay and jitter, enhances the packet delivery fraction, reduces the routing expenses and
optimizes the routing selection. However, in this paper, we only conduct the emulation experiment in a
simple traffic model. At the further research, we will improve the vehicle movement models and check
the performance of the optimized protocol (EIM-AODV) under the more real and complex traffic
models, so that we can define the applicable environment, choose the transmitted strategy of the
message control under different traffic situations, and increase the efficiency of EIM-AODV.
7. Acknowledgment
This paper is supported by “the Fundamental Research Funds for the Central Universities” and the
National 863 Program (NO.2011AA110303) .
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