Recent Years rapid development in wireless communication networks has made Car to Car (C2C) and Car to Infrastructure Communications (C2I) possible in Mobile Ad hoc Networks (MANETs). This has given birth to a new type of high mobile MANET called Vehicular Ad hoc Networks (VANET) creating a fertile area of research aiming for road safety, efficient driving experience and infotainment (Information and Entertainment).
Creating an efficient safety system on the road is a very important and critical
concern for human today, each year nearly 1.3 million people die as a result
of road traffic accidents more than 3000 deaths each day and more than half
of these people are not travelling in a car, the injuries are about fifty times
of this number (WHO, 2011), Malaysia also has its dangers
with a very high per fatality rates 26 people per 100,000 are killed in traffic
crashes, each year, there are about 6,300 fatal accidents as stated by the accidents
statistics website (Accidents, 2011). The number of cars
in 2004 is approximately estimated as 750 million cars around the world (Raya
et al., 2006), with an annually constant increase by 50 million car
around the world (Worldometers, 2011), with this constant
raise, the estimated number of cars nowadays exceeding one billion, this raise
the possibility to increase the number of crashes and deaths on the roads, road
traffic accidents are predicted to become the fifth leading cause of death in
the world, resulting in an estimated 2.4 million death each year as stated by
WHO (2011), besides traffic congestion makes a huge waste
of time and fuel, this makes developing an efficient safety system an urgent
need on the road. The new techniques in this system should aim to make the intelligent
vehicle to think, communicate with other vehicles and act to prevent hazards.
VANET safety applications depend on exchanging the safety information among vehicles (C2C communication) or between Vehicle to infrastructure (C2I Communication) using the control channel (Fig. 1).
VANET safety communication can be made by two means: Periodic Safety Message (called Beacon in this document) and Event Driven Message (called Emergency Message in this document), both sharing only one control channel. The Beacon messages are status messages containing status information about the sender vehicle like position, speed, heading
etc. Beacons provide fresh information about the sender vehicle to the surrounding vehicles in the network helping them to know the status of the current network and predict the movement of vehicles.
Emergency Messages are messages sent by a vehicle detect a potential dangerous situation on the road; this information should be disseminated to alarm other vehicles about a probable danger that could affect the incoming vehicles. VANET is a high mobile network where the nodes are moving in speeds that may exceed 120 km h-1, which means that this vehicle move 33.33 m sec-1, even if these vehicles are very far from the danger, they will reach it very soon, here milliseconds will be very important to avoid the danger. For instance, in 2008, a serial crash happened on the highway between Dubai and Abu Dhabi, involving 250 vehicles causing three deaths and 277 injured people, including, 10 serious injuries, again in the 2nd of April 2011, on the same highway, another serial vehicle crash. 127 vehicles crashed causing one death and 61 injured (Fig. 2).
When the first crash happened there should be a technique to alarm the incoming and speeding vehicles about this danger and this will save peoples lives and money. Sending the alarm to the incoming vehicles helps in conditions, especially when vehicles moving in high speeds, bad weather conditions and low road visibility.
|| VANET structure
|| Dubai highway crashes 2011
Emergency messages in VANET are sent in broadcast fashion where all the vehicle
inside the coverage area of the sender should receive the message. The coverage
area is not enough as it is hardly reaches a 1000 m (which is the DSRC communication
range) due to attenuation and fading effects. Away vehicles from the danger
should receive this critical information to avoid the danger. Furthermore, the
probability of message reception can reach 99% in short distances and can be
as low as 20% at half of the communication range (Torrent-Moreno
et al., 2004). Therefore, there should be a technique to increase
the emergency message reception with high reliability and availability, furthermore,
it is assumed that each vehicle equipped with a GPS device to retain the current
position (Wang et al., 2008).
VANET broadcasting: Duo to the high mobility of vehicles, the distribution
of nodes within the network changes rapidly and unexpectedly that wireless links
initialize and break down frequently and unpredictably. Therefore, broadcasting
of messages in VANETs plays a crucial rule in almost every application and requires
novel solutions that are different from any other form of ad hoc networks.
Broadcasting of messages in VANETs is still an open research challenge and needs
some efforts to reach an optimum solution.
Broadcasting requirements are: high reliability and high dissemination speed with short latency in single-hop as well as multi-hop communications. Problems associated with regular broadcasting algorithms are: the high probability of collision in the broadcasted messages, the lack of feedback and the hidden node problem.
Using a predefined rout in MANET (Manickam and Shanmugavel,
2007; Lakshmi and Sankaranarayanan, 2006; Hussain
et al., 2007; Dan-Yang et al., 2009)
is not practical in VANET as vehicles always moving, thus, fixed routing techniques
In this study, we concerned with making a comparison for broadcasting the emergency message in VANET.
Emergency message broadcast: Several protocols are proposed considering
dissemination of the safety information such as (Durresi
et al., 2005; Costa et al., 2006;
Biswas et al., 2006; Briesemeister
et al., 2000) which intend to deliver the information to all vehicles
within senders coverage area (up to 2000 m) with low delay. Durresi
et al. (2005) authors propose building a hierarchical structure for
the vehicles located in the same direction in order speed the dissemination
of emergency message.
However, with the high mobility of VANET any hierarchical structure will not
last long. Costa et al. (2006) authors proposed
to choose the message forwarders depending on the use of a probabilistic method.
This approach is not proven to be a valid especially it depends on probability.
Furthermore, Mobility prediction method presented by (Meng
et al., 2008) is not suitable in high mobile network like VANET.
Emergency message rebroadcast: Wu et al.
(2010), authors proposed Transmission Range Adaptive Broadcast (TRAB) where
the selection of the forwarders depends on choosing the vehicles that have the
largest coverage area and can rebroadcast the message to a large number of vehicles.
This protocol depends on the forwarders coverage area. The coverage area
for all vehicles depends on the transmission power and channel status. The transmission
power for the emergency message is the highest and the channel status for the
adjacent vehicles is the same. The coverage area for the vehicles depends on
the progress from the original sender (Torrent-Moreno, 2007a),
so there is no novelty in this protocol.
Another approach of study adopted by Weiner (2010) where
authors proposed to broadcast the warning for none-mobile vehicles in a single
hop and concentrated on how to deliver the information to the driver and how
the driver will react to the warning message.
At this approach, authors didnt give their attention on how the emergency message is broadcasted, furthermore, the vehicles are static, no mobility.
Ching-Yi and Shou-Chih (2010), proposed a street-based
broadcast scheme that utilizes neighbors information by exchanging hello
messages among vehicles, when any probable danger is detected, a warning message
is broadcasted to all neighbors. The farthest vehicle is selected as a forwarder
depending on the information gained from the hello message, if the preselected
forwarder receives the message, it will rebroadcast it.
Depending on just one forwarder is not enough in a high mobile network like VANET. Furthermore, authors didnt depend on beacons to gain the information. They proposed to use hello message, which creates a chance to increase the channel load.
The contention period schemes (which is a waiting time that the receiver waits
before rebroadcasting the original message received from the sender) are proposed
by many researchers (Qiong and Lianfeng, 2010; Biswas
et al., 2006; Torrent-Moreno, 2007b; Torrent-Moreno
et al., 2009; Qiong and Lianfeng, 2010; Biswas
et al., 2006; Fubler et al., 2003;
Briesemeister et al., 2000).
Qiong and Lianfeng (2010) authors proposed the Link-based
Distributed Multi-hop Broadcast (LDMB), in which all the receivers of the emergency
message are potential forwarders. Each forwarder computes and waits for contention
time using Eq. 1, if the contention time ends the forwarder
will start to rebroadcast the emergency message.
Torrent-Moreno (2007b) and Torrent-Moreno
et al. (2009) where authors proposed position-based message forwarding
strategy by sending the emergency message in a broadcast fashion and selecting
the best forwarder available. All vehicles receiving that message are potential
forwarders. In order to decide which node forwards the message all receivers
will be assigned a contention window (waiting time); the contention window size
will be the smallest for the farthest node and the biggest size for the nearest
node, in other words, this protocol will give priority for the farthest node
to be the next forwarder.
The problem of the last two protocols that all the message receivers will compute the waiting time and wait to make the rebroadcast even the closest vehicles to the sender will do and this will make the entire network vehicles busy for any message received.
Another protocol proposed by Torrent-Moreno (2007a)
called Emergency Message Dissemination for Vehicular (EMDV) protocol, by enabling
the farthest vehicle within the transmission range to make the rebroadcasting
of the emergency message.
|| Sender utilizing EMDV
Choosing one forwarder vehicle is not appropriate in a high mobile network
like VANET as the position is always changing and the receiver vehicle may become
out of range when sending the message or simply the receiver cant receive
the message because of the channel problems like jam or denial of service, Fig.
Biswas et al. (2006) proposed that the receivers
of the message will select random waiting times and make acknowledgment to avoid
the re-transmissions from nodes closer to the original sender.
The acknowledgment scheme causes delay to the rebroadcast.
Fubler et al. (2003) proposed the Contention-Based
Forwarding (CBF) protocol where a vehicle sends a packet as a broadcast message
to all its neighbors. On receiving the packet, neighboring vehicle will contend
for forwarding the packet. The node having the maximum progress to the destination
will have the shortest contention time and will first rebroadcast the packet.
If other nodes receive the rebroadcast message, they will stop their contention
and delete the previously received message. This protocol mainly proposed for
forwarding the periodic safety message (Beacons).
The problem of this protocol that there should be a management technique to
manage the contention for all the neighboring vehicles and there is a chance
that the nearest vehicle to the sender may not hear the rebroadcast of another
vehicle, here this vehicle will rebroadcast the message and this called (hidden
node problem (Khan et al., 2008) also it may
lead to broadcast storm problem that makes the protocol useless.
Briesemeister et al. (2000) suggested that the
emergency message will be rebroadcasted by the receivers located at farther
distances from the sender by the selection of shorter waiting times, Eq.
where, d is the distance from the original sender, MaxWT: maximum waiting time, Range: transmission range.
Emergency message rebroadcast by network segments: Another way to rebroadcast
the message is to divide the network into segments proposed (Korkmaz
et al. 2004; Fasolo et al., 2006;
Zhou et al., 2010; Dai et
al., 2009; Zorzi and Rao, 2003).
Korkmaz et al. (2004) proposed a protocol called
Urban Multi-hop broadcast (UMB) aiming to maximize the message progress and
avoid broadcast storm, hidden node and reliability problems. The protocol assigns
the duty of forwarding and acknowledging the broadcast packets to only one vehicle
by dividing the road portion inside the transmission range into segments and
choosing the vehicle in the furthest non-empty segment without prior topology
information. The source node transmits a broadcast control packet, called Request
to Broadcast (RTB), which contains the position of the source and the segment
size. On receiving the RTB packet, nodes compute the distance between the sender
and the receiver. Then, nodes transmit a channel jamming signal, called black-burst,
that contains several time-slots equal to their distance from the source (in
number of segments): the farther the distance, the longer the black-burst. Each
node transmits its black-burst and senses the channel; if there is no other
black-burst in the channel it concludes that it is the farthest node from the
source. Then the node returns a Clear-to-Broadcast (CTB) control packet, containing
its Identifier (ID), to the source.
The Smart Broadcasting Protocol (Fasolo et al.,
2006) addressed the same objective as UMB using a different methodology.
Upon reception of a RTB message, each vehicle should determine its segment and
set a random back-off time. Each segment has its own contention window size,
i.e., if this segment has contention window size (4) TS (time-slot); vehicles
in the furthest segment should randomly choose a back-off time between (0) to
(3) TS. Vehicles in the next nearer segment choose a value between (4) to (7)
TS and so on, as vehicles near the sender should wait for longer time.
Vehicles will decrement their backoff timers by one in each time-slot while listening to the physical channel. While waiting, if any vehicle receives a valid CTB message, it will exit the contention time phase and listen to the incoming broadcast. On the contrary, if any node finishes its backoff timer, it will send the CTB containing its identity and rebroadcast any incoming broadcast.
While, Zorzi and Rao (2003) proposed the Geographic
Random Forwarding (GeRaF) protocol, which divides the network into equally adjacent
sectors, the transmitter (source) elects the sectors starting from the farthest
one, by sending RTB message, all the nodes in the elected sectors reply by CTB
message, if one node reply the CTB message, then this node will become the next
forwarder, if there are more than one node sent the CTB message the source issue
a collision message and make a collision-resolution procedure to elect the next
forwarder depending on a probabilistic rule.
Samara et al. (2011) proposed a novel technique
aiming to enhance the performance of the emergency message by assigning the
rebroadcast job to a limited number of vehicles and this to avoid the hidden
node and the flood problems.
The criteria of selection depending on the vehicles progress and on the number of vehicles in the last none-empty segment and assure that the number is sufficient to carry the rebroadcast of the emergency message so the message will reach to a larger number of vehicles.
Other approaches: Another idea based on RTB and CTB presented by Yuanguo
et al. (2010) where authors proposed a Cross Layer Broadcast Protocol
(CLBP) in which the sender vehicle broadcasts an RTB packet to all neighboring
vehicles and waits for the CTB packet from one of its neighbors, depending on
the information received, the furthest node will be chosen as the forwarder
of the emergency message and then. The emergency message will be broadcasted,
when the preselected forwarder receives the emergency message, it will rebroadcast
it, also it will send an acknowledgment for the original sender.
It is worth noting that, the potential forwarder vehicles wait the longest
time before rebroadcasting the emergency message. The previously mentioned protocols
are using the RTB and CTB handshake to select the forwarder before sending the
message and this may lead to long latency, especially for saturated traffic
Another approach adopted by Sommer et al. (2011),
where authors proposed the Adaptive Traffic Beacon (ATB) in which vehicle utilizing
the beacon message to send the emergency information, first. Vehicle detects
a probable danger, then it will check the channel status, if the channel is
idle, then it inserts the emergency information for the ready beacon and finally,
it broadcasts the beacon.
This approach helps to reduce the channel load, bus sending the emergency information using the beacon gives lower priority to the emergency information, as the emergency message has the highest priority and should be processed first upon receive and receiving the emergency information with the beacon information may lead the receiver to mix between the information and ignore it in some times.
Korkmaz et al. (2004) proposed Based Broadcast
(LBB) protocol, were the sender transmits a frame to all other vehicles and
when the receiver receives the frame, it decides which action to take depending
on the senders location and message type.
The protocol also depends on a repetition strategy to achieve a reliable delivery for the message broadcasted.
The main problem for this approach is that it is an only a single-hop broadcast; so the information will be broadcasted to a limited number of vehicles. It is unlikely that the protocol would support the multi-hop relaying of broadcast messages. If multi-hop relaying is used, as it would exhaust the network bandwidth.
Table 1 compares the previously mentioned protocols in the field of performance of the emergency message system.
|| Comparison table for protocols in the field of performance
of emergency message system
In this study, we presented a comparison between various protocols, which aim to broadcast the safety information for other vehicles. A summary table for all the protocols also presented. We conclude that the traditional techniques in increasing the emergency message performance score good results, but there still a room for improvement and the intelligent approach should be adopted to give the vehicles better selection criteria and hence better probability in receiving and rebroadcasting the emergency message.
This work is part of Universiti Sains Malaysia short term grant No. 304/PKMP/6363/1090.