Post on 26-Oct-2021
Review of Routing Protocols in VANETs
by
Rana Moazzam Tufail
B.E. Dawood College of Engineering and Technology, Karachi, Pakistan, 2008
A Project Submitted in Partial Fulfillment of the Requirements for Degree of
MASTER OF ENGINEERING
in the Department of Electrical and Computer Engineering
c©Rana Moazzam Tufail,2016
University of Victoria
All rights reserved. This thesis may not be reproduced in whole or in part, by
photocopy or other means, without the permission of the author.
i
Supervisory Committee
Dr. Fayez Gebali, Supervisor
Department of Electrical and Computer Engineering
Dr. Samer Moein, Departmental Member
Department of Electrical and Computer Engineering
ii
Abstract
Vehicular Ad Hoc Network (VANET) is becoming an important technology which col-
laborating ad hoc network, wireless LAN (WLAN) and cellular technology to attain
intelligent communication mechanism. Due to rapidly changing topology, obstacles in
communication network and limited mobility in VANET, there is a need of intelligent and
efficient routing protocol which promise improved efficiency in terms of minimizing delay,
increase throughput and reliability. A review of most recent protocols is presented by
using few parameters of the network, location verification, clustering, routing technique,
delay, control overhead and forwarding strategy. The review discusses the advantages
and disadvantages of routing protocols for vehicular ad hoc networks. It inspects the
need behind the design of these routing protocols. The review also includes Physical
layer and MAC protocol structure for current vehicular ad hoc networks. Finally the
review concludes discussing issues with routing protocol and Physical layer and MAC
protocol with regard to VANETs.
Contents
Supervisory Committee . . . . . . . . . . . . . . . . . . . . . . . . . . . . i
Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ii
Table of Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv
List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v
List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vi
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii
Dedication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viii
1 Introduction 1
2 Routing Protocols 4
2.1 Topology Based Routing Protocols . . . . . . . . . . . . . . . . . . . . . 5
2.1.1 FSR-Fisheye State Routing Protocol . . . . . . . . . . . . . . . . 5
2.1.2 AODV-Ad hoc On Demand Routing Protocol . . . . . . . . . . . 5
2.1.3 DSR-Dynamic Source Routing Protocol . . . . . . . . . . . . . . . 6
2.1.4 DSDV-Destination Sequenced Distance Vector Routing Protocol . 7
2.1.5 TORA-Temporally Ordered Routing Algorithm . . . . . . . . . . 7
2.1.6 ZRP-Zone Routing Protocol . . . . . . . . . . . . . . . . . . . . . 8
2.1.7 DYMO-Dynamic On-demand Routing Protocol . . . . . . . . . . 8
2.1.8 Pros and Cons of Topology Based Protocols . . . . . . . . . . . . 9
2.2 Position Based Routing Protocols . . . . . . . . . . . . . . . . . . . . . . 9
2.2.1 AEGRP-An Enhanced Geographical Routing Protocol . . . . . . 9
2.2.2 WNPRP-Wagon Next Point Routing Protocol . . . . . . . . . . . 10
2.2.3 GeoSVR-A Stateless Map Based Routing Protocol . . . . . . . . . 10
2.2.4 CAR-Connectivity-Aware Routing Protocol . . . . . . . . . . . . 10
2.2.5 Pros and Cons of Position Based Protocols . . . . . . . . . . . . . 11
2.3 Broadcast Based Routing Protocols . . . . . . . . . . . . . . . . . . . . . 11
iii
iv
2.3.1 EAEP-Edge Aware Epidemic Routing Protocol . . . . . . . . . . 11
2.3.2 DV-CAST-Distributed Veehicular Broadcast Routing Protocol . . 12
2.3.3 SRB-Secure Ring Broadcast Routing Protocol . . . . . . . . . . . 12
2.3.4 DADCQ-Distribution-Adaptive Distance With Channel Quality
Routing Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.3.5 UMB-Urban Multi-hop Broadcasting Routing Protocol . . . . . . 12
2.3.6 Pros and Cons of Broadcast Based Protocols . . . . . . . . . . . . 13
2.4 Multicast Based Routing Protocols . . . . . . . . . . . . . . . . . . . . . 13
2.4.1 ROVER-Robust Vehicular Routing Protocol . . . . . . . . . . . . 13
2.4.2 DG-CASTER-Direction-Based Geocast Routing Protocol for Query
Dissemination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.4.3 Pros and Cons of Multicast Based Protocols . . . . . . . . . . . . 14
3 Physical Layer of VANET 15
3.1 Physical Layer Architecture . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.2 Modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3.3 Challenges of PHY Layer . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
4 MAC Protocol of VANET 19
4.1 Medium Access Control . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
4.2 Challenges of MAC Protocol . . . . . . . . . . . . . . . . . . . . . . . . . 22
5 Conclusion 23
List of Figures
1.1 VANET Communication Model [1] . . . . . . . . . . . . . . . . . . . . . 1
1.2 Cellular, Ad hoc and Hybrid Networks [2] . . . . . . . . . . . . . . . . . 2
2.1 Taxonomy of VANET Routing Protocols . . . . . . . . . . . . . . . . . . 4
2.2 Route Discovery Mechanism [2] . . . . . . . . . . . . . . . . . . . . . . . 6
2.3 Route Maintenance Mechanism [3] . . . . . . . . . . . . . . . . . . . . . 7
2.4 Height of each node for updated message delivery [4] . . . . . . . . . . . 8
3.1 IEEE 802.11p Channel Frequency Band [5] . . . . . . . . . . . . . . . . . 15
3.2 IEEE 802.11p Protocol Stack and Sub-layer of PHY [6] . . . . . . . . . . 16
4.1 Data link layer of WAVE Protocol Stack [7] . . . . . . . . . . . . . . . . 20
4.2 Node Priority Process [8] . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
v
List of Tables
3.1 Comparison of PHYs implementations in IEEE 802.11a and IEEE 802.11p[21] 17
4.1 EDCA parameter settings for applications in IEEE 802.11p [8] . . . . . . 21
vi
vii
Acknowledgements
I would like to express my honest gratitude and deepest appreciation to my supervisor,
Dr. Fayez Gebali for his guidance, time, knowledge and support in the pursuit of my
studies and in the completion of this project. I am deeply thankful and grateful to my
lovely parents, my brother Mohsin, without his support and motivation, it would not
have been possible. I would also like to thank my wife, my sisters and all those who have
supported me throughout this entire process, by keeping me harmonious, motivated and
helping me put the pieces together. I will be grateful forever for their love.
viii
Dedication
This work is dedicated to my mother and my father, they are my strength, courage and
belief. I love you both.
Chapter 1
Introduction
VANET has become very challenging technology in recent years. It’s unique due to highly
dynamic in nature and intermittent connectivity. Figure 1.1 explains the communication
types in VANET. Two types of communication takes place, Vehicle to Vehicle (V2V) and
Vehicle to Infrastructure (V2I). Topology of network has changed from virtual to real as
nodes are replaced by vehicles acting as a router and client at the same time to share
information in between using wireless links. Operating mechanism of VANET is based
on information collected from traffic and road environment such as traffic congestion,
accidents, warning messages etc. There are two types of communication in Vehicle to
Vehicle, single hop communication and multiple hop communication. There are various
research projects which are related to VANET such as CarNet, NoW, DRIVE, Fleet Net
and CarTALK. There are number of applications such as Net access, Security distance
warning, Driverless Vehicles, Cooperative driving, Auto Parking, Driver help and Vehi-
cle collision warning. Need of efficient routing protocol is necessary in order to adapt
dynamic mobility of VANETs.
Figure 1.1: VANET Communication Model [1]
2
Figure 1.2 shows the architecture of Vehicular Ad hoc Networks(VANETs), categorized
into three major parts (a) Pure Cellular, (b) Pure Ad hoc, (c) Hybrid. The
network of Pure Cellular Architecture works in such a way that the access points and
cellular tower are connected to the internet to facilitate vehicular applications. In this
architecture vehicles can easily communicate by connecting internet with a wireless ac-
cess point or cellular gateway. Because of some geographic boundaries nodes can only
communicate with each other. However, the sensors help in informing about the traffic
conditions and also help in solving police crimes. In Pure Ad Hoc architecture, the nodes
execute vehicle to vehicle communication with each other. Roadside Cellular gateways
and access points help the vehicles which have wireless networking devices in communi-
cating with one another. Numerous applications in parts of urban monitoring, driving
assistance, safety and entertainment have used communicating units to access vigorous
information outside their network and share it through ad hoc infrastructure less com-
munication. As far as the hybrid architecture is concerned, it offers richer contents and
better flexibility in content sharing. In hybrid architecture the nodes act as servers and
they share the information like peers. These nodes are mobile thus, they make data
transmission less consistent.
Figure 1.2: Cellular, Ad hoc and Hybrid Networks [2]
Highly dynamic topology is one of the most important feature of VANETs. The topol-
ogy always keeps on varying because the vehicles move at a high speed. For instance
if there is 250m radio range between two vehicles then their link would last for almost
10 seconds. Vehicles follow a certain mobility pattern that is a function of the under-
lying roads, traffic lights, speed limit, traffic condition and drivers’ driving behaviors.
Because of the particular mobility pattern, evaluation of VANET routing protocols only
makes sense from traces obtained from the pattern. In frequently disconnected network
3
the link between the vehicles can rapidly disappear as the nodes transmit information.
This problem is aggregated by heterogeneous node density e.g Urban environment and
Rural environment. In addition non busy hours result in low node density, which results
in disconnectivity. A robust routing protocol needs to recognize the rapidly changing
topology and provides alternate paths to ensure smooth communication. In VANETs,
propagation is not free due to many obstacles on and off the road like buildings, trees,
pedestrians and vehicles. A VANET propagation model should be smart enough to take
obstacle in consideration which can cause wireless communication interference. Nodes in
VANETs are not subject to power and storage limitation as in sensor networks, another
class of ad hoc networks where nodes are mostly static. Nodes are assumed to have
ample energy and computing power. Therefore, optimizing duty cycle is not as relevant
as it is in sensor networks. Nodes are assumed to be equipped with sensors to provide
information useful for routing purposes. Many VANET routing protocols have assumed
the availability of GPS unit from on-board Navigation system. Location information
from GPS unit and speed from speedometer provides good examples for plethora of in-
formation that can possibly be obtained by sensors to be utilized to enhance routing
decisions.
Chapter 2
Routing Protocols
A routing protocol governs the way that two communication entities exchange informa-
tion. It includes the procedure of establishing a route, decision in forwarding and action
in maintaining the route or recovering from route failure. This section describes different
routing protocols proposed in the literature where a single data packet is transported to
the destination node without any duplication due to the overhead. As shown in Figure
2.1, protocols are classified into four categories, Topology Based, Position Based, Broad
cast and Geo cast based.
Figure 2.1: Taxonomy of VANET Routing Protocols
5
2.1 Topology Based Routing Protocols
Routing protocols use already available links to transmit the data in networks. Dynamic
routing decisions in the network are made by efficient routing protocols. Topology based
routing Protocols are classified into Proactive and Reactive.
Proactive routing carries the distinct feature, regardless of the request from the node, the
routing information such as the next forwarding hop is maintained in the background.
Flooding of control packets in the network are constant to maintain the path or link
among any pair of nodes. Due to that table is built in a node with each entry in table
points to next hop node toward a specific destination. Advantage of table driven routing
is that there is no searching or route discovery as destination path is already maintained
in background.Though it provides minimal latency for real time applications, most of
its bandwidth is consumed by unused paths, that creates overhead particularly in high
mobility. Protocols are normally based on shortest path algorithm.
Reactive routing protocols are opposite in nature to Proactive routing protocols, table
is not maintained when topology changes. In Reactive routing, route only initiates
when nodes want to communicate with each other. It helps minimizing the overhead
on network as this is the only communication taking place in the network. In order to
send data, query packets are flooded into the network in search of a route to destination.
Path is stored until that other node is irresponsive.
2.1.1 FSR-Fisheye State Routing Protocol
FSR [2] is a link state routing and maintains full topology map at each node, periodi-
cally exchange HELLO packets and periodically exchange of topology tables within local
neighbors instead of flooding the network. Updates are frequently sent to nearby desti-
nation then to remote destination. In order to reduce size of update routing message,
topology table use different frequencies for different entries depending on hop distance
to the current node.
2.1.2 AODV-Ad hoc On Demand Routing Protocol
Figure 2.2 explains the route establishment process in Ad hoc On Demand Distance
Vector (AODV) [4]. Route Request (RREQ) is generated in search of destination, each
node which receives RREQ, store sending node address in routing table. When request
finally reaches to destination, a Request Reply (RREP) is sent back to the same path. To
6
keep the update routing information and to prevent loops AODV uses Sequence number
maintained at each node and carried by all routing packets.
Figure 2.2: Route Discovery Mechanism [2]
2.1.3 DSR-Dynamic Source Routing Protocol
DSR [2] objective is to provide highly reactive process by implementing routing tech-
nique with very low overhead and quick reaction to frequent topology changes. DSR does
not require periodic HELLO messages as it is beaconless. DSR dynamically sends the
packets in network, upon receiving the request destination node send reply and carries
in header the route traversed packet, due to that path is established between source and
destination node. Source node can receive and store multiple replies from destination
which can be utilized in case of link termination. Here DSR has an advantage over
AODV, instead of flooding the network in case of failure it has an alternate route to
re-establish communication. Figure 2.3 showing available links to destination and infor-
mation of route error is delivered through same path. Source have multiple routes to
reach destination in case of link failure.
7
Figure 2.3: Route Maintenance Mechanism [3]
2.1.4 DSDV-Destination Sequenced Distance Vector Routing
Protocol
DSDV [9] deals with the routing loop problem. It provides loop free route by using short-
est path algorithm, it carries destination sequence number in packet header. Protocol
is carried by two types of packets, full dump and Incremental. First type, full dump
packets contain routing information of all nodes which are broadcasted to neighbors and
incremental packet deliver updates. Bandwidth is affected in full dump packets and the
incremental packets affect overhead in networks. Both types make DSDV unsuitable for
highly Dynamic VANETs.
2.1.5 TORA-Temporally Ordered Routing Algorithm
(TORA) [4] is a source initiated on demand routing protocol and it finds multiple routes
from a source node to a destination node. The three basic functions of TORA are Route
Creation, Route Maintenance and Route Delete. Route creation is done by QRY and
UPD packets. QRY keeps the destination address for which the algorithm is running.
UPD keeps the height of node I (Hi) for packet broadcasting. The height of destination
is set to 0 and all other nodes’ height set to NULL. The source broadcasts a QRY along
destination node’s ID. Node when receives a reply packet, will update its height only
when height in reply packet has minimum of all heights from reply packets it has received
till yet. After that, Reply packet will be rebroadcasted by the node. Invalid routes are
erased by flooding clear packet (CLR) in the network. The advantages of TORA are that
8
the execution of the algorithm gives a route to all the nodes in the network and minimize
communication overhead on topological changes. Maintenance of routes is complexed as
TORA allow route to every node in network, particularly in highly active VANETs.
Figure 2.4 is showing route Re-establishment on link failure 5-7, new reference level is 5.
Figure 2.4: Height of each node for updated message delivery [4]
2.1.6 ZRP-Zone Routing Protocol
ZRP [10] is a combination of proactive and reactive routing protocol. Network is di-
vided into different zones, each zone contains number of nodes. Proactive routing is
used if the packet is destined within the zone area and Reactive routing is used outside
of zone. Longer routes are affected by overhead in proactive routing, ZRP minimizes
control overhead for longer routes and eliminating the delays within zone. Disadvantage
of ZRP protocol is that it’s not suitable for high density and rapidly changing topology
of VANETs, because it works with proactive approach in large size zones and reactive
in small zones.
2.1.7 DYMO-Dynamic On-demand Routing Protocol
DYMO [11] protocol is a reactive multi hop routing protocol. Like AODV protocol,
sequence number is used to provide loop free paths. In DYMO a route request pro-
cess aims to maintain information about all intermediate nodes. In addition, each node
participating in an ongoing route discovery process have to gather information about a
requested node as well as intermediate nodes. Specifically at higher density level, which
9
happens more often in VANETs, routing and transport protocols can cause the network
overhead. Congestion is un avoidable when establishing a new path in the network and
retransmission of packets will only create more congestion.
2.1.8 Pros and Cons of Topology Based Protocols
In order to route packets from source to destination, Topology based protocols utilize
link information available in the network.
Pros
Discovery is not required.
Low latency for real time applications e.g Audio/Video streaming.
No periodic messages.
Support unicast, multicast and broadcast message.
Cons
Frequent network changes may cause congestion in network.
Huge amount of available bandwidth consumed by unused paths.
More control overhead as no control messages being triggered even on link failure.
2.2 Position Based Routing Protocols
In geographic (position-based) routing, node makes a decision on position of packet
destination and next hop neighbor’s position. Neighbor’s position is determined by
periodically sent Beacon messages. Nodes are neighbors if they fall under same radio
range. Each node knows its’ location in Geographic routing and OBU (On board Unit)
having GPS, so location of destination is already known to sender. As geographical
routing protocol do not follow traditional protocol mechanisms i.e sharing of link state
information with neighbor node or maintenance of routing table, it means less overhead
and more scalability and more suitable for dynamic environment like VANETs.
2.2.1 AEGRP-An Enhanced Geographical Routing Protocol
The AEGRP [12] selects a best route based on road segments with variables like traffic
saturation, road length, distance and velocities of vehicle. Each packet calculates the
10
path based on given road variables. Neighbors update the sender about vehicle velocities
and distance on receiving the broadcast request message which contains a query about
number of intersections, lanes and road length. On finding multiple routes to destination,
source will chose the route with higher density due to better transmission coverage. If
road densities are same, then priority will be given to shorter distance. On failure
of finding neighbor node, it carry and forward packet till the discovery of appropriate
neighbor. Protocol has a prediction behavior for neighbor discovery and it does that by
calculating self velocity,distance and position. Network is suppose to work better with
high velocities of vehicle.
2.2.2 WNPRP-Wagon Next Point Routing Protocol
The WNPRP protocol [13] assumes that the range of a Wagon in the network is around
500 m. Also each Wagon in the network should be able to gain sufficient knowledge
about the nearby nodes. This is done by sending ‘Hello Message’ periodically with the
nearby Wagons. This help in gaining the information such as the position of Wagon,
speed and direction at which the Wagon is moving. The source from where the Wagon
starts and the destination point is marked with the help of GPS. Wagon gather the
information about network from its location. If data need to be transmit from source to
destination, source will filter and discard the information about vehicles going on other
route. Vehicles on the same route would be considered to avoid dispersion of packets.
2.2.3 GeoSVR-A Stateless Map Based Routing Protocol
GeoSVR [14] is combination of node location and digital map. Vehicle density is taken
into account before route setup. If next hop is destination, source will transmit the
packets directly. Second step is to select next hop, protocol selectively finds a neighbor
within a range to avoid packet loss caused by wireless channel. Disadvantage of this
protocol is strict mechanism of choosing nodes which can cause delay and loss of packets.
2.2.4 CAR-Connectivity-Aware Routing Protocol
CAR [15] is topology based on destination location, route selection process, data forward-
ing and path maintenance, using the concepts of Anchor and Guards. Every node in
CAR transmits route finding request, on receiving request each node updates hop count,
minimum and average neighbor list and send it back to sender node. If link breaks in
11
between the transmission CAR uses a technique GUARD to recover from link failure.
CAR shows better performance in packet delivery and minimizing routing overhead in
the network.
2.2.5 Pros and Cons of Position Based Protocols
Pros
Scalability
Maintenance and discovery of routes is not required.
Efficient in rapidly changing mobility pattern.
Low overhead.
Cons
Obtaining exact location.
Does not guarantee connectivity indoors or underground locations e.g tunnels.
Obstacles on highways
2.3 Broadcast Based Routing Protocols
Protocol is used for flooding broadcast messages in the network. In case of emergency
information needs to be broadcasted so other vehicles should know about it. Protocol
broadcast the message to all neighbor nodes which can intensify the transmission. Pro-
tocol has low packet loss ratio and more reliable in transmitting important information
in the network.
2.3.1 EAEP-Edge Aware Epidemic Routing Protocol
EAEP [1] improves reliability, uses bandwidth efficiently and propagates information
in an efficient manner. Protocol eliminates overhead of periodic Hello messages being
exchanged among vehicles and simplifies the network maintenance. Each node push its
own position to transmits packets in order to eliminate beacon messages. EAEP after
receiving this information use number of transmission from front and back nodes to
calculate the probability for either nodes retransmit the packets or not. EAEP addresses
12
flooding issue but the disadvantage is, it does not deal with link failures in network and
increase packet delivery ratio.
2.3.2 DV-CAST-Distributed Veehicular Broadcast Routing Pro-
tocol
DV-CAST [16] keeps an update info about neighbors in order to initiate communication.
Protocol works on multi hop scheme. DV-CAST gets information about the network
from periodic beacon messages, it deals with different parameters of the network e.g
vehicle density state, traffic lights, neighbor nodes etc. When source cannot find enough
connected nodes it won’t broadcast, the packet will be stored till more nodes gets into
broadcast range. Packet will be discarded if no node is found. Protocol enable message
duplication awareness in nodes using flag parameter. DV-CAST is suitable for both high
and low traffic density because it reduces broadcasting overhead. Disadvantage is data
transmission delay and high control overhead.
2.3.3 SRB-Secure Ring Broadcast Routing Protocol
SRB make node rings based on their receiving power, rings are grouped as Inner, Outer
and Secure rings [2]. Protocol controls retransmissions in the network and limit them to
the rings to minimize overhead in order to achieve more reliable and stable network.
2.3.4 DADCQ-Distribution-Adaptive Distance With Channel
Quality Routing Protocol
DADCQ [17] aims for large networks with large node distribution. Nodes are selected
on their geographical location before broadcasting a packet. Receiving node will make a
decision to rebroadcast message on destination location, packet will not be rebroadcast
if destination is close which minimizes network delay and increase network efficiency.
Disadvantage is it creates a message overhead.
2.3.5 UMB-Urban Multi-hop Broadcasting Routing Protocol
UMB protocol [18] is developed to eliminate the issue of hidden node and packet collision
simultaneously initiate communication in multi-hop broadcast. Nodes in UMB protocol
13
do not account previous network knowledge to forward packets and their acknowledg-
ment. Protocol tries to reach to the farthest node while broadcasting. UMB is efficient
in a network with high data loss and high traffic density.
2.3.6 Pros and Cons of Broadcast Based Protocols
Pros
Due to smaller number of nodes it has a high efficiency on highways.
Reliability, since packet delivered to destination via multiple nodes.
Effective in minimizing overhead due to broadcast storm mechanism.
Cons
Due to reachability beyond transmission range it consumes significant network
bandwidth.
Nodes receive duplicate messages due to flooding(Broadcast nature of protocol)
Cause longer data transmission delays in network.
2.4 Multicast Based Routing Protocols
Multicast protocols use multi hop communication to send messages from sender to par-
ticular cluster nodes. They are sub-classified into two classes Cluster based routing and
geocast based routing Geocast Based Routing Protocol: Geocast routing protocols [9]
belongs to a multicast routing protocol which based on sending packets from a source to
a particular group of destinations. In geocast routing protocol one node can broadcast
to other nodes fall under same geographical range, marked as zone of relevance (ZOR).
Nodes under same geographical area are called members, if member goes out of that
boundary then packet will be dropped. Zone of forwarding (ZOF) is point of interaction
between zone and non-zone members. ZOF developed to provide a reliable packet’s de-
livery in highly dynamic topology. Disadvantage of these protocols is transmission delay
due to intermittent connectivity in network.
2.4.1 ROVER-Robust Vehicular Routing Protocol
ROVER [9] is a geographical multicast protocol. ROVER has somewhat similar AODV
protocol mechanism, but more efficient and consistent as it only flood network with
14
control packets and unicasts the data packets. Another difference is that ROVER reply
back to the node it received the packet rather then sending it to source node, which helps
in creating more stable path to forward packets. ROVER assumes that each node have
geographic location, identity and a map. Node initiate route discovery within its ZOR
by sending route request message which contains source address, ZOR address, location
and route sequence number. Another node only accepts the packet if it falls under his
ZOR or ZOF, otherwise it drops the packet. Node on accepting the packet will reply
including its ID and retransmits the packet. ROVER is considered as reliable routing
protocol in VANETs, except couple of drawbacks, higher control overhead and delay in
data delivery caused by retransmission of packets.
2.4.2 DG-CASTER-Direction-Based Geocast Routing Proto-
col for Query Dissemination
DG-CASTOR [19] gives an idea of link availability. Protocol does a node prediction
in the network, identifies neighbors which have tendency to communicate with source
at a particular time period. Protocol is designed for commercial use in VANETs. The
main aim of DG-CASTOR is to build an essential commonality that is based on future
location prediction of moving nodes in the network. This prediction behavior is known
as Rendezvous group.
2.4.3 Pros and Cons of Multicast Based Protocols
Pros
Reduced Power consumption Reduced transmission overhead.
Reduced control overhead.
Assure packet delivery in highly dynamic topology.
Cons
Consume bandwidth.
Packet transmission delay due to link failure.
Chapter 3
Physical Layer of VANET
The Vehicle to anything (V2X) communication system is an essential part of the Intelli-
gent Transport Systems (ITS). Commercial and safety applications e.g traffic efficiency,
driverless cars are matter of concern and need enhancements. The IEEE community
is working on a new standard technology IEEE 802.11p modified for ITS communica-
tion [17]. The IEEE 802.11p is expansion of IEEE 802.11 and also known as Wireless
Access in the Vehicular Environment (WAVE). It uses the mechanism initially provided
by IEEE 802.11 to operate in the Dedicated Short Range Communication (DSRC). Fea-
ture of DSRC is higher transfer rates and low communication delays for small areas. It
provides exchange of data between vehicles (V2V) and vehicle to roadside infrastructure
(V2I) up to 1000m with transmission rate 3 Mbps to 27 Mbps and node speed up to
161 mph. IEEE 802.11p operates on about 9 channels and the frequency band used by
each channel is described in Figure 3.1. CH172-5.860 GHz and CH184-5.920 GHz are
safety dedicated channels. The first addresses to security solutions and other protect
against congestion on other channels. Transmission broadcast and link creation is done
by Channel CH178-5.890GHz which is a control channel. There is 5 MHz in the begin-
ning of the band used as guard band (GB).
Figure 3.1: IEEE 802.11p Channel Frequency Band [5]
16
3.1 Physical Layer Architecture
The physical layer (PHY) is an interface between the MAC protocol and the media re-
sponsible for sending and receiving frames. The PHY of the IEEE 802.11p is similar to
the one of IEEE 802.11a [6]. It consists of two sub layers as shown in Figure 3.2. Physical
Layer Convergence Protocol (PLCP) and Physical Medium Access (PMD). PLCP com-
municates with the MAC and also converts the Packet Data Unit (PDU) coming from
MAC to form an OFDM frame. The Physical Medium Access (PMD) defines the details
of transmission and reception of individual bits on physical medium.e.g radio channels
and fiber links. PMD is responsible to handle data encoding and perform modulation.
Figure 3.2: IEEE 802.11p Protocol Stack and Sub-layer of PHY [6]
3.2 Modulation
DSRC uses Orthogonal frequency division multiplexing (OFDM) modulation to multi-
plex data, it divides the radio signal to multiple smaller sub-signals [6]. Main reason for
using OFDM is, it utilizes the spectrum efficiently by allowing overlap. Transmission of
all the sub carriers are simultaneous. OFDMA- Orthogonal frequency division multiple
access offer shared access to multiply users by assigning subsets of subcarriers to indi-
vidual users, allows lower data rates from multiple users. However, negative impact can
be caused by high mobility such as message reception failure or packet errors. Message
reception failure is due to nodes/receivers move out of sender transmission range dur-
ing safety related message transmission. The other disadvantage is increase in packet
17
error rates and subsequently lower channel capacity due to high mobility cause intense
Doppler’s spread on OFDM. Failure or delay in IEEE 802.11p networks could lead to
hazardous situations, so those networks have to be very efficient and robust.
Regardless of many benefits, Physical layer encounter various challenges which are unique
in comparison to other wireless networks such as : Collision Avoidance among vehi-
cles in high mobility environment. Latency for VANET safety applications has to be
50ms and must not over 100ms. Doppler’s spread caused by high packet error rate
and OFDM being sensitive to frequency offset may effect with lower channel capacity
due to high mobility. To address these issues of IEEE 802.11p, few changes has been
done in Physical layer parameter. Sub-carrier is using half clocked mode. For example,
single IEEE 802.11a OFDM channel consists of 52 sub-carriers, 48 among them used for
transmit data and 4 for pilot carrier, but single channel of IEEE 802.11p uses equal num-
ber of sub-carriers changing the bandwidth/channel from 20 MHz to 10 MHz resulting
in decrease of Doppler’s spread and interference. Data rate in 802.11p is 3 to 27 Mbps
which is 6 to 54 Mbps in 802.11a. Comparison between parameters of IEEE 802.11a and
IEEE 802.11p is shown in table 3.1. All the time parameters are double due to change
in bit rate ( 20MHZ to 10MHz), subcarrier spacing is half and rest of the parameters are
same as in IEEE 802.11a.
Table 3.1: Comparison of PHYs implementations in IEEE 802.11a and IEEE 802.11p[21]
18
3.3 Challenges of PHY Layer
Still there are several challenges faced by 802.11p like: Effect of noise in bit and symbol
energy, multipath effects, channel variation, channel estimation, network coverage range
and bit rate enhancement techniques. Though these issues are partially addressed. An
efficient PHY will eliminate these problems. PHY should be robust and scalable with low
latency and minimum BER. Different transmission scenarios like urban, highways may
cause PHY to perform within variable channels because rain, dust and other environmen-
tal elements affect the transmission method. Apart from that, various technical factors
can affect the PHY performance (Transmission quality), like unused carries, modulation,
encoding, data rate and frame size.
Chapter 4
MAC Protocol of VANET
VANET creates a challenging scenario with quickly changing environment where move-
ment of nodes are very high. Moreover it’s hard to keep track of nodes due to high
mobility. In absence of Access point/Base station in decentralized network, it is difficult
to manage the scalability problems. Frequency reuse or cell structure of cellular network
is not possible to large extent. Moreover, efforts are being made in US and Europe to use
single standard of frequency channel for transmission. Thus, the same radio spectrum
will be shared by most vehicular communication links in a limited area, which will cause
interference. The MAC method selects when a node has access to use the common com-
munication channel. Which MAC method need to be used in a specific communication
network is subjected to network topology and application. In a centralized network AP
or BS has information about all the nodes within their range and traffic has to route
through them. So, distribution of resources like frequencies and time slots can be done
by MAC protocol. But, in ad hoc networks where there is no central mechanism and
extreme movement of nodes in a given area, it is hard to implement an efficient MAC
protocol. In VANET situation, MAC mechanism should be self- organizing, scalable and
reliable. Moreover, absence of centralized network in ad hoc scenario, MAC mechanism
has to be scalable so it can accumulate increasing number of nodes and resources. In
VANET traffic safety applications, there are no controlled number of nodes and not
known in advance. The MAC protocol for that reason has to satisfy the fairness, relia-
bility and delay requirements even in high node density network.
WAVE protocol stack is shown in Figure 4.1. Layer is divided into two parts, MAC and
logical link control (LLC) layer. LLC is responsible for point and multipoint communi-
cation between wireless and wired channels.
19
20
Figure 4.1: Data link layer of WAVE Protocol Stack [7]
4.1 Medium Access Control
The IEEE 802.11p implicates the contention-based channel access EDCA (Enhanced
Distributed Channel Access) as the MAC algorithm, it is a basic type of Distributed
Coordination Function (DCF) by 802.11. EDCA applies Carrier Sense Multiple Access
(CSMA) with Collision Avoidance (CSMA/CA) [8]. In CSMA/CA a node initially listen
to the channel before starting communication and if the channel is free for Arbitration
Inter-frame space(AIFS), the node will start to transmit by selecting a random back-off
time. If the medium is or becomes busy in between that period the node will perform a
backoff procedure, i.e. node will defer the access for randomized time period. To ensure
significant safety communications and to find a reliable method of message transmission
in rapidly changing topology, the 802.11p MAC protocol use various Access Classes
(ACs) i.e queues for data traffic as shown in Figure 4.2. Data traffic is classified into
four ACs : Background traffic (BK), Best Effort traffic (BE), Video traffic (VI) and Voice
traffic (VO). Different ACs use different AIFSN and CW values, as shown in Table 4.1,
it describes a set of four AC’s for station operation with mechanism to calculate CWmin
and CWmax for each AC’s.
21
Figure 4.2: Node Priority Process [8]
Table 4.1: EDCA parameter settings for applications in IEEE 802.11p [8]
Ranking of transmission in EDCA is realized by a new Interframe Space (IFS) instead
of AIFS, which is an extension of the back-off procedure in DCF [5]. The Short Interframe
Spacing (SIFS), PCF Interframe Space (PIFS), and DCF Interframe Space (DIFS), are
new AIFS values for various Access Class (AC) that are brought in EDCA.
22
4.2 Challenges of MAC Protocol
MAC protocol needs more enhancement in order to meet the highly dynamic topology
of VANETs. Future work can be done on following areas:
Better Throughput : Safety related messages are necessary to be transmitted
between vehicles sporadically and to have a better traffic control, Vehicle to RSU
communication needs improvement. Therefore, high throughput is very important
in VANETs.
Scheduling optimization: The proposal of multiple channel configuration has
also been given rather than using single channel like Control Channel (CCH) and
Service Channel (SCH). The method will guarantee the minimum delay while trans-
mitting safety messages and making sure their delivery in dense traffic scenario.
Traffic control: By using Back-off algorithms in MAC protocol the increase in
traffic flow cause more contention periods. This can trigger packet collision be-
fore reaching to destination, or there is a possibility that node want to broadcast
would unable to initiate communication and in result cause maximum packet loss,
therefore to avoid collisions and packet loss, traffic control needs to be taken care
of.
Chapter 5
Conclusion
There are number of routing protocols designed for VANET. Many of them addressing
to specific situation and specific issue. For example, CAR routing protocol address the
issue where node receives an inaccurate info about neighbors and their location. On
the other hand DSR works efficiently in low traffic scenarios and reacts well to rapidly
changing topology, not effective when mobility is very high. Regardless of the fact that
protocols are dealing with particular problems in routing environment, there is no agreed
ground to authenticate their performance. Simulation tools or environments are not yet
able to test routing protocols performance with regard to VANETs. In summary, new
protocols are being designed, they are progressing and becoming established. Further,
PHY layer and MAC protocol for VANET are discussed. Review has been done about
parameters of both layers that are reformed in new technology IEEE 802.11p, finally
highlighted the issues and possible research areas.
23
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