VANETS Vehicular Adhoc NETworkS

Vehicular Ad-hoc NETwork (VANET)
Speaker: Yi-Ting Mai
Contact info. :wkb@wkb.idv.tw
Date: 2010/05/04

2
Outline
Overview of VANETs
Physical Layer and MAC protocols for VANETs
Broadcast Routing Protocols for VANETs
Applications for VANETs

3
Why Vehicular Networks?
Safety
– On US highways (2004):
• 42,800 Fatalities, 2.8 Million Injuries
• ~$230.6 Billion cost to society
– Combat the awful side-effects of road traffic
• In the EU, around 40,000 people die yearly on the roads; more
than 1.5 millions are injured
• Traffic jams generate a tremendous waste of time and of fuel
– Most of these problems can be solved by providing
appropriate information to the driver or to the vehicle

4
Why Vehicular Networks? (cont.)
Efficiency
– Traffic jams waste time and fuel
– In 2003, US drivers lost a total of 3.5 billion hours
and 5.7 billion gallons of fuel to traffic congestion
Profit
– Safety features and high-tech devices have become
product differentiators

7
A taxonomy of vehicular
communication systems

8
Inter-vehicle communication (IVC)
Systems
IVC systems are completely infrastructure-free;
only onboard units (OBUs) sometimes also
called in-vehicle equipment (IVE) are needed.

9
IVC systems
Single-hop and multihop IVCs (SIVCs and
MIVCs).
SIVC systems are useful for applications
requiring short-range communications (e.g., lane
merging, automatic cruise control)
MIVC systems are more complex than SIVCs but
can also support applications that require long-
range communications (e.g., traffic monitoring)

10
IVC systems
a) Single-hop IVC system b) Multihop IVC system

11
Vehicular Communication
Future Vehicular Communication Scenario
Internet
Internet Gateway
Vehicle

12
Vehicular Communication-DSRC
In 2003, FCC established the service and license
rules for Dedicated Short Range
Communications (DSRC) Service.
– DSRC is a communication service that uses the 5.9 GHz
band (5.850-5.925 GHz band) for the use of public safety
and private application.
– The vehicular related services and communication
standards enable vehicles and roadside beacons to form
VANETs (Vehicular Ad Hoc Networks) in which the
mobile nodes (vehicles) can communicate each other
without central access points.

13
VANETs vs. MANETs
A VANET consists of vehicles to form a network which
is similar to a Mobile Ad Hoc Network (MANET).
However, there are following differences between these
two networks.
– Vehicles mobility
• Vehicles move at high speed but mobility is regular and predictable
– Network topology
• High speed movement makes network topology dynamic
– No significant power constraint
• Recharging batteries from vehicle
– Localization
• Vehicles position estimate accurately through GPS systems or on-
board sensors

14
Features of VANETs
The characteristics of VANETs can be summarized after
comparing with the MANETs.
– Dynamic topology
• Nomadic nodes with very high speed movement cause frequent
topology variation
– Mobility models
• Vehicles move along original trajectories completely different from
typical MANET scenarios
– Infinite energy supply
• Power constraint can be neglected thanks to always recharging
batteries
– Localization functionality
• Vehicle can be equipped with accurate positioning systems (GPS and
GALILEO) integrated by electronic maps

15
Operating Environment
According to the environments of operating
vehicles, the VANETs can be established in the
following situations:
– City environments, disaster situations, extreme
weather conditions, and so on.
• For instance: City environments, have certain unique
characteristics:
– Many tall buildings obstructing and interfering the transmission
signals,
– In the highway scenario, vehicles are closer together than, thus
incur interference if their transmission range are large,
– The topology is usually two dimensional (e.g. with cross streets).

16
Scopes of VANETs (1/2)
Communication range of VANETs
– Short/medium-range communication systems (vehicle-to-
vehicle or vehicle-to-roadside)
Applications of VANETs
– The VANETs vision includes vehicular real-time and safety
applications, sharing the wireless channel with mobile applications
from a large, decentralized array of commercial service providers.
– VANET safety applications include collision and other safety
warnings.
– Non-safety applications include real-time traffic congestion and
routing information, high-speed tolling, mobile infotainment, and
many others.

17
Scopes of VANET (2/2)
VANET research issues
– Safety and non-safety applications
– Roadside-to-vehicle and vehicle-to-vehicle communication
– Communication protocol design
– Channel modeling
– Modulation and coding
– Power control and scalability issues
– Multi-channel organization and operation
– Security issues and countermeasures
– Privacy issues
– Network management
– Simulation frameworks & real-world testbeds

18
Threat model
Presented in SeVeCom (Secure Vehicular
Communication) project
An attacker can be:
– Insider / Outsider
– Malicious / Rational
– Active / Passive
– Local / Extended
Attacks can be mounted on:
– Safety-related applications
– Traffic optimization applications
– Payment-based applications
– Privacy

19
Attack 1 : Bogus traffic information
Traffic
jam
ahead
• Attacker: insider, rational, active

20
Attack 2 : Generate “Intelligent
Collisions”
SLOW
DOWN
The way
is clear• Attacker: insider, malicious, active

21
Attack 3: Cheating with identity,
speed, or position
Wasn’t me!
• Attacker: insider, rational, active

25
Protocols of Layers in VANETs
In this topic, we introduce the physical layer and the
802.11 related MAC protocols. Afterwards, the routing
protocols between vehicles are presented.Finally, the
applications of VANETs are proposed.
– The physical layer and the 802.11 related protocols.
• The physical layer and the MAC layer of DSRC/802.11p
• 802.11 DCF
– Routing protocols
• Position-based Routing (Unicast)
• Geocasting Routing (Multicast)
• Broadcast Routing
– Applications of VANETs.

Physical Layer and MAC protocols
for VANETs

27
Physical/MAC Layers
DSRC/802.11p
– Dedicated Short Range Communication (DSRC) was
released in 2002 by the American Society for Testing
and Materials (ASTM).
– In 2003, the standardization moved to IEEE Forum
and changed the name from DSRC to WAVE
(Wireless Ability in Vehicular Environments), which
was also known as 802.11p.

28
DSRC/802.11p Physical Layer (1/4)
DSRC/802.11p
– The standard of 802.11p is based on IEEE 802.11a
PHY layer and IEEE 802.11 MAC layer
• Seven 10 MHz channels at 5.9GHz
• one control channel and six service channels
Vehicle to
vehicle
Service
channel
Service
channel
Control
channel
Intersection
CH 172 CH 174 CH 182CH 180CH 178CH 176 CH 184
5.855
5.925
5.915
5.905
5.895
5.885
5.875
5.865
Frequency (GHz)
Optionally combined
service channels

29
DSRC/802.11p vs. 802.11a
– 802.11a is designed for high data rate multimedia
communications in indoor environment with low user
mobility.
– DSRC PHY uses a variation of OFDM modulation
scheme to multiplex data.
• high spectral efficiency, simple transceiver design and
avoids multi-path fading

30
DSRC/802.11p vs. 802.11a
– DSRC/802.11p reduces the signal bandwidth from
20MHz to 10MHz.
• all parameter values are doubled in time domain in order
to increase the robustness (e.g. timeout increase) to ISI
caused by the multi-path delay spread and Doppler
spread effect
– Data rates are between 6 and 27 Mbps
– Transmit power level are changed to fit requirements
of outdoor vehicular communications
• communication ranges up to 1000 meters

31
Parameters DSRC/802.11p 802.11a
Information data rate Mb/s 3, 4.5, 6, 9, 12, 18, 24, and
27
6, 9, 12, 18, 24, 36, 48, and
54
Modulation BPSK, QPSK, 16-QAM, 64-
QAM
BPSK, QPSK, 16-QAM, 64-
QAM
Coding rate 1/2, 1/3, 3/4 1/2, 1/3, 3/4
Number of subcarriers 52 (=48+4) 52 (=48+4)
OFDM symbol duration 8μs 4μs
Guard time 1.6μs 0.8μs
FFT period 6.4μs 3.2μs
Preamble duration 32μs 16μs
Subcarrier frequency
spacing
0.15625MHz 0.3125MHz

32
Revolution and Design in 802.11
DCF
The revolution of 802.11 DCF can be described
in the following.
– The design of avoiding collisions: The design to
solve the collisions including collisions incurred by
the terminal problem.
– The improvement design to IEEE 802.11 DCF

33
The Design of Avoiding Collisions
The design of avoiding collisions
– In mobile wireless networks, the objectives of MAC protocols
is to avoid collisions, process contention, and re-tramsit lost
packets to increase the overall throughput. In previous works,
the design of avoiding collisions can be described in the
following.
• Carrier Sense Multiple Access Protocols, CSMA: A mobile node uses
carrier sensing technology to detect whether there is any node using
the channel before transmitting data to avoid collisions.
• The problems in the CSMA: hidden- and exposed- terminal problems
• Terminal problems:
– Hidden terminal problem
– Exposed terminal problem

34
Medium Access Control (MAC)
LAN(Ethernet)
– CSMA/CD （ Carrier Sense Multiple Access with
Collision Detection ）
WLAN(802.11)
– CSMA/CA (Carrier Sensing Multiple
Access/Collision Avoidance)

35
CSMA/CD
CSMA/CD (Carrier Sense Multiple
Access/ Collision Detection)

37
CSMA/CA
CSMA/CA (Carrier Sense Multiple Access/
Collision Avoidance)
MH
MH
MH
MH
Sender Receiver

38
Hidden-Terminal Problem
The hidden-terminal problem occurs when node
C sends data to node B, as shown in the
following Figure.
A B C

39
Hidden-Terminal Problem (cont.)
MH
MH
MH
MH
Sender Receiver

40
Exposed-Terminal Problem
The exposed-terminal problem occurs when
node C is exhibited to transmit data to node D.
A C DB
Broadcast ranges of each node
(Interfere)

41
Exposed-Terminal Problem (cont.)
MH
MH
MH
MH
x
Sender Receiver

42
CSMA/CA (cont.)
ReceiverSender
RTS
CTS
MH
MH
MH
MH
Data
ACK

43
The Designs to Solve the Hidden-
Terminal Problem
The Designs to Solve the Hidden-Terminal
Problem
– The design of using busy tone channel
– The design of MACA (IEEE 802.11 DCF)

44
The Design with Busy Tone Channel
Protocol
– Each node equipped with an extra busy tone channel to send
out the busy signals when the node is processing data
transmission.
– When a node would like to transmit data, it detects weather
there are nodes issuing the signals by other nodes in its
range.
– If a node detects no signal, it can process the transmission.
Problems
– Needed an extra busy tone channel.
– The hidden-terminal is solved, but the exposed-terminal
problem still exists.

45
IEEE 802.11 DCF
To solve the hidden-terminal problem, MACA proposed
the Multiple Access Collision Avoidance protocol, which
is adapted by the IEEE 802.11 MAC to be the IEEE
802.11 DCF.
– Contention period
– Handshake period
– Data period
– ACK period
data ACK
CTS
contention
4 1
8 4
6 3 SIFSSIFS
Defer Access
handshake
RTS SIFS data
NAV
ACK
Sender
Receiver
Others

46
Contention Period of IEEE 802.11
DCF
Contention period
– Interval Frame Space, IFS
• Short IFS, SIFS) ： CTS, ACK, or Poll Response
• PCF (PIFS)
• DCF (DIFS)

47
Handshake period of IEEE 802.11
DCF
Handshake period
– In MACA, before processing data transmission, a sender
broadcasts a RTS (Request To Send) signal to inform its
neighbors that it will send out data.
– When a neighbor except the sender and the receiver receives
the RTS signal, it use the NAV (Network Allocation Vector) to
exhibit itself to issue signals to avoid occurring interference of
data transmission.
– When the receiver receives the RTS, it will reply a CTS (Clear
To Send) signal if it accepts the RTS request.
– Similarly, when a neighbor of the receiver except the sender
receivers the CTS, it uses the NAV to exhibit itself to send
any signal.

48
Data and ACK Periods of IEEE
802.11 DCF
Data period
– After completing the handshaking period, the sender
and the receiver can transit data, while the neighbors
of these two nodes are exhibited by the NAV until the
finishing data transmission.
ACK period
– After the completion of data transmission, the
receiver sends a ACK to the sender to show that the
data has been received.
– At the same time, all neighbors are in the listening
status for contending the channel.

49
IEEE 802.11 DCF and Problems
With the protocol (IEEE 802.11 DCF) mentioned
above can solve the hidden-terminal problems
The problems of IEEE 802.11 DCF
– The exposed-terminal problems exists.
– The number of contention nodes during the
contention period increases.
– The length of backoff time period.

50
Exposed-Terminal in IEEE 802.11
DCF
With IEEE 802.11 DCF, the nodes are exhibited
by NAV increase. Therefore, the problem of
exposed-terminal becomes more serious than
CSMA.
– In CSMA, only node C is exhibited to send or receive
data.
– In IEEE 802.11 DCF, nodes C and D are exhibited.
AC B D AC B D
(a) (b)

51
The Power Control Design
The design of controlling power to improve the
exposed-terminal problem.
– With detecting the strength of signals, the power of
data transmission can be controlled to fit the
distance between two nodes.
– With the decrease of exhibited area, the exposed-
terminal problem can be improved.
AC B E AC BD E FD F

52
The Power Control Design
Problems:
– With controlling power, the problem of exposed-
terminal can be improved, the hidden-terminal
problem may occur.

53
The Spilt-Channel Design to Improve
the Problem of Contending for the
Channel
Spilt-channel design
– Two pipeline stages of contending for the channel.
• Nodes that would like to send data contending at the first
stage. If nodes pass the first one, they can contending for
the channel at the second stage.
• The number of nodes contending for the channel is
reduced.
– To avoid occurring starvation, the protocol uses the
weight schemes to make some nodes enter the
second stage directly.

54
Influence of the Backoff Time
The length of backoff time:
– If the node density in IEEE 802.11 DCF is high, to
avoid collisions in the contention period, the backoff
time should be increase.
– If the node density in IEEE 802.11 DCF is low, too
long backoff time incurs the time waste of waiting.

55
Dynamic Adjustment of Backoff Time
Schemes: Dynamic adjustment for the backoff
time to reduce the waste of bandwidth utilization.
– Three kinds of the dynamic adjustments
– Successful history records.
– Polling the neighbors
– Statistical method: With the statistic list, the length of
backoff time can be decided according to the statistic
list.

56
DSRC/802.11p MAC Layer (1/2)
DSRC/802.11p MAC
– MAC layer of DSRC is very similar to the IEEE
802.11 MAC based on CSMA/CA with some minor
modifications.
– DSRC involves vehicle-to-vehicle and vehicle-to-
infrastructure communications.

57
DSRC/802.11p MAC Layer (2/2)
Vehicle-to-Vehicle
– relative speed : low
– absolute speed: high
– multi-hop relay
Vehicle-to-Infrastructure
– high download rates over
a short duration
(a) distributed mobile multihop network

58
ADSL/WiFi
WorkstationWiMax/3.5G
DSRC/802.11 DSRC/802.11
V-V communication
V-I communication
V-V communication
V-I communication
TTS Server
Communication architecture

Broadcast Routing Protocol for
VANETs

60
Broadcast Routing
In Inter-Vehicle Communication Systems
(IVC) ， broadcasting is an efficient method to
spread messages.
The reasons of occurring broadcast storm
– In a broadcasting network, the situations of
contentions and collisions often take place if an
efficient broadcasting scheme is not used.
– The result incurred by broadcasting is called
broadcast storm.

61
Broadcast Storm
In VANETs, broadcast is used for disseminating
the traffic information ：
– Detour route
– Accident alert
– Construction warning
– etc…
Some messages will be periodically broadcasted
by roadside unit (RSU) for several hours or even
some days.
– The problem of broadcast storm in VANET is more
serious than that in MANET

62
Broadcast Routing
Message Dissemination
– Ideal solution: Minimum Connected Dominating Set, which
minimizes packet rtx and preserves network connectivity.
– Realistic solutions: trade-off between robustness and
redundancy.
The important concern in designing a broadcast scheme
in VANET.
– How to design broadcast algorithm to efficiently transmit
messages to the target nodes.
– To design a broadcast algorithm to make the desired vehicles
to receive the message as soon as possible.

63
Four Broadcasting Strategies
Different broadcasting strategies to select the
forwarding nodes:
– Probability-based
– Location-based
– Neighbor-based
– Cluster-based

64
Broadcast Routing
1. Probability-based:
– A given PDF determines the decision, for example
depending on the number of copies a node has
received.
– The strategy is often dynamic.
– PDF = probability distribution function

65
Broadcast Routing
Probability-based
Car A
PDF = 0.8
Car B
PDF = 0.5
Forwarding Node choose

66
Broadcast Routing
Location-based
– The selection criterion is the amount of additional
area that would be covered by enabling a node to
forward.
– Some proposal also computes position prediction as
useful input information.

67
Broadcast Routing
Location-based Target
Car B
wants to turn right
Car A

68
Broadcast Routing
Neighbor-based
– A node is selected depending on its neighbors status
(for instance, the status concerns how a neighbor is
connected to the network).

69
Broadcast Routing
Neighbor-based
Target
Car B
Car A
Collect the information of neighbors

70
Broadcast Routing
Cluster-based
– Nodes are grouped in clusters represented by an
elected cluster-head. Only cluster-heads forward
packets.
– Nodes in the same cluster share some features (e.g.,
relative speed in VANETs).
– Reclustering on-demand or periodically.

71
Broadcast Routing
Cluster-based
Cluster-Header
Cluster-Header
Gateway-Node

73
Assistance for Safe Navigation
Traffic safety
– Detecting dangerous situations
– Sending warning messages to other cars using ad-
hoc networking
Traffic management services
– Traffic congestion
– Weather forecast
– Road works

74
Assistance for Safe Navigation (1/3)
There are some components must be included
into a smart car.

75
Overview of the demonstrator routing
architecture

76
A danger situation:
– The system sends the warning message immediately
after there are cars accident occurring.

77
RDS/DVB/DAB
GSM/GPRS/3G/
3.5G/WiMAX
WiFi/DSRC
Service
terminals
Signal
exchanging
facilities
車內網路
智慧車輛智慧車輛
智慧駕駛智慧駕駛
Ubiquitous UseUbiquitous Use
Intelligent Vehicle
•Intelligent Driving
•Advanced Safety Features
Intelligent Vehicle
•Intelligent Driving
•Advanced Safety Features
Innovated Services
Vehicle Infotainment Service UNS LifeUNS Life
ETC/
CVO
Mobile
Business
services
Multi-Modal
Navigation/
Reservation
E-call/
Maintenance
& warrantee
LBS/
Social Networking
Safety Warning/
Mitigation
智慧道路智慧道路
WiFi/Cellular/DSRC
GPS/RDS/DVB/DAB
+
Urban Nomadic/pedestrians
Telematics
整合車、家庭與公室應用內辦
Source: adapted from TEEMA, 2007/12
車載產業及智慧交通願景

VANETS Vehicular Adhoc NETworkS

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