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Vehicular Ad-hoc NETwork (VANET)
Speaker: Yi-Ting MaiContact info. :wkb@wkb.idv.twDate: 2010/05/04
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Outline
Overview of VANETsPhysical Layer and MAC protocols for VANETsBroadcast Routing Protocols for VANETsApplications for VANETs
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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
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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
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What is a VANET?
Roadside base station
Inter-vehicle communications
Vehicle-to-roadside communications
Emergency event
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A taxonomy of vehicular communication systems
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Inter-vehicle communication (IVC) Systems
IVC systems are completely infrastructure-free; only onboard units (OBUs) sometimes also called in-vehicle equipment (IVE) are needed.
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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)
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IVC systems
a) Single-hop IVC system b) Multihop IVC system
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Vehicular Communication
Future Vehicular Communication Scenario
Internet
Internet Gateway
Vehicle
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Vehicular Communication-DSRCIn 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.
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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
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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
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Operating Environment
According to the environments of operating vehicles, the VANETs can be established in the following situations:– City environments, disaster situations, extreme weather c
onditions, and so on.• For instance: City environments, have certain unique characteri
stics:– Many tall buildings obstructing and interfering the transmission signals, – In the highway scenario, vehicles are closer together than, thus incur int
erference if their transmission range are large, – The topology is usually two dimensional (e.g. with cross streets).
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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 application
s, 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.
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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
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Threat model
Presented in SeVeCom (Secure Vehicular Communication) projectAn 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
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Attack 1 : Bogus traffic informationTraffic
jam ahead
Attacker: insider, rational, active
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Attack 2 : Generate “Intelligent Collisions”
SLOW DOWN
The way is clear Attacker: insider, malicious, active
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Attack 3: Cheating with identity, speed, or position
Wasn’t me!
Attacker: insider, rational, active
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Attack 4: Jamming
Roadside base station
Jammer
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Attack 5: Tunnel
Physical tunnel or jammed area
Wrong information
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Attack 6: Tracking
A
* A at (x1,y1,z1)at time t1
* A communicates with B
* A refuels at time t2 and location
(x2,y2,z2)
1
2
AB
A
* A enters the parking lot at time
t3* A downloads from server X
3
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Protocols of Layers in VANETsIn 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
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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.
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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
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DSRC/802.11p Physical Layer (2/4)
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
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DSRC/802.11p Physical Layer (3/4)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
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DSRC/802.11p Physical Layer (4/4)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
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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
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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
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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)
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CSMA/CD
CSMA/CD (Carrier Sense Multiple Access/ Collision Detection)
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CSMA/CD (cont.)
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CSMA/CACSMA/CA (Carrier Sense Multiple Access/ Collision Avoidance)
MH
MH
MH
MH
Sender Receiver
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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
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Hidden-Terminal Problem (cont.)
MH
MH
MH
MH
Sender Receiver
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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)
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Exposed-Terminal Problem (cont.)
MH
MH
MH
MH
x
Sender Receiver
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CSMA/CA (cont.)
ReceiverSender
RTS
CTS
MH
MH
MH
MH
DataACK
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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)
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The Design with Busy Tone ChannelProtocol– 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.
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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
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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)
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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.
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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.
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IEEE 802.11 DCF and Problems
With the protocol (IEEE 802.11 DCF) mentioned above can solve the hidden-terminal problemsThe problems of IEEE 802.11 DCF– The exposed-terminal problems exists.– The number of contention nodes during the contentio
n period increases.– The length of backoff time period.
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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)
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The Power Control DesignThe 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
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The Power Control Design
Problems:– With controlling power, the problem of exposed-
terminal can be improved, the hidden-terminal problem may occur.
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The Spilt-Channel Design to Improve the Problem of Contending for the
ChannelSpilt-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.
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Influence of the Backoff Time
The length of backoff time:– If the node density in IEEE 802.11 DCF is high, to a
void 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.
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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.
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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.
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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
(b) centralized one-hop network(a) distributed mobile multihop network
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Hot SpotADSL/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
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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.
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Broadcast StormIn 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 t
han that in MANET
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Broadcast RoutingMessage Dissemination– Ideal solution: Minimum Connected Dominating Set, which minimizes pa
cket 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 th
e target nodes.– To design a broadcast algorithm to make the desired vehicles to receive
the message as soon as possible.
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Four Broadcasting Strategies
Different broadcasting strategies to select the forwarding nodes:– Probability-based– Location-based – Neighbor-based– Cluster-based
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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
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Broadcast Routing
Probability-based
Car APDF = 0.8
Car BPDF = 0.5
Forwarding Node choose
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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.
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Broadcast Routing
Location-based Target
Forwarding Node choose
Car Bwants to turn right
Car A
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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).
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Broadcast Routing
Neighbor-basedTarget
Forwarding Node choose
Car B
Car ACollect the information of neighbors
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Broadcast Routing
Cluster-based– Nodes are grouped in clusters represented by an ele
cted 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.
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Broadcast Routing
Cluster-based
Cluster-Header
Cluster-Header
Gateway-Node
Forwarding Node choose
Applications for VANETs
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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
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Assistance for Safe Navigation (1/3)
There are some components must be included into a smart car.
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Assistance for Safe Navigation (2/3)
Overview of the demonstrator routing architecture
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Assistance for Safe Navigation (3/3)
A danger situation:– The system sends the warning message immediately
after there are cars accident occurring.
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Hot Spot
RDS/DVB/DABGSM/GPRS/3G/
3.5G/WiMAX
WiFi/DSRCService terminals
Signal exchangingfacilities
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 ServicesVehicle Infotainment Service UNS LifeUNS Life
ETC/CVO
MobileBusiness services
Multi-ModalNavigation/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
整合車內、家庭與辦公室應用
Source: adapted from TEEMA, 2007/12
車載產業及智慧交通願景