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