intelligent transportation systems (its) and inter...

Intelligent Transportation Systems (ITS) and Inter-vehicular Communication (IVC)

Internet access

entertainmentAccesso

cooperative gaming

p2p communication (es. VoIP, chat)V2V

tourist information

toll service

traffic informationITS services

road information

Collision avoidanceSafety

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Non-Safety Applications

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Preferential routes


Emergency vehicles routes

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ITS - Top Application: SAFETY

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Safety Applications

• Cooperative Forward Collision Warning• Road Condition Warning• Emergency Electronic Brake Lights

• Pedestrian Crossing Information$ Traffic Signal Violation Warning$ Visibility Enhancer• Public Safety

Cooperative Forward Collision WarningTraffic Signal Violation Warning

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Safety Applications

Pedestrian Crossing Information

Road Condition Warning

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Cooperative Trajectory Planning

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Projects on Intelligent Transportation Systems

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2006 – Frank Kargl – CCC Ulm

Vehicular networks: which technologies for which applications ?

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Vehicular networks: a short range transmission approach

Considering each vehicle as a node of a mobile ad-hoc network

Two communication patterns:

� Vehicle to Vehicle (V2V)� Vehicle to Infrastructure (V2I)

Vehicular Ad-Hoc NETwork(VANET)

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A standard for short range communication in VANET? Why not !?!

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• High mobility and rapid but predictable topology changes• Frequent fragmentation• Small effective nework diameter but potentially large scale• Low spatial redundancy• Vehicles not completely reliable• Not significant power constraints• Variable network density

Vehicular Networks: characteristics

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Vehicular Networks: Challenges

� Physical: tx power setting, directional antennas, MIMO,..

� Data Link: Lack of centralized management entity, hidden and exposed terminal problems, multichannel

� Network : routing, mobility management

� Transport: flow control� Applications

� Security � QoS� Efficient and Reliable Messages

Diffusion

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Supporting broadcasting in IEEE 802.11 basedVehicular ad hoc networks

Broadcast communication is predominant in VANET!

Alert messages Periodic messages

In general, broadcast means transmitting a packet to all nodes in a network

What does “broadcast”mean in a VANET?

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Broadcasting in a VANET is the capability to send messages to every node within an Area of Interest (Message Range)

Message range is fixed by applications

Broadcasting in a VANET means that messages should be sent to every node within a Message Range by a Message Lifetime

But.. Safety applications also have strict delay requirements (Message Lifetime)

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Broadcast reliability issues at medium access layer in IEEE 802.11 based Vehicular ad hoc networks

802.11 is known for not being able to manage the medium very efficiently, especially in case of broadcast messages.

Providing reliable delivery of broadcast messages in a VANET introduces several key technical challenges:

� The hidden terminal problem is not alleviated because of the lack of the RTS/CTS exchange

� No retransmission is possible for failed broadcast transmissions – A failed unicast transmission is detected by the lack an of acknowledgment (ACK) from the receiver. However, acknowledgments are not used for a broadcast message.

� The contention window (CW) size does not change because of the lack of MAC-level recovery. The lack of detection of failed broadcast transmissions results in the size of the CW being held constant

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350m

� 8 lanes highway with 4 lanes per direction in a circular fashion

� cars distributed uniformly along the circle, and all of them move at a constant speed without changing lanes

� all lanes have a different speed, assigned randomly with values between 55km/h and 120km/h.

� Each node sends 10 UDP packets per second. Message Size: 200/ 500 bytes

� Channel Models: Two-ray-ground / Nakagami� Communication ranges: 100/ 200m

dynamic simulation scenario

Broadcast reliability at medium access layer in IEEE 802.11 based VANET: performance analysis

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A broadcast message will not be received by all neighbors within a circular transmission range of a sender but it will suffer collisions both due to two or more direct neighbors accessing the channel at the same time and due to the well-known hidden terminal problem

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Some solutions:

� change IEEE 802.11 mechanisms

� Retransmission and relay strategies

Broadcast reliability at medium access layer in IEEE 802.11 based VANET: performance analysis

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Enhancing broadcast reliability at medium access layer: some approaches

Distributed TDMA

RTS/CTS based

Busy tone

Repetitive transmission

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Msg Range

Ideal Tx Range

If Tx_Range � Msg_Range

One hop Broadcast transmission

Can I adjust TX power to cover the Msg Range?

But….

� Is it sensible and feasible to adjust Tx_power for each packet?

� Tx_power limit� Impact on spatial reuse� Which propagation model has to be

taken into account for fixing TX_power? ��

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Msg Range

Ideal Tx Range

Propagation effects (path loss, shadowing, multipath fading,..),hidden nodes, collisions,…

Some vehicles could not correctly receive the messages !!! ��


Msg Range

Ideal Tx Range

In Urban scenarios, vehicles behind buildings could not correctly receive the messages

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Multi-hop is useful…: � when the message range is larger than the Tx Range� For increasing the probability of reception of the messages � For reaching vehicles hidden behind obstacles

Msg Range

Tx Range

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Message diffusion issues in IEEE 802.11 basedVANET: Simple Flooding

The simplest way to disseminate a message via multi-hop is simple flooding : Every node rebroadcasts the message

Msg Source

Relayer

Multi-hop is useful … : � when the message range is larger than the Tx Range� For increasing the probability of reception of the messages � For reaching vehicles hidden behind obstacles

Simple flooding leads to the broadcast storm problem:� many redundant transmissions� High contention� Many collisions

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Simple Flooding: In conclusion

Directions to alleviate the broadcast storm problem and increase the reliability:

� inhibit some hosts from rebroadcasting

� differentiate the timing of rebroadcasts

� Multiple copies (redundant) of the same message may be received

� Heavy contention could exist because rebroadcasting hosts are probably close to each other

� Collisions are more likely to occur because the RTS/CTS dialogue is inapplicable and the timing of rebroadcasts is highly correlated

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Broadcasting reliability and optimality at network layer : Minimum Connected Dominating Sets

Given S a set of nodes, a Dominating Set D(S) is a set of nodes such that each node from S either belongs to D(S) or has a neighboring node in D(S)

A broadcasting scheme at network layer is reliable if nodes that rebroadcast the message belong to a connected dominating set

But ………Is it so easy?

An Optimal Broadcasting scheme uses a connected dominating set of minimal size (MCDS)

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Some Approaches:� probabilistic: each node rebroadcast with probability p and a random delay� counter-based: each node rebroadcasts if it received the message from less

than C neighbors � distance-based: each node rebroadcasts if the distance to each previous

forwarder is greater than D� location-based: each node rebroadcast if the additional covered area is > A� cluster-based: only heads in each cluster rebroadcast� Neighbor elimination and self pruning: each node rebroadcasts if it can reach

additional neighbors

Broadcasting reliability and efficiency at network layer: the heuristic approach

The problem of finding a connected dominating set of minimal size is NP-complete!!!

But ………even if every node has global knowledge about the network……

Heuristics

�They can require neighbors knowledge or not

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Efficient Broadcasting in VANET: the spatial differentiation approach

Each vehicle that receives a message waits, before rebroadcast, a time WT inversely proportional to its distance from the source

d

WT(d)

Range

MaxWT

0

Tx Range

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When a vehicle receives a duplicated message, it discards the new message

But, if the old message has not been transmitted yet, the older one is discarded and the new one is treats based on its direction of arrival:

• if the direction is the same of the original message, a new tx attemptingphase starts for the new message

• otherwise, the new message is discarded


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Observations:

How can receivers know their distance by the source?Uhm……

� each vehicle knows its own position by using GPS system!

� the information on position of the source node could be added to the message


NOTE: The spatial differentiation approach requires that each node only knows its own position! No knowledge on neighbors positions is required. So that, no periodic information exchange is needed among nodes!

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Observation: Suppression time (ST) has to be taken into account.

ST is the period of time elapsing from the expiration of the winner’s timer up to the time when its packet is received by another node

STB = QDB + BTB + PD

If (WTA – WTB) < STB node A could have already queued the packet for transmission when it overhears the copy transmitted by the winner B thus uselessly rebroadcast the packet


Uhm……

PHY

MAC

…

APPL WTA

PHY

MAC

…

APPL WTB

A B

BTB

QDB

Problem 1

In order to avoid this problem, WTs should be chosen so that the difference between WTs of two nodes is always greater than ST ....but this is very difficult!

WT = Waiting TimeQD = MAC Queuing DelayBT = Backoff TimePD = Propagation Delay

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Due to channel errors and interference some contending nodes might uselessly retransmit the packet because they do not overhear the copy

transmitted by the winner B


B

Problem 2

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d

CWinterval

Range0

PHY

MAC

…

APPL

BT

Tx Range S4 S2 S1S3

Efficient Broadcasting in VANET: spatial differentiation approach using MAC access priority

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RTB

CTB

DATA

CTB

BTA

A

B

S

BTB B overhears CTB from A and cancels its CTB tx attempt

Fw=A

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A scheme based on MAC control packets and Backoff Timer differentiation

� The sender node (S) starts a Forwarder Election Procedure (FEP) by sending a Request–To–Broadcast (RTB) control packet containing its position.� Each potential forwarder determines the sector Sr it belongs to, randomly picks a Backoff Time (BT) in the associated contention window interval and starts a timer: the higher the distance, the shorter the timer value.� When the timer expires the node replies with a Clear–To–Broadcast (CTB).� Upon receiving a CTB, S transmits the data packet which carries the identifier of CBT sender which will be the next forwarder.

Tx Range S4 S2 S1S3S

B A

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B A

Upon receiving the RTB packet, nodes transmit a channel jamming signal,called Black–Burst(BB), that covers a number of time–slots equal to their distance fromthe source (in number of sectors): the further the distance, the longer the black–burst.

RTB

CTB

DATABBA

A

B

S

BBB

RTB~~ ~~QD

Finished the BB transmission it checks the channel status:• If there are still ongoing transmissions, it assumes that a farther node is contending. So that, it

exits the contention phase.• if the channel is sensed idle, the node returns a Clear–to–Broadcast (CTB)

The potential relay node (the farther) waits the longest time before retransmitting!!!

Spatial differentiation by using Black Bursts

Uhm……

What happens if there are many election procedures at the same time? When node check the channel status, they might overhear BB from other election procedures. So that they erroneously assume that some other contending node is transmitting a longer BB and do not send the CTB [11], [13]

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A

B

Due to interference or CTB collisions, control packets could be lost

RTBBTMAX

RTBS

CTB

CTBA

B

After the maximum Backoff time S retransmits the RTB

Spatial differentiation approach with RTB/CTB:RTB retransmission

Collision

A and B may choose the same BT

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Due to interference, DATA packet could be lost

RTB DATAS

BTB

CTB CollisionA

B

BTA

CTB

Node B may not sense

CTB and DATA

A retransmission strategy is needed In case DATA packet is lost

Two acknowledgment approaches:

� implicit acknowledgment

� explicit acknowledgment

Spatial differentiation approach with RTB/CTB:Data retransmission

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S overhears RTB transmitted by node A and stops retransmission timer

RTB DATAS

BTB

CTBA

B

BTA

CTB

RTB~~ ~~QD

How to set Retransmission

timer? Node S should set retransmission timer according to the queuing delay (QD).

But …QD could vary!

Too low R-Tx timer values => unnecessary retransmissions

Too high R-Tx timer values => high delay

Spatial differentiation approach with RTB/CTB:Data implicit acknowledgment strategy

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Upon receiving DATA, the elected relay A sends an ACK control packet to S

RTB DATAS

BTB

CTBA

B

BTA

CTB

ACK

The source S can set its retransmission timer value according to DATA tx time

Due to ACK packet transmission, an additional overhead is introduced

Spatial differentiation approach with RTB/CTB:Data explicit acknowledgment strategy

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RTB/CTB handshake grants a more accurate differentiation

RTB/CTB handshake can help alleviating the hidden terminal problem

RTB/CTB introduces an additional delay (RTB + CTB Tx time) and overhead

Pros and Cons of using RTB/CTB based schemes

RTBBTMAX

RTBS

CTB

CTBA

BCollision

RTB DATAS

BTB

CTB CollisionA

B

BTA

CTB

Due to channel errors or collisions, control packets and data packets could be lost

FEP FailureThe retransmissions introduce additional overhead

If a maximum number of retries is reached, diffusion stops

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DATA

BTA

A

B

S

ACKBTB

B overhears ACK from A and cancels its ACK tx attempt

ACK

The DATA/ACK scheme (DAS) (1/2)

� The sender node (S) starts a Forwarder Election Procedure (FEP) by directly sending a DATA packet containing its position.� Each potential forwarder determines the sector Sr it belongs to, randomly picks a Backoff Time (BT) in the associated contention window interval and starts a timer: the higher the distance, the shorter the timer value.� When the timer expires the node replies with a ACK packet.


B A

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It employs only one control packet. Hence, it does not introduce much overhead

By using a control packet, DAS permits to untie the message transmission time from the contention mechanism thus enabling possible semantic control on the messages

DAS Does not guarantee that, at each hop, the elected forwarder is unique

The DATA/ACK scheme (DAS) (2/2)DATA

BTAAB

S

ACKBTB

ACK

� diffusion process can only stop in case no node in the message propagation direction is able to correctly receive the message and the maximum number of retries is reached

Diffusion process rarely stops

DATABTA

AB

S

BTB

DATA

DATA

This scheme does not !

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r [1] S. Basagni, M. Conti, S. Giordano, I. Stojmenovic – “Mobile Ad Hoc Networking”r [2] M. Torrent-Moreno, M.Killat, H. Hartenstein - “The Challenges of Robust Inter-Vehicle

Communications”r [3] J. J. Blum, A. Eskandarian, L. J. Hoffman – “Challenges of Intervehicle Ad Hoc

Networks”r [4] S. Yousefi, M. S. MIousavi, M. Fathy – “Vehicular Ad Hoc Networks (VANETs):

challenges and perspectives”r [5] M. Torrent-Moreno, D. Jiang, H. Hartenstein – “Broadcast reception rates and

effects of priority access in 802.11-based vehicular ad-hoc networks”

r [6] S.-Y. Ni, Y.-C. Tseng, Y.-S. Chen, and J.-P. Sheu, "The broadcast storm problem in a mobile ad hoc network“

r [7] Williams, T. Camp – “Comparison of Broadcasting Techniques for Mobile Ad Hoc Networks”

r [8] L. Briesemeister, L.Sch¨afers, G. Hommel- “Disseminating Messages among Highly Mobile Hosts based on Inter-Vehicle Communication”

r [9] C.-F. Chiasserini, R. Gaeta, M. Garetto, M. Gribaudo, M. Sereno, “Efficient Broadcasting of Safety Messages in Multihop Vehicular Networks”

r [10] A. Zanella, E. Fasolo, “An effective broadcast scheme for alert message propagation in vehicular ad-hoc networks”

References (1/2)

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r [11] G. Korkmaz, E. Ekici, F. OzgunerBlack-”Burst-Based Multihop Broadcast Protocols for Vehicular Networks”

r [12] Nathan Balon and Jinhua Guo– “Increasing Broadcast Reliability in Vehicular Ad Hoc Networks”

r [13] G. Ciccarese, M. De Blasi, P. Marra, C. Palazzo, L. Patrono, “On the use of control packets for intelligent flooding in VANET”

r [14] G. Ciccarese, M. De Blasi, P. Marra, C. Palazzo, “A timer based intelligent flooding scheme for VANETs”

References (2/2)

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