IEEE VTC2005-Fall
Marc Torrent-Moreno, Moritz Killat and Hannes Hartenstein
DSN Research Group
Institute of Telematics
University of Karlsruhe
VTC-Fall‘05
Marc Torrent-Moreno
Universität Karlsruhe (TH)

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Vehicular ad-hoc networks
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Challenges of inter-vehicle communications
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Impact on different types of communications
– Periodic messages
– Event driven messages
– Bidirectional links
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Summary and Conclusions
VTC-Fall‘05
Marc Torrent-Moreno - 2
Universität Karlsruhe (TH)

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Increasing interest: Projects, consortiums, standardization bodies ...
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Goals
– Safety
– Environmental
– Comfort
– Infotainment
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Main characteristics
– 802.11p (DSRC) as underlying technology
– Mainly geo- and broad-cast
– Lack of centralized entity
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Not yet an agreed of applications with specific requirements
– Reliability
– Robustness
– Delay
– Fairness
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Underestimated implications from real radio channel effects
VTC-Fall‘05
Marc Torrent-Moreno - 3
Universität Karlsruhe (TH)

VTC-Fall‘05
Marc Torrent-Moreno
Universität Karlsruhe (TH)

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Lack of centralized management entity
– Difficult to make an efficient use of the resources, 802.11 [1]
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High mobility
– Compromise iterative optimization algorithms (e.g., Li et al.
[2], ADHOC-
NOW’04) and predefined routes (e.g., Füßler et al.
[3], ELSEVIER’04)
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Hidden terminal problem
– Broadcast mediums specially sensible
– Torrent-Moreno et al. [4],
VANET’04
Priority
Non-priority
VTC-Fall‘05
Highly populated scenario with 2.56Mbps inside communication range in a 6Mbps medium
Distance [m]
Marc Torrent-Moreno - 5
Universität Karlsruhe (TH)

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Channel congestion
– Scenarios with high number of nodes when high penetration rates
– 802.11p [5] considers channel hopping (restricted time for safety)
– Qu et al.
[6], VANET’04, study different repetition strategies and introduce feasibility areas: x
Can we achieve … ?
- Prob. Reception Failure = 0.01%
- Channel Busy Time = 50%
Exemplary Configuration:
- Message Rate = 200 ms
- Message Size = 250 bytes
- 30meter between cars
4 lane highway
150meters message range
= 40 Interfers
Interferer number
VTC-Fall‘05
Marc Torrent-Moreno - 6
Universität Karlsruhe (TH)

Radio channel characteristics
– In VANETs: multiple mobile and reflecting objects able to degrade the strength and quality of the receiving signal (Fading)
– Deterministic radio propagation models are invalid to evaluate the performance of communication protocols (e.g. Takai et al . [7], MOBIHOC’01)
No Speed
10 miles
85 miles
125 miles
Transmission power trade-off when trying to increase safety :
Transmission power
Prob. Reception of single transmission
, Prob. of collision for all transmissions
SNR (dB)
BER vs. SNR at 12Mbps and 400ns RMS delay
Yin et al.
[8], VANET’04
VTC-Fall‘05
Prob. Successful reception without interferences
Model and adjustments from Taliwal et al.
[9], VANET’04
Marc Torrent-Moreno - 7
Universität Karlsruhe (TH)

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Why Collisions may occur in a 802.11 medium?
– Two or more nodes have the same remaining back-off time
– Two or more nodes can not sense the transmission of each other
• Nodes far away from each other
• Multi-path Fading and Shadowing
Link Layer
Desynchronization
Node in the middle X sends a message with:
Communication Range = 200m
Carrier Sense Range = 400m
VTC-Fall‘05
As close as 85m!
Marc Torrent-Moreno - 8
Universität Karlsruhe (TH)

VTC-Fall‘05
Marc Torrent-Moreno
Universität Karlsruhe (TH)

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Motivation: Systems are able to detect non-safe situations with an accurate and updated information of all surrounding vehicles
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Requirements: - minimum message inter-arrival time
- minimum reliability
- minimum coverage area
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Trade-offs when trying to increase safety:
– Packet generation rate: #packets #collisions
– Transmission power: TxPower Interferences
• Information aggregation looks like a reasonable solution (safety value of information decreases with distance)
VTC-Fall‘05
Marc Torrent-Moreno - 10
Universität Karlsruhe (TH)

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Fairness
– Ultimate goal is SAFETY
– Classical ad hoc networks approaches:
• Capacity
• Connectivity
Not valid to maximize safety
– All nodes might have different safety requirements or ‘danger level’
– Torrent-Moreno et al [10],
VANET’05 :
• Centralized algorithm:
• Limit channel capacity
• Fairly share bandwidth
• Max-min TxPower
• Per node maximality
VTC-Fall‘05
Marc Torrent-Moreno - 11
Universität Karlsruhe (TH)

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Motivation: Needed in emergency situations to disseminate information
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A clever strategy required to handle:
– Broadcast storm problem (Ni et al .[11], MOBICOM’99)
– Multi-path fading and shadowing effects
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Challenge: Make sure all cars receive the information within specific time in the most efficient manner
VTC-Fall‘05
Marc Torrent-Moreno - 12
Universität Karlsruhe (TH)

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Motivation: Applications, e.g., for traffic information or access to stationary nodes few hops away
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P(2-ways link) = P(1-way link) 2 , assuming independence
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One hop link:
• Resulting overhead:
• Link probability
• Required prob.
of reception
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Multi-hop: Avoid unicast flows, e.g., CBF ( Füßler et al.
[3], Elsevier’04)
VTC-Fall‘05
Marc Torrent-Moreno - 13
Universität Karlsruhe (TH)

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Challenges of robust inter-vehicle communications:
– Hidden terminal problem
– High channel load
– Radio channel characteristics
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Impact on:
– Broadcast-periodic messages
– Event driven messages (data dissemination)
– Bidirectional links
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PHY/MAC models have crucial impact on protocols’ performance
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Need accurate models, also following technological advances
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Consider probabilistic models when designing robust communication protocols
VTC-Fall‘05
Marc Torrent-Moreno - 14
Universität Karlsruhe (TH)

VTC-Fall‘05
Marc Torrent-Moreno
Universität Karlsruhe (TH)

[1] “IEEE Std. 802.11-1999, Part11: Wireless LAN Medium Access Control (MAC) and
Physical Layer (PHY) specifications” IEEE Std. 802.11, 1999 edition.
[2] X. Li, T. Nguyen, and R. Martin, “An Analytic Model Predicting the Optimal Range for Maximizing 1-Hop Broadcast Coverage in Dense Wireless Networks” in Proc.
ADHOC-NOW , July 2004.
[3] H. Füßler, J. Widmer, M. Käsemann, M. Mauve, and H. Hartenstein, “Contention-
Based Forwarding for Mobile Ad-Hoc Networks”, Elsevier's Ad Hoc Networks 1 (4),
November 2003.
[4] M. Torrent-Moreno, D. Jiang, and H. Hartenstein, “Broadcast Reception Rates and
Effects of Priority Access in 802.11-Based Vehicular Ad-Hoc Networks,” in Proc.
ACM VANET , October 2004.
[5] “Dedicated Short Range Communications working group,” http://grouper.ieee.org/groups/scc32/dsrc/index.html.
[6] Q. Xu, T. Mak, J. Ko, and R. Sengupta, “Vehicle-to-Vehicle Safety Messaging in
DSRC,” in Proc. ACM VANET , October 2004.
VTC-Fall‘05
Marc Torrent-Moreno - 16
Universität Karlsruhe (TH)

[7] M. Takai, J. Martin, and R. Bagrodia, “Effects of Wireless Physical Layer Modeling in Mobile Ad Hoc Networks,” in Proc. ACM MOBIHOC , October 2001.
[8] J. Yin, T. ElBatt, G. Yeung, B. Ryu, S. Habermas, H. Krishnan, and T. Talty,
“Performance Evaluation of Safety Applications over DSRC Vehicular Ad Hoc
Networks,” in Proc. ACM VANET , October 2004.
[9] V. Taliwal, D. Jiang, H. Mangold, C. Chen, and R. Sengupta, “Empirical
Determination of Channel Characteristics for DSRC” in Proc. ACM VANET ,
October 2004.
[10] M. Torrent-Moreno, P. Santi, H. Hartenstein, “Fair Sharing of Bandwidth in
VANETs”, in Proc. ACM VANET , September 2005.
[11] S.-Y. Ni, Y.-C. Tseng, Y.-S. Chen, and J.-P. Sheu, “The Broadcast Strom
Problem in a Mobile Ad Hoc Network,” in Proc. ACM MOBICOM, August 1999.
VTC-Fall‘05
Marc Torrent-Moreno - 17
Universität Karlsruhe (TH)