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PE-WASUN2

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Razvan Stanica

on 17 June 2013

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Transcript of PE-WASUN2

Loss Reasons in Safety VANETs and
Implications on Congestion Control Razvan Stanica*, Emmanuel Chaput**, and André-Luc Beylot**

* INSA Lyon **INP Toulouse 9th ACM International Symposium on Performance Evaluation of Wireless Ad Hoc, Sensor, and Ubiquitous Networks Paphos
25.10.2012 What are Safety Vehicular Networks?

Why are Safety Message Lost?

How can we Control Congestion?

Are these solutions Efficient? Complementary? Implementable? Razvan Stanica
PE-WASUN 2012 Safety Vehicular Networks Razvan Stanica
PE-WASUN 2012 Enabled Applications:
Cooperative Collision Warning
Emergency Electronic Brake Lights
Lane Change Assistant
... With only two types of messages:
Cooperative Awareness Message (CAM)
Decentralized Environmental Notification (DEN) Message Loss Reasons Radio Propagation Problems Razvan Stanica
PE-WASUN 2012 Synchronized Transmissions Concurrent Transmissions Expired Messages The physical channel in a vehicular environment is extremely noisy Happen when two (or more) one-hop neighbors begin transmitting in the same slot Collisions between hidden terminals, a station accesses the channel during any slot of an ongoing transmission CAMs (a.k.a. beacons) have a limited lifetime. When a new message reaches the MAC layer, the old beacon is dropped. Congestion Control Mechanisms Razvan Stanica
PE-WASUN 2012 Beaconing Frequency Data Rate Transmission Power Contention Window Carrier Sense Threshold Principle: Frames transmitted with a higher data rate occupy the channel for less time
Problem: Complex modulations are difficult to use on the noisy vehicular channel
Impact on losses:
Propagation - increased probability
Expiration - reduced probability
Synchronization - no effect (probability depends on number of neighbors, beaconing period and back-off interval)
Concurrency - reduced probability (transmission duration is an essential parameter) Principle: Use a lower transmission power under high density
Problem: We still need to cover a certain safety range
Impact on losses:
Propagation - increased probability
Expiration - reduced probability (less contenders)
Synchronization - reduced probability
Concurrency - reduced probability, higher impact (hidden nodes get closer) Principle: Give a larger choice for back-off under high vehicular density.
Problem: A higher back-off time results in a higher delay.
Impact on losses:
Propagation - no effect
Expiration - increased probability
Synchronization - reduced probability
Concurrency - reduced probability Principle: Use a higher carrier sense threshold (be more aggressive) under high density.
Problem: Carrier sense adaptation is difficult to implement.
Impact on losses:
Propagation - no effect (assuming the reception threshold is constant)
Expiration - reduced probability (more transmission opportunities)
Synchronization - no effect (as long as the carrier sense threshold remains lower than the reception threshold)
Concurrency - increased probability (transmitters get closer) Results using
Congestion Control JiST/SWANS framework

StreetRandomWaypoint mobility model

Fixed beaconing frequency (10Hz)

Fixed data rate (6 Mb/s) Standard scenario Transmission Power = 40 dBm (50% reception probability at 500m)
Contention Window = 7 (standard IEEE 802.11p value)
Carrier Sense Threshold = -95dBm (Reception Threshold= -75 dBm) Reception Ratio Transmission Power Contention Window Carrier Sense Threshold Razvan Stanica
PE-WASUN 2012 The impact of collisions with one-hop neighbors decreases (less neighbors)
Collisions with hidden nodes become more important (two-hop neighbors get closer)
A lower transmission power leads to more propagation problems Razvan Stanica
PE-WASUN 2012 Simulation Scenario Principle: Transmit less often under high density, when the speed is also lower.
Problem: Incompatible with some safety applications (e.g. left-turn assistant).
Impact on losses:
Propagation - no effect
Expiration - reduced probability (longer periods)
Synchronization - reduced probability (lower load)
Concurrency - reduced probability Expired messages appear as the contention window increases
Major impact on the number of synchronized transmissions
Collisions with hidden nodes become the most important loss reason A small decrease in synchronized transmissions
Hidden nodes get closer
Conclusion Congestion control is essential for safety vehicular communications.
The methods currently proposed are not well understood.
We look at the relation between congestion control mechanisms and loss reasons for safety messages.
A decentralized congestion control framework should focus on multiple, complementary parameters.
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