Presentation at MOBIWAC 2012, about a MAC protocol to reduce the collision in Vehicular Networks (VANETS). The paper won the Best Paper Award.

1. DySCO: a Dynamic Spectrum
and Contention Control
Framework for Enhanced
Broadcast Communication in
Vehicular Networks
Marco Di Felice, Luca Bedogni
and Luciano Bononi
Department of Computer
Science
University of Bologna - Italy

2. Outline
System model
Analytical Results
The DYSCO framework
Performance evaluation
Conclusions
Introduction

3. Introduction
VANETs – Vehicular Adhoc NETworks
●
Safety related messages to reduce car accidents
●
Notification messages for better route planning
WAVE – Wireless Access in Vehicular Environment
●
DSRC frequencies
●
IEEE 802.11p and 1609.4 protocols
●
Switch every 50 ms from CCH to one of the SCH
50ms 100 ms 150ms 200ms 250ms 300ms
CCH SCH CCH SCH CCH SCH

4. Issues
Synchronous collisions
●
At the start of each CCH, every one could transmit a
packet
Bandwidth shortage
●
At the end of each CCH, one could not had the time to
transmit everything
Underutilization of the spectrum
●
Even in congested scenarios, every 50 ms the CCH or
the SCHs are not used

5. Contributions
Analytical model to investigate the PDR of
broadcast applications
●
Derive probabilities of succesfull transmissions
Study on the impact of changing
communication parameters
●
Change CW and rate to lower collisions
Evaluation in a scenario
●
By simulation

6. WAVE - IEEE 802.11p 1609.4
DSRC frequencies – V2V and V2I
●
1 control channel (CCH) 10MHz wide
●
6 service channels (SCH) 10 MHz wide
802.11p
●
EDCA mechanism
1609.4
●
On top of the 802.11p MAC protocol
●
Strict synchronization between the vehicles
●
Safety related messages are transmitted on the CCH
●
The lenght of the interval is 50 ms

7. System Model
Each vehicle is equipped with a SDR – WAVE stack
No queuing mechanism
QoS requirements

8. Analytical model
W is the CW size
 is the probability of a succesfull transmission
 Is the backoff state with CW equal to i
B0
Bi

9. Evaluation
We can derive the probability of transmitting
a message as
The packet delivery ratio is given by
More metrics are on the paper
τ=P(B0)=
2
W+1
PDR=
NumPkts−Succesfully −Transmitted
NumPkts−Transmitted
=
κ⋅psp
λ

10. PDR computation
Probabilities of:
Idle slot
Busy slot
Succesfull transmission
Collision
pi=(1−τ)
N
pb=1−pi
ps=N⋅τ⋅(1−τ)
N −1
pc=1−pi−ps

11. PDR computation
Probabilities of:
Succesfull transmission
knowing that the slot is
busy
Avg of transmitted
SAFETY messages
psp =
ps
pb
=
N⋅τ⋅(1−τ)N−1
1−(1−τ)
N
κ=min(λ ,
E[Taccess ]
Tcch
)

12. Evaluation

13. PDR varying the CW

14. PDR varying the CW

15. DYSCO – RSU operations
Centralized
●
Distributed as a future work
During the SCH interval → NETCONF packet
●
Propagates the network configuration
< W , RCCH , Tstamp >

16. DYSCO - algorithm
Step 1
●
Try to determine the best CW
Step 2
●
Verify if current rate is enough
●
If not, using cognitive technology, try to
increase it
W >
2⋅(N −1)
ϵ
RCCH⩾
S⋅λ⋅β
TCCH−α−λ

17. Comparison varying the density

18. Comparison varying the density

19. Comparison varying the load

20. Comparison varying the scenario

21. Conclusions
Simultaneous collisions
Untransmitted security packets
Characterization of MAC/PHY parameters
Outcomes
●
Reduced collisions
●
Better spectrum utilization
●
Fullfilling of QoS requirements
Major issue
in VANETs

22. Future works
Extend the algorithm to a distributed fashion
●
To minimize the risks of a centralized architecture
Inclusion of additional network parameters
Extend the study also for non safety related
applications

23. THANK YOU FOR
YOUR ATTENTION
Marco Di Felice <difelice@cs.unibo.it>
Luca Bedogni <lbedogni@cs.unibo.it>
Luciano Bononi <bononi@cs.unibo.it>

24. Collisions probability varying the CW

25. Comparison varying the scenario