1
TBD: Trajectory-Based Data Forwarding
for Light-Traffic Vehicular Networks
Jaehoon Jeong, Shuo Gu, Yu Gu, Tian He and David Du
Computer Science and Engineering
University of Minnesota
{jjeong,sguo,yugu,tianhe,du}@cs.umn.edu
June 23rd, 2009
IEEE ICDCS’09, Montreal, Quebec, Canada
Motivation
2

The vehicular networking is getting a hot research topic.


Internet Access, Driving Safety, Data Dissemination, etc.
The environments for the vehicular networks
Every vehicle has a DSRC device for wireless communication.
 Every vehicle has a GPS-based navigation system for driving
information.
 The Internet Access Points (APs)
are sparsely deployed in road
networks.


The objective in this paper

The vehicles can deliver their packets to APs through the
multi-hop forwarding with the help of other vehicles.
Problem Definition
3
Light-Traffic Road Network
Forwarding Path
Carrier-1’s
Moving
Trajectory
Delay-1
Delay-1 < Delay-2
Next hop?
Delay-2
Carrier-2’s
Moving
Trajectory
Problem Definition
4
Road Network with Unbalanced Traffic Density
Light Traffic Path
Delay-1 > Delay-2
Forwarding Path
Next hop?
Delay-2
Heavy Traffic Path
Heavy Traffic Path
Delay-1
Contribution and Challenges
5

Contribution
 Data
forwarding based on Vehicle Trajectory
 With
vehicle trajectory, TBD outperforms the existing scheme
(VADD, Infocom’06) only using vehicular traffic density.

Challenges
A
more accurate link delay model than VADD’s
 Mathematical
 End-to-End
 E2E
model for the link delay
delay model based on vehicle trajectory
delay modeling based on (i) vehicular traffic density
and (ii) individual vehicle trajectory
Link Delay Model
6

Objective


To compute the expected link delay over a one-way road segment.
Road Segment for Link Delay Model

Given the vehicle arrival rate


and the vehicle speed
v,
v
 How to compute the Forwarding Distance l f ?
Forwarding Distance for Vehicle Arrivals
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Vehicle nk 1 arrives
at time t k 1 .
dist(n1, n0 )  R
l f (Forwarding Distance)
Forwarding Distance over Time
Forwarding Distance
dist(t1, t0 )  R / v
Forwarding Distance for Vehicle Arrivals
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Forwarding Distance
dist(t1, t0 )  R / v
T0  dist(t1, t0 )  R / v
Forwarding Distance
k
l f  v  Th
h 1
where Th  R / v
Link Delay Model Comparison between
VADD and TBD
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
VADD Link Delay Model
the vehicle arrival rate  and the vehicle speed v,
the forwarding distance is the sum of the network components.
 This model is inaccurate since it misses the following fact
 Given


Only the first network component can be used for data forwarding.
Performance Comparison
 For
Average Forwarding
Distance, TBD is much closer
to the Simulation result
than VADD.
E2E Delay Model
10

Objective
 To
compute the expected end-to-end delay from a
Vehicle to an Internet Access Point (AP).

Road Network Graph for Data Forwarding
 Given a vehicle’s trajectory,
how to compute the E2E delay?
 Since node1 and node2
have different trajectories,
their E2E delays are different.
Expected Delivery Delay
at Intersection (VADD Model)
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
D1, 2 :Expected Delivery Delay (EDD)
at Intersection 1
 Where
a packet is sent towards Intersection 2.
Link Delay for
Road Segment (1,2)
D1, 2  d1, 2  P2,1D2,1  P2,3 D2,3  P2,7 D2,7
Dij  dij  E[delivery delay at j by forwarding or carry]
Expected Delivery Delay
at Intersection (VADD Model)
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
Average Forwarding Probability ( Pij )
 The
probability that a packet at intersection i can be
delivered towards neighboring intersection j.
 We consider all the possible moving directions of the
current packet carrier at intersection i.

How to compute P2,3 ?
Pij' : forwarding probability
Moving Direction-1
Moving Direction-3
Moving Direction-2
that the packet carrier at i can
forward its packet to another
carrier moving towards j.
Packet Delivery
Direction
Expected Delivery Delay
at Intersection (VADD Model)
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
Limitation of EDD at Intersection
 The
vehicle trajectory is not used to
They are very close to
compute the EDD.
each other.
 Node1 and Node2 have the same EDD regardless of their
different trajectories.
 Thus, we cannot determine which node is a better next carrier.

How to involve the vehicle trajectory into EDD computation?
 The
main idea is to divide the delivery process recursively into
two steps:
1.
2.
The packet forwarding process at the current carrier.
The packet carry process by the current carrier.
Expected Delivery Delay for
Vehicle Trajectory (TBD Model)
14

Vehicle Trajectory: 1  2  3
Case 1: The packet is forwarded at intersection 1.
 Case 2: The packet is carried to intersection 2 and
is forwarded at intersection 2.
 Case 3: The packet is carried to intersection 3 and
is forwarded at intersection 3.

Expected Delivery Delay for
Vehicle Trajectory
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D  P1', 2 D1, 2  P1',6 D1,6
 P1c, 2 (C1, 2  P2' ,1D2,1  P2' ,3 D2,3  P2' ,7 D2,7 )
 P1c, 2 P2c,3 (C1,3  P3', 2 D3, 2  P3', 4 D3, 4  P3',8 D3,8 )
where Pi,j' : forwarding probability for (i, j),
Pic, j : carry probability for (i, j), and
C1,k : carry delay for path 1  k.
Forwarding Protocol
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
TBD Forwarding Rule
 Within
a connected component, packets are forwarded
to the vehicle with a minimum EDD.
Performance Evaluation
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
Evaluation Setting
 Performance
Metric: Average Delivery Delay
 Parameters: (i) Vehicular traffic density, (ii) Vehicle speed,
and (iii) Vehicle speed deviation.

Simulation Environments
 36-intersection
road network (4.2 miles X 3.7 miles)
 Vehicle mobility model: Random-Way Point
 Vehicle speed distribution: N(40,5) MPH
 Communication range: 200 meters
 Time-To-Live (TTL): infinite (i.e., no timeout)
Average Delivery Delay Comparison
between TBD and VADD
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TBD outperforms VADD under the light traffic, such 20~50 vehicles.
As the traffic density increases, two schemes are converged.
Conclusion
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
In this talk, the data forwarding scheme called TBD is
introduced based on the vehicle trajectory:


Also, the link delay model is introduced for TBD data
forwarding scheme:


Data Forwarding from Vehicle to AP.
This link delay model can be used for other VANET routing or
forwarding schemes.
As future work, the multiple-hop Internet access will be
investigated in the vehicular networks:

Vehicle trajectory will be used for the data forwarding for the
Internet access.
Future Work: Reverse Data Forwarding
for Internet Access
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Target Point
Challenge in Reverse Data Forwarding
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
As packet destination, the vehicle is moving, not static.
 The
packet from AP needs to be delivered to the vehicle,
considering the rendezvous point along the vehicle trajectory.
 The reason of the
target missing?
 Inaccurate estimation
of the vehicle arrival
Target Point
Target
Missing!
 How to provide this
reverse forwarding?