A traffic-aware infrastructurebased Architecture for intervehicules File Sharing
Amine K., Sadeq Ali M., Mesut G.
2008 IEEE
學生：羅英辰、董藝興
學號：M97G0216、M98G0106
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Introduction

Recent studies have shown in which extent
peer-to-peer file sharing traffic occurs in the
global Internet.

Vehicular ad hoc networks (VANET)
represent an emerging environment that
illustrates the combination of these trends.
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
The rest of this paper is organized as follows.
 Firstly
we present the proposed overlay
organization of access points and subsequently
we propose a clustering strategy for that
organization.
 Finally, we give an overview of the protocol
that has been implemented to support the
architecture.
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Architecture
The main goal in the design of the architecture
was to provide searching for shared files not
only among vehicles during opportunistic
meetings, but also among all vehicles in the
VANET.
 The main scenario assumed by our architecture
is illustrated by Fig. 1.
 Fig.1 uses CarTorrent protocol.

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In this scenario car 1 is looking for a file that is
shared by car 2.
 Use access points to avoid some important
restriction.

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
To fulfill this requirements, it is required to
build and manage shared-file indexes in a
distributed way in order to avoid bottlenecks.

We will show the importance of synchronizing
concurrent requests and deliveries to reduce as
much as possible the overhead and decrease
the download time of files.
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Architecture - Overlay Protocol
To use P2P overlay protocols such as Chord, is
a good way for implementing vehicular
network and the high mobility of the nodes
demand for a distributed management of filerequests.
 Chord uses consistent hashing to map nodes
onto an m-bit circular identifier space.

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
A thorough analysis of the overhead has shown that
the standard Chord-model with a single ring would
result in heavy message-overhead, especially for the
updating of car-positions.

As it was clear that most message-traffic will be
caused by the management for the car-positions, we
decided to migrate this functionality to n the super
nodes (clusterheads), at the same time keeping the
file-management decentralized.
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Architecture - Clustering
A key point in the architecture is to build
clusters and choose supernodes.
 To build clusters in such a way that intercluster vehicle-traffic is minimized, as it will
be reflected through an inter-supernode
message overhead.
 Clusters have the most possible maximum
coherence and the most possible minimum
coupling to other clusters.

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
We decided to build clusters using the k Medoid clustering algorithm.
•
•
i=1…n, j=1…k
Cp is the pth cluster such that d(xi,mp) ≤
d(xi,mj) with j = 1. . . k and i = 1. . . n.
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
The algorithm could be resumed in the
following four steps:
1.
2.
3.
4.
Choose k points to be the initial cluster-heads
(Medoids).
Assign each node to the closest Medoid.
When all nodes have been assigned, try swapping
cluster-nodes with their cluster-heads and see if the
costs are decreased.
Repeat Steps 2 and 3 until the Medoids no longer
move.
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
The coherence of a cluster, in our case, is measured
by the traffic of vehicles inside it rather than distances
between its nodes.

Let σst = σts denote the number of shortest paths from
s ∈ V to t ∈V , where σss = 1 by convention.
The following measure denotes the betweenness CB(v)
for vertex v as defined by Freeman .

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Architecture-Protocol Details

It is clear that with the changes made to the
Chord protocol, we have to adjust the basic
functionalities of this protocol to cope with the
splitting of file-indexes between clusters and
the introduction of clusterheads in their
organization.
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Protocol Details - Synchronization of
requests

The schema of synchronization deployed
therefore can be resumed in four rules:
1.
2.
3.
4.
Requests for each file have to be temporarily registered
in the corresponding file-index.
As soon as a gateway retrieves a file, it sends a
message to clear the list of requests registered for this
file (or check whether it has already been cleared).
If the requests-list is empty this gateway abandons the
delivery of this file otherwise it takes into
responsibility to transmit the retrieved file to all
requesting cars.
From time to time, gateways have to check whether the
non yet retrieved files have been already retrieved by
another gateway by checking the requests-cache.
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
In this context of dissemination of fully
autonomous delivery-messages, we introduced
the notion of responsibility-based (prioritybased) delivery process to be able to control
and manage the refreshing process of
deliveries in a decentralized way.
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Protocol Details - Prediction of NextCar-Positions

The prediction of next car positions is used in:
 First
stage to avoid flooding access points with filerequests and hence quickly retrieve the sought-after
files.
 Second stage to accelerate the deliverance of files to
cars by forwarding the retrieved files to probable
next-positions of requesting cars.
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Simulation
We have defined a grid of 5 blocks at both the
horizontal and the vertical axis, where each
block represents 1 km.
 Cars have a speed of about 50km/h with a turn
probability of 0.7.
 The number of access points has been varied
from 13 to 23 and thereof 2 to 6 supernodes
should be chosen.(Fig. 3b)

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All nodes use IEEE 802.11b MAC operating at
5 Mbps.
 The transmission range is about 250 m.
 The radio model has been specied according to
the results of the drive-thru-measurements.

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Performance Evaluation

In our performance evaluation, we have
focused on two aspects.
The first aspect is the clustering strategy.
 Protocol Details.

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Conclusions and Future Work

Using a suitable clustering strategy, we could prove
that taking into account the form of car-traffic has a
direct influence on reducing the overhead and the
download time of files.

We want also to enhance the prediction strategy for
car-movements by identifying redundant paths in cartraffics. On the mobile side, we are developing a
protocol that uses our architecture to support a direct
exchange between cars as it is the case in the
CarTorrent project.
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