Tonghong Li, Yuanzhen Li, and Jianxin Liao

Department of Computer Science Technical University of Madrid, SpainBeijing University of Posts &

Telecommunications Beijing, China

IEEE ICDCSw, 2009

A Contention-Based Routing Protocol for Vehicular Ad Hoc Networks

in City Environments

1112/04/19 Speaker: I-Hsin Liu

OutlineIntroductionContention Based Routing Protocol

(CBRP)SimulationConclusion

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IntroductionVANETs can be roughly classified in

two categories:Broadcasting the information from the

vehicle to all surrounding vehicles.Most safety applications such as accident

alerts.Delivering the information to a

particular destination through multi-hop.

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IntroductionThis paper proposes a contention

based routing protocol for VANETs in city environments.

Different from the position based routing, this protocol does not require the node to maintain its neighbors’ locations.

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GoalThis paper proposes a contention

based routing protocol for VANETs in city environments.

The goal of paper is Increasing delivery ratioDecreasing end-to-end delay

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OverviewCBRP works in two modes:

Street mode.Junction mode.

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OverviewWhen a packet is carried by a vehicle in

a street, CBRP operates in street mode.Using contention based forwarding to

deliver the packet greedily to the next junction.

When a packet is in the junction area, CBRP operates in junction mode.It first performs junction selection in order

to determine the next junction.7

Overview

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Destination

Street Mode

Street Mode

Junction Mode

Junction Mode

AssumptionWe assume that each vehicle is equipped

with a navigation system and a GPS receiver.Position.Moving direction.

In every navigation system, there is a digital city map.real traffic network.

Each node know the position of destination.

Each node know the ID of destination.Destination is fixed.

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Contention Based Routing Protocol

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Data forwarding in street mode

Junction selectionData forwarding in

junction mode

Junction SelectionEvery packet includes in the packet

header.The ID and position of the node that has

just forwarded the packet.Previous junction (PJ)Current junction (CJ)Source IDSource’s positionDestination IDDestination’s positionPacket ID

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Destination

Currentjunction

Previousjunction

Junction SelectionWhen is junction selection performed?

Case 1: when a packet is sent by the source node.

Case 2:when a node goes from street mode to junction mode.

In order to aid junction selection,a weight is assigned to each street based

on its traffic density.i.e. more vehicles, less weights assigned.

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Junction SelectionA path can be computed by using

Dijkstra’s least-weight path algorithm. Case 1:from the source to the destination.Case 2:from the CJ to the destination.

We choose the first junction in the Dijkstra’s least-weight path as the new CJ.

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CC

DD

EE

DestinationSourceNode

1 32

8

3CJPJ C

Contention Based Routing Protocol

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Data forwarding in street mode

Data forwarding in junction mode

Junction selection

Data forwarding in street modeA node that receives a packet at

first time. determines whether or not it is a

candidate next hop of Previous hop(Phop).

Nodes in the shaded area are candidate next hops of Phop, since they are closer to CJ than Phop.

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Data forwarding in street mode

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Destination

PJ

Street Mode

CJShaded Area

A

B

S

Data forwarding in street modeNote that if a node is in shaded

area and does not move towards CJ.We do not classify it as a candidate

next hop even though it is in the shaded area.

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PJ

Street Mode

I3

Shaded Area CJ

A

S

Destination

Junction Mode

Data forwarding in street modeNote that if a node is in CJ’s

junction area and does not move towards CJ.We do not classify it as a candidate

next hop even though it is in the shaded area.

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PJ

Street Mode

I3

Shaded Area CJ

A

S

Junction Mode

Destination

CJ PJ

Discard the packet

Data forwarding in street modeAfter classifying itself as a non

candidate next hop, a node discards the packet immediately.

Otherwise, it waits for a delay period before making a decision whether to rebroadcast a packet or not.

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Data forwarding in street modeThe delay of a node is calculated

based on the progress it provides towards the packet’s CJ.

The packet progress of a node i is defined as Pi=dist(Phop, CJ)-dis(i, CJ)

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maximum forwarding delay.

radio range.

Destination

Street Mode

CJ

dist(Phop, CJ)

dis(A, CJ)Phop

A

RPdelay i /1

Data forwarding in street modeDuring the delay period, the node listens

to other retransmission of the same packet.

If the node receives the same broadcast from other node closer to CJ than itself in the delay time, it drops the packet.

Otherwise, the node forwards the packet at the end of the delay period.Avoid sending duplicates.

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Data forwarding in street modeIf no further rebroadcasts happen

during τ periodit means that no next hop neighbors are

currently present due to the partitioned network.

In this circumstance, Phop holds the packet in its buffer.repeats the broadcast after waiting t time. The process is repeated until the next hop

neighbor appears.22

Data forwarding in street modeIn this case, we can set t as

follows:

assume that Phop’s current speed is Vthe maximum speed in the street

between CJ and PJ is max V.

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Destination

Street Mode

CJ

Phop

VPJ

Vmax

Data forwarding in street modeThe idea behind the choice of t is:

(1) the packet can not be rebroadcasted too often, which will jam the network.

(2) t can not be set too large, as it will miss the opportunity to forward the buffered packet further.

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Simulation

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Simulation

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Position Based Routing Protocol

Simulation

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Simulation

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ConclusionIn this paper, we present a

contention-based routing protocol designed specifically for vehicular networks in city environments.This protocol does not require the

node to maintain its neighbors’ locations.

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Thank You