The 5th PSU-UNS International Conference on Engineering and

Technology (ICET-2011), Phuket, May 2-3, 2011 Prince of Songkla University, Faculty of Engineering

Hat Yai, Songkhla, Thailand 90112

Abstract: This paper is concerned with the experimental

testbed of vehicles with the ad hoc network capability.

Based on the ns-3 platform, a new code has been

developed to emulate the message forwarding of incident

warning system application. The developed code is

located upon the protocol stacks of UDP and IP in the

Linux operating system. Preliminary experiments have

been here reported on an ad hoc network testbed with

four vehicle nodes each with an IEEE 802.11b/g wireless

interface card. The testbed consists of 2-hop ad hoc

scenario with a simple flooding protocol as a warning

message dissemination mechanism. Message delivery

ratio and message receiving delay have been reported in

the scenarios with and without interference effects.

Despite of small testbed size, the obtained results suggest

interesting findings that can only be confirmed by the

testbed, but not from simulations. Finally, possible future

research towards the effect of the network mobility and

dynamic topological environments is given.

Key Words: Ad Hoc Network / Incident Warning / ns-3

1. INTRODUCTION

Nowadays, ad hoc communication technologies have

become essential for the emerging Intelligent

Transportation System (ITS) developments. Vehicular

Ad-hoc Networks (VANETs), expected to enhance

communication capability amongst moving vehicles,

have enabled such ITS applications as road traffic

information system, accident locating system or obstacle

warning system [1]. With increasing concerns on road

safety, there is a great amount of research attempts

concentrating on incident detection/warning applications,

where vehicles must be able to relay nececessary

warning messages about the detected incident location to

all relavant vehicles nearby.

For a succesful implementation of incident warning

applications, VANET protocol designer must try to

spread warning messages in time that the vehicles

moving towards the incident location can change their

way from the affected area. Further, due to the limited

sprectrum for wireless communications and expectedly

large protocol overheads in VANET, the number of

warning messages to be fowarded should be kept as

minimum as possible to prevent the collision of data

frames as well as the resultant wireless signal

interference.

Since nodes in a VANET can move, network

topology always changes over time. With vehicles

moving as platoons, the resultant topology is often

fragmented. Such fragmentation causes slow data

receptions especially for data transmission across

fragmented subnetwork boundaries. For saftefy

application, one needs not specify destinations for data

transmission so various flooding protocols are often

proposed as the dissemination method from a sending

node to the others in its transmission range. However,

this can also affect overall system performance since too

many flooded messages can lead to undesired data

collision likelihood.

To overcome the aforementioned challenges, there

exist important research literatures (e.g.[2], [3], [4])

proposing various protocol improvements. To evaluate

the proposed protocols, network simulators (e.g. ns-2 [5],

OMNeT++ [6], Jist/SWANS [7]) have been proved to be

an indespensible tool. However, great cares must be

taken in selecting the simulation platforms. In particular,

it has been found [8] that different network simulation

programs can lead to different simulation results even

with the same system parameter configurations.

A good simulation platform must not only be able to

give insights from simulated results, but also the ease of

confirming those insights with the realistic VANET

experiements. In this regard, ns-3 [9] is an interesting

simulation/emulation platform that allows the written

simulation codes to be imported directly to real devices.

This facilitates the need to better combine the simulation

capability and the actual testbed implementation. In

Preliminary ns-3 Evaluation of Ad Hoc

Network Testbed for Incident Warning

System Application

Piangpoon Jakkaew1, Patrachart Komolkiti

2, Chaodit Aswakul

1*

1 Department of Electrical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok 10330,

Thailand 2 Department of Computer and Network Engineering, Assumption University, Bangkok 10240, Thailand

*Authors to correspondence should be addressed via email: chaodit.a@chula.ac.th

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addition, comparing with other simulators, ns-3 has been

reported as the best platform in terms of memory usage

and simulation run time [10].

This paper reports our early findings from a small

testbed of the incident warning system by using special

capability of ns-3. Application codes have been newly

developed in ns-3 and imported into real computing

devices. The focus is on modeling the scenario of indoor

wireless ad-hoc network with a static topology and a

simple flooding protocol as a warning message

dissemination. Delay of individual nodes in receiving

necessary incident warning messages have been analyzed

in various scenarios with controllable interference effects

of transmitted wireless signal. Experiment is

implemented on testbed and simulation.

The structure of the paper is as follows. Section 2

introduces the incident warning protocol being employed

in this research. Section 3 presents the experimental

implementation. This section also reports on the

experimental settings. Section 4 gives the obtained

experimental results. Finally, Section 5 gives conclusions

and the future direction of research.

2. INCIDENT WARNING PROTOCOL

Nodes in the testbed are here classified into three

types. Firstly, incident warning node is defined as the

node that detects an incident and starts distributing

warning messages to all the other nodes. Secondly,

forwarding node is defined as the node that receives the

distributed warning messages and can help forward

warning messages further. Lastly, interference node is

defined as the node transmitting its packets via wireless

signals, which interfere with the signals from the incident

warning node and the forwarding nodes. Fig. 1 depicts a

simple timing diagram of incident warning protocol with

an incident warning node V0 and forwarding nodes V1-

V2. Nodes V0 and V2 are in the wireless coverage area

of V1, but V0 and V2 are not in the coverage area of

each other. All nodes in our first testbed here are non-

moving. Node V0 must try to forward the incident

warning message to V1-V2.

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Fig. 1. Timing diagram of incident warning protocol with

one incident.

In Fig. 1, starting at time t = 0 when the incident has

been detected, V0 floods a warning message to all

neighbor nodes within its transmission range

periodically, say every 0.05-0.5 seconds according to the

European ITS Communication Architecture [11]. Using

these periodic values, nodes in incident warning

application can prepare themselves while moving

towards the location of the detected incident [11]. Each

warning message has a unique message ID being

assigned by the incident warning node V0. When

forwarding nodes V1-V2 receive a warning message,

they check the message ID. To prevent the overhead

explosion of message flooding ([1], [12]), each

forwarding node sends the received message further only

when that message has been received by the node for the

first time, i.e., the ID of message is not redundant to the

IDs of previously received messages.

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Fig. 2. Timing diagram of incident warning protocol with

interference.

Fig. 2 shows another timing diagram example of

incident warning protocol in an interference scenario.

Like Fig. 1, V0 and V2 are in the wireless coverage area

of V1, but V0 and V2 are not in the coverage area of

each other. In addition, V0, V1 and V2 are in the

wireless coverage area of V3. Here, V0 is an incident

warning node, V1 and V2 are forwarding nodes and V3

is an interference node. Suppose that the interference

node can interfere all nodes in the system. Node V3

distributes packet every 0.01 second to forwarding node

V1. If the incident warning node V0 and the interference

node V3 sense that the channel is not busy, then V0 and

V3 will send their packets simultaneously, causing the

layer-2 collision of the packets from V0 and V3, and

hence other nodes cannot receive those packets. It can be

also expected then that the wireless signal transmitted

from interference nodes can increase the layer-1 signal

interference within the vicinity of the incident-warning

operation. The goal of this paper is to study the impact of

such interference scenario using real computing devices,

not their simulation.

To implement this mechanism of incident warning

nodes and forwarding nodes, a new code has been

developed in this research in ns-3 platform. As shown in

Fig. 3, the developed code is located on the layer of

incident warning application and uses the actual protocol

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stacks of UDP and IP in the Linux operating system.

This architecture of ns-3 platform can be implemented in

emulating mode and also be readily extensible for the

real implementation of the incident warning protocol by

replacing the simulated part with the actual network

interface on real devices. In this paper, incident warning

system is implemented in real devices.

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Fig. 3. Implemented protocol stacks of incident warning

system.

3. EXPERIMENTAL IMPLEMENTATION

To study the impacts of interference scenarios, the

developed small testbed of incident warning application

consists of four notebook computers to form a static

network testbed. The specification of all notebook

computers is summarized in Table 1. The operating

system is the Linux Ubuntu 9.10. The wireless network

cards are of 2 dBi gain, 16±1 dBm with wireless signals

transmission and -80 dBm receiver sensitivity. All

experiments have been conducted on channel 1 of 802.11

b/g, with the central channel frequency at 2.412 GHz.

There are two background or interference traffics we

could not eliminate: the control traffic and the other

wireless access points within the experimental area. The

control traffic is due to the ssh program, which is used to

remotely control all nodes in the system.

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Table 1. Computing node specification

The experimental parameters are shown in Table 2.

The main goal of the experiment is to compare network

performance parameters at interference scenario and

non-interference scenario. Incident warning messages

and interference packets have the same packet size, 1024

bytes. Testbed environment is static in its topology and

located in about 30-meter long and 15-meter wide area

on the rooftop of our 20-floor building. This location has

been chosen so as to minimize the chance of being

interfered by external wireless access points. For such a

small test area, we have to reduce the transmission range

of nodes by using aluminium foil box as show in Fig. 4.

For MAC layer, we used the default values of available

wireless interface card inside each testing computer to all

MAC parameters, such as the carrier sense, the

retransmission counter, the contention window and the

RTS/CTS threshold. Fig. 5 shows the 3-node, 2-hop ad

hoc network used in our experiment. The ad hoc network

includes an incident warning node and two forwarding

nodes. For interference scenario, one interference node

has also been added into our system as show in Fig. 6.

And the actual configuration of the node topology is

depicted in Fig. 7. All the nodes in the system are in their

transmission range of the interference node.

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Table 2. Parameter configuration

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Fig. 4. Node in experiment.

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Fig. 5. Topology of nodes.

In ns-3, many physical layer models are available.

This experiment uses the default model, constant speed

propagation delay model [9] and the log distance

propagation loss model [9]. The value of the speed

propagation delay is 3x108 m/sec. Log distance

propagation loss model calculates the reception power

with a so-called log-distance propagation model as:

0

100 log10d

dnLL (1)

where n is the path loss distance exponent; 0d is

reference distance (m); 0L is path loss at reference

distance (dB); d is distance (m) and L is path loss

(dB). The path loss distance exponent n is 3, reference

distance 0d is 1 m and path loss at reference distance

0L is 46.6777 dB.

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Fig. 6. Topology of nodes with interferences.

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Fig. 7. Actual configuration of testbed topology on the

rooftop.

4. EXPERIMENTAL AND SIMULATION

RESULTS

The goal of the experiment is to study the VANET

system in the actual network environment with real

wireless signals being transmitted and hence subject to

possible interferences. Two network performance

metrics have been used. Firstly, the message receiving

delay is measured at the incident warning application

layer of protocol stack from the incidence warning node

to all the forwarding nodes. Secondly, the message

delivery success ratio is measured from the number of

non-redundantly received messages at forwarding nodes

over the total number of messages sent by the incident

warning node. The experiment compares the results of

testbed and simulaiton in details, as depicted in Tables 3-

8.

The main conjecture of the test is, as the interference

node injects more messages to the system, it should

induce more collisions and backoffs. The results confirm

the conjecture that both the message receiving delay and

and message delivery success ratio are degraded by

interference. However, the impact varies with scenarios.

Particularly, when comparing between similar settings of

simulations and testbed experiments, the results from

simulations are always superior. For instance, the

message receiving delay with interference in Tables 3

shows that, with the more frequent incident generating

rate, the delay is higher. This is due to higher traffic load

in the system, causing longer delay. On the other hand,

when observing the results of testbed experiments from

Tables 4 in the similar settings, the absolute values of the

delay is much higher. Nevertheless, there are virtually no

difference of delay for different incident generating rate.

In terms of the message delivery success ratio, the results

from simulations, shown in Tables 5 and 7, are 100%

when there are no interference. However, as shown in

Tables 6 and 8, in a real environment of testbed using the

2.4 GHz ISM band, the results vary greatly. Also, the

message delivery success ratio of the interference

scenario in Tables 6 and 8 show significant drops with

the more frequent incident generating rate. While both

values of the incident generating rate used in this work

are within the suggested range by the standard [11], the

resultant message delivery success ratio at the more

frequent incident generating rate may not be acceptable.

This finding strengthens the importance of testbed

implementation, as many imperfections in real

environment can not be modeled in simulations.

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Table 3. Simulation: message receiving delay

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Table 4. Testbed: message receiving delay

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Table 5. Simulation: message deliviery success ratio of

node V1 (%)

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Table 6. Testbed: message deliviery success ratio of node

V1 (%)

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Table 7. Simulation: message deliviery success ratio of

node V2 (%)

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Table 8. Testbed: message deliviery success ratio of node

V2 (%)

5. CONCLUSION

This paper reports our early findings from a small

testbed of the incident warning system by using special

emulation and simulation capability of ns-3 platform.

Application codes have been newly developed in ns-3

and ported into real computing devices. The focus is on

modeling the scenario of indoor wireless ad-hoc network

with a static topology and a simple flooding protocol as

warning message dissemination. Based on the obtained

values of message delivery ratio and message receiving

delay from both of the testbed and simulation

experiments, interference causes increased message

receiving delay and decreased message delivery success

ratio. This is because of traffic interference. However, for

the same experiment settings of the testbed and

simulations, simulations may not give the actual realistic

values. It should be noted that, the simulation

performances are always better than the testbed

performance. For the message receiving delay of

interference scenario with different incident warning

message generating rate, the testbed does not get the

different message receiving delay. But the higher

message delivery success ratio is achieved at the incident

generating rate of 0.5 seconds per message. It can be

concluded that, proper incident warning message

generating rate is critical to a good performance.

Therefore, despite of small testbed size, the obtained

results suggest interesting findings that can only be

confirmed by the testbed, but not from simulations. In the

future, our ongoing research will focus on the study of

the network mobility and dynamic topological

environments in the onroad real-vehicle testbed

environment.

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