Preliminary ns-3 Evaluation of Ad Hoc Network Testbed for Incident Warning System Application

Transcription

1 The 5 th PSU-UNS International Conference on Engineering and 343 Technology (ICET-211), Phuket, May 2-3, 211 Prince of Songkla University, Faculty of Engineering Hat Yai, Songkhla, Thailand 9112 Preliminary ns-3 Evaluation of Ad Hoc Network Testbed for Incident Warning System Application Piangpoon Jakkaew 1, Patrachart Komolkiti 2, Chaodit Aswakul 1 * 1 Department of Electrical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok 133, Thailand 2 Department of Computer and Network Engineering, Assumption University, Bangkok 124, Thailand *Authors to correspondence should be addressed via chaodit.a@chula.ac.th 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 82.11b/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

2 344 addition, comparing with other simulators, ns-3 has been reported as the best platform in terms of memory usage and simulation run time [1]. 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 V and forwarding nodes V1- V2. Nodes V and V2 are in the wireless coverage area of V1, but V and V2 are not in the coverage area of each other. All nodes in our first testbed here are nonmoving. Node V must try to forward the incident warning message to V1-V2. Fig. 1. Timing diagram of incident warning protocol with one incident. In Fig. 1, starting at time t = when the incident has been detected, V floods a warning message to all neighbor nodes within its transmission range periodically, say every.5-.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 V. 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. 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, V and V2 are in the wireless coverage area of V1, but V and V2 are not in the coverage area of each other. In addition, V, V1 and V2 are in the wireless coverage area of V3. Here, V 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.1 second to forwarding node V1. If the incident warning node V and the interference node V3 sense that the channel is not busy, then V and V3 will send their packets simultaneously, causing the layer-2 collision of the packets from V 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

3 345 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. 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. Table 2. Parameter configuration 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 9.1. The wireless network cards are of 2 dbi gain, 16±1 dbm with wireless signals transmission and -8 dbm receiver sensitivity. All experiments have been conducted on channel 1 of b/g, with the central channel frequency at 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. Table 1. Computing node specification Fig. 4. Node in experiment. 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, 124 bytes. Testbed environment is static in its topology and located in about 3-meter long and 15-meter wide area on the rooftop of our 2-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 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 3x18 m/sec. Log distance propagation loss model calculates the reception power with a so-called log-distance propagation model as: d L L 1nlog 1 (1) d where n is the path loss distance exponent; d is reference distance (m); L 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 d is 1 m and path loss at reference distance L is db.

4 346 Fig. 6. Topology of nodes with interferences. 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 1% 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. Table 3. Simulation: message receiving delay 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 Table 4. Testbed: message receiving delay Table 5. Simulation: message deliviery success ratio of node V1 (%)

5 347 Table 6. Testbed: message deliviery success ratio of node V1 (%) Table 7. Simulation: message deliviery success ratio of node V2 (%) 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.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. 6. REFERENCES [1] T. L. Willke, P. Tientrakool, and N. F. Maxemchuk. A Survey of Inter-Vehicle Communication Protocols and Their Applications. IEEE Communiocation Surveys and Tutorials, vol. 11, no. 2, pages 3 2, 29. [2] F. Hrizi, and F. Filali. Achieving Broadcasting Efficiency in V2X Networks with a Distance-based Protocol. ComNet 29, pages 1 8, 29. [3] S. Khakbaz, and M. Fathy. A Reliability Broadcast Method for Vehicular Ad hoc Networks Considering Fragmentation and Intersection Problems. The Second International Conference on Next Generation Mobile Applications, Services, and Technologies, pages , 28. [4] Y. Qiangyuan, and G. Heijenk. Abiding Geocast for Warning Message Dissemination in Vehicular Ad Hoc Networks. ICC Workshops 8, pages 4 44, 28. [5] The network simulator ns-2. [6] OMNET++ Community Site. [7] JiST Java in Simulation Time / SWANS Scalable Wireless Ad hoc Network Simulator. [8] D. Cavin, Y. Sasson, and A. Schiper. On the Accuracy of MANET Simulators. In Proceedings of the second ACM international workshop on Principles of mobile computing, POMC 2, pages 38-43, 22. [9] ns-3. Available: [1] E. Weingartner, H. Lehn, and K. Wehrle. A performance comparison of recent simulators. IEEE ICC, 29. [11] R. Bossom, R. Brignolo, T. Ernst, K. Evenson, A. Frotscher, W. Hofs, J. Jaaskelainen, Z. Jeftic, P. Kompfner, T. Kosch, I. Kulp, A. Kung, A. K. Mokaddem, A. Schalk, E. Uhlemann, and C. Wewetzer. D31 European ITS Communication Architecture. Information Society TechnologiesCommunication for e Safety, 29. [12] P. Muhlethaler, A. Laouiti, and Y. Toor. Comparison of Flooding Techniques for Safety Applications in VANETs. ITST 7 7th International Conference on ITS, 6 8, 27.