Georgios Z. Papadopoulos

Adding value to WSN simulation using the IoT-LAB experimental platform

Validation of protocols and mechanisms is an essential step to the development of object networks in critical domains. Most papers still provide evaluation either obtained through theoretical analysis or simulations campaigns. Yet, simulators and formal models fail to precisely reproduce the unique specificities of the deployment environments those networks have to evolve in. Also, by putting no limits to code complexity and execution, those tools prevent users to apprehend the actual limits of WSN nodes and to propose realistic communication protocols and applications. In this paper, we highlight to what extent the addition of experimentations can significantly improve the value of performance evaluation campaigns. Along with the recent tendency to have algorithmic and protocol proposals facing real environments, it is questionable whether the so obtained results should be considered as scientific or empirical ones. In the former case, reproducibility, stability over time, topology management to cite a few, are a must have for testbeds and real deployments that are used. In the latter, the results should be viewed as a proof-of-concept only, far from independent of the used hardware and encountered conditions at the experimentation time but still critical from the development cycle standpoint. Through some experiments over the IoT-LAB testbed, we aim at demonstrating to what extent some of the simulation setup and conditions from reality could be emulated. We also provide insight on how to obtain the best out of it in a quick and efficient manner. We show that such testbeds would satisfy many expectations (e.g. scientific tool and proof-of-concept validator), thus minding and bridging some of the gaps between theory and practice in WSN. To this end, we here give an overview of available simulation tools, and guidelines on how to transpose simulation setups to the open large-scale IoT-LAB platform.

Performance evaluation methods in ad hoc and wireless sensor networks: a literature study

Verification of theoretical analysis is a vital step in the development of an application or a protocol for wireless networks. Most proposals are evaluated through mathematical analysis followed by either simulation or experimental validation campaigns. In this article, we analyze a large set of statistics in articles published (674 papers in total) in the top representative ad hoc and wireless sensor networks related conferences (i.e., ACM/ IEEE IPSN, ACM MobiCom, ACM MobiHoc, and ACM SenSys) during the period 2008-2013. We focus on the evaluation methodologies provided by researchers. More specifically, our goal is to explore the role of simulators and testbeds in the theoretical analysis of a model throughout the protocol development procedure. We show that there is a tendency for more and more researchers to rely on custom or open testbeds in order to evaluate the performance of their proposals. Simulators indeed fail to reproduce actual environmental conditions of deployed systems. Experimentation with real hardware allows our research community to mind the gaps between simulation and real deployment. Still, as the experimental apFproach through custom testbeds results in a low reproducibility level (i.e., 16.5 percent), we investigate to what extent such performance evaluation methods will be able to bridge those gaps. We finally discuss experimental testbeds and their potential to replace simulators as the cornerstone of performance evaluation procedures.

Experimental Validation of a Distributed Self-Configured 6TiSCH with Traffic Isolation in Low Power Lossy Networks

Time Slotted Channel Hopping (TSCH) is among the proposed Medium Access Control (MAC) layer protocols of the IEEE 802.15.4-2015 standard for low-power wireless communications in Internet of Things (IoT). TSCH aims to guarantee high network reliability by exploiting channel hopping and keeping the nodes time-synchronized at the MAC layer. In this paper, we focus on the traffic isolation issue, where several clients and applications may cohabit under the same wireless infrastructure without impacting each other. To this end, we present an autonomous version of 6TiSCH where each device uses only local information to select their timeslots. Moreover, we exploit 6TiSCH tracks to guarantee flow isolation, defining the concept of shared (best-effort) and dedicated (isolated) tracks. Our thorough experimental performance evaluation campaign, conducted over the open and large scale FIT IoT-LAB testbed (by employing the OpenWSN), highlight the interest of this solution to provide reliability and low delay while not relying on any centralized component.

T-AAD: Lightweight traffic auto-adaptations for low-power MAC protocols

After many successful deployments, Wireless Sensor Networks (WSNs) now appear open for a paradigm shift in traffic resulting from applications. Consequently to the traditional event or time-driven a priori known traffic patterns, those networks face occasional, bursty and unanticipated multi-hop data transmissions. This specific case is rarely addressed by preamble-sampling Medium Access Control (MAC) protocols. Such homogeneously configured solutions avoid network disconnections and isolated nodes, yet lead to long periods of channel occupancy, increased delays and energy-consumption. We here present T-AAD, a Traffic Auto-ADaptive mechanism, specifically designed to address the previously reported phenomenon through the introduction of heterogeneous node configurations in the network. T-AAD dynamically and locally adapts the MAC configuration depending on the actual and expected traffic load, without endangering network connectivity nor overall network performances. T-AAD is therefore compliant with Low Power Listening (LPL) based MAC protocols. In this paper, we evaluate our proposed mechanism and we demonstrate that it provides significant gains in terms of delay and energy consumption, when compared to respective research work available in the literature (e.g. MaxMAC, AADCC).

Enhancing ContikiMAC for bursty traffic in mobile sensor networks

This work focuses on Medium Access Control (MAC) protocols for Wireless Sensor Networks (WSNs). We investigate preamble-sampling solutions that allow asynchronous operation. We introduce anycast transmission to ContikiMAC where a mobile node first emits an anycast data packet whose first acknowledging node will serve as responsible to forward it towards the sink. Once this link is established, burst transmission can start, according to the respective burst handling mechanism of ContikiMAC. Although it is considered as negligible in the literature, such an anycast-based on-the-fly attachment actually results in high packet duplication at the sink. Our contribution is twofold. We first present that even a basic anycast-based ContikiMAC would fail to handle bursty traffic from mobile nodes mainly due to increased traffic and channel occupancy. We then propose lightweight adaptations that drastically reduces the duplications in the network thus, improving network performance while reducing energy consumption.

Leapfrog collaboration: Toward determinism and predictability in industrial-IoT applications

Recent standardization activities bring high Quality of Service (QoS) and predictability to Internet of Things (IoT), which are “going industrial”. Critical applications such as industrial process control, smart grid or vehicle automation require deterministic transmissions with properties such as on-time data deliveries and end-to-end reliability close to 100%. Traditional radio technologies based on collision detection and retransmission introduce unpredictable delays, and can not ensure reliable delivery within a narrowly bounded time. This paper proposes to exploit spatial diversity and packet redundancy to compensate for the inherently lossy wireless medium. We introduce “Leapfrog Collaboration”, a communication mechanism which takes advantage of communication overhearing, and in which parallel transmissions over two paths are scheduled. Promiscuous listening between the paths enables nodes to possibly overhear transmissions on the other. We evaluate the delay and jitter of the communication by simulation using Contiki OS and show that Leapfrog Collaboration outperforms the default retransmission-based approach of IEEE802.15.4-TSCH by up to 28% and 54%, respectively, while providing high network reliability.

Wireless Medium Access Control under Mobility and Bursty Traffic Assumptions in WSNs

In Wireless Sensor Networks (WSNs) the nodes can be either static or mobile depending on the requirements of each application. During the design of Medium Access Control (MAC) protocols, mobility may pose many communication challenges. These difficulties require first a link establishment between mobile and static nodes, and then an energy efficient and low delay burst handling mechanism. In this study, we investigate preamble-sampling solutions that allow asynchronous operation. We first introduce anycast transmission to ContikiMAC where a mobile node emits an anycast data packet whose first acknowledging node will serve as responsible to forward it towards the sink. Once this link is established, burst transmission can start, according to the respective burst handling mechanism of ContikiMAC. Although it is considered as negligible in the literature, such an anycast-based on-the-fly operation actually results in high packet duplication at the sink. Hence, we demonstrate that even a basic anycast-based M-ContikiMAC would fail to handle bursty traffic from mobile nodes mainly due to increased unnecessary traffic and channel occupancy. We then propose Mobility-Enhanced ContikiMAC (ME-ContikiMAC), a protocol that reduces packet duplications in the network by more than 90 % comparing to M-ContikiMAC. Moreover, our results in a 48-node scenario show that ME-ContikiMAC outperforms a number of state-of-the-art solutions (including MoX-MAC and MOBINET), by terms of reducing both delay and energy consumption.

Importance of Repeatable Setups for Reproducible Experimental Results in IoT

Performance analysis of newly designed solutions is essential for efficient Internet of Things and Wireless Sensor Network (WSN) deployments. Simulation and experimental evaluation practices are vital steps for the development process of protocols and applications for wireless technologies. Nowadays, the new solutions can be tested at a very large scale over both simulators and testbeds. In this paper, we first discuss the importance of repeatable experimental setups for reproducible performance evaluation results. To this aim, we present FIT IoT-LAB, a very large-scale and experimental testbed, i.e., consists of 2769 low-power wireless devices and 127 mobile robots. We then demonstrate through a number of experiments conducted on FIT IoT-LAB testbed, how to conduct meaningful experiments under real-world conditions. Finally, we discuss to what extent results obtained from experiments could be considered as scientific, i.e., reproducible by the community.

Optimizing the handover delay in mobile WSNs

In Wireless Sensor Networks (WSNs), during the design of Medium Access Control (MAC) protocols, mobile nodes may pose many communication challenges. These difficulties require first a link establishment between a mobile and static node, and then an efficient and effective data packet transmissions. In this study, we propose MobiXplore, a MAC scheme that allows a seamless transfer of communication (handover) to achieve low reconnection and handover delays. Finally, we present the design, implementation and evaluation of our proposal.

Thorough IoT testbed characterization

In this paper, we explore the role of simulators and testbeds in the development procedure of protocols or applications for Wireless Sensor Networks (WSNs) and Internet of Things (IoT). We investigate the complementarity between simulation and experimentation studies by evaluating latest features available among open testbeds (e.g., energy monitoring, mobility). We show that monitoring tools and control channels of testbeds allow for identification of crucial issues (e.g., energy consumption, link quality) and we identify some opportunities to leverage those real-life obstacles. In this context, we insist on how simulations and experimentations can be efficiently and successfully coupled with each other in order to obtain reproducible scientific results, rather than sole proofs-of-concept. Indeed, we especially highlight the main characteristics of such evaluation tools that allow to run multiple instances of a same experimental setup over stable and finely controlled components of hardware and real-world environment. For our experiments, we used and evaluated the FIT IoT-LAB facility. Our results show that such open platforms, can guarantee a certain stability of hardware and environment components over time, thus, turning the unexpected failures and changing parameters into core experimental parameters and valuable inputs for enhanced performance evaluation.