Fast network joining algorithms in industrial IEEE 802.15.4 deployments
Introduction
Nowadays the Internet of Things (IoT) is at the ground floor of many novel applications and services, based on capillary interactions among smart objects [1], [2], [3], [4], [5], [6]. Its adoption in industrial environments sets new requirements to satisfy, which does not usually emerge in plain IoT scenarios, such as: (i) wirelike reliability; (ii) ultra low power (years of battery lifetime or energy harvesting capabilities); and (iii) hard constraints on data latency and throughput [7], [8].
Low power and short range wireless communication technologies are key drivers for industrial IoT systems, since they can enable centralized and distributed sensing and actuation operations thanks to their inherent capabilities of creating networks of smart nodes [9]. In this field, the IEEE 802.15.4 MAC is a leading standard [8]. Its scope has been further extended in the 2015 release [10], which, among other features, includes the TSCH1 to improve reliability and energy efficiency of short range wireless communications in harsh radio conditions.
In 2013, due to the relevance of using this powerful access scheme, the new Internet Engineering Task Force (IETF) “IPv6 over the TSCH mode of IEEE 802.15.4e” (6tisch) working group has been chartered to define new standards for enabling the usage of IEEE 802.15.4 TSCH also in IPv6 Low-power Lossy Networks (LLN).
When TSCH is enabled, a IEEE 802.15.4 LLN is composed by a set of synchronized nodes arranged in a multi-hop topology. Accordingly, all nodes share a common time slotted baseline, organized as a periodic sequence of slotframes. In this way, it is possible to wake up each single node only when strictly necessary, thus minimizing network duty cycle and energy consumption [12], [13]. The effectiveness of this TDMA scheme can be further improved by adopting channel hopping, which can strongly mitigate the impact of noise and interference.2 To this end, in TSCH each node switches the physical channel at each consecutive timeslot by following a pre-assigned sequence, referred to as logical channel. The resulting overall reliability is improved because any error occurring in a given timeslot, due to noise and interference on the used physical channel, can be recovered at the next slot by using a different channel (unless there is a wideband interference). Moreover, since 16 logical channels are defined, it is also possible to enable simultaneous transmissions by neighboring nodes (provided they use different logical channels) and spatial frequency reuse, without incurring collisions [12], [16].
In order to capitalize the advantages brought by TSCH, it is necessary to ensure that the network quickly converges towards a global synchronization point, where all nodes share the same time-slotted baseline. According to the standard, the initial synchronization can be reached by configuring at least one node as synchronizer, usually the Personal Area Network (PAN) coordinator, which is in charge to broadcast Enhanced Beacon (EB) frames. Each EB advertises the Absolute Slot Number (ASN), which is the information on the total number of slots elapsed from the boot up. In this way, as soon as a new joining node receives an EB, it can synch up to the slot-frame structure of the network. After, it can also start to send EBs on its own in order to broaden the diameter of the network.
It is worth to note that EBs can be transmitted using TSCH to increase the communication resilience in noisy environments. At the same time, this choice can inflate the time spent by a new node to join the network. In fact, the joining node and the synchronizer one are usually not aligned on the same transmission/reception frequencies (i.e., some extra time could be required in order to allow their switching sequences to intersect at the same channel). In other words, while the synchronizers is sending an EB on a given physical channel, the joining node might be listening another frequency, so that both nodes are forced to remain awake for a long time, till the synchronization is gained, thus worsening also energy efficiency. Furthermore, the transmission of EB from multiple nodes can incurs in collisions.
These problems can be faced using a proper scheduling strategy that drives the transmission of EBs in order to avoid collisions and quicken as much as possible the joining phase. Unfortunately, the most of contributions proposed so far for the basic version of the IEEE 802.15.4 standard do not immediately apply to the TSCH [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33]. Note that in [34] a similar problem has been discussed with reference to the Time Synchronized Mesh Protocol, i.e., an ancestor of TSCH; as a possible solution, it was proposed to increase the number of nodes involved in sending beacons to reduce the time needed for the join phase of a new node.
Recently, several novel contributions have been also formulated for TSCH [35], [36], [37]. In [35] it is proposed to increase the slots available for EB transmissions in order to improve the chance to match the channel the joining node is listening to. A completely different approach is pursued in [37], proposing a model to calculate the near-optimal schedule for EB transmissions. In the latter case, the schedule needs to be calculated a-priori and made known to every joining node before the network bootstrap. Finally, in [36] lean distributed coordination schemes have been proposed to enable the transmission of EBs from multiple network nodes.
The present contribution extends the findings in [36] in several direction: (i) four novel mechanisms to speed up joining operations are proposed and implemented3 within the OpenWSN stack [38]; (ii) their performance has been analytically modeled with closed form expressions as a function of node density, communication reliability, and beacon transmission frequency; (iii) their effectiveness and the agreement between analytical and experimental results have been successfully evaluated in different scenarios.
Both theoretical and experimental results demonstrate that: (i) the joining process can be made quicker and quicker by increasing the node density, which contributes an higher aggregate transmission rate of EBs; (ii) a significant speed up can be achieved by coordinating, on a distributed basis, the transmission of EBs sent by network nodes; (iii) a further performance improvement can be pursued by allowing a higher beacon transmission frequency by nodes powered by the mains (i.e., without any energy constraints) with respect to nodes supplied by batteries.
The rest of the paper is organized as follows. In Section 2, an overview on the IEEE 802.15.4 standard is provided with a major emphasis on TSCH and synchronization mechanisms. In Section 3 and Section 4, the new synchronization algorithms are illustrated and analytically modeled, respectively. Section 5 describes the testbed, reports experimental outcomes, and validates theoretical findings. Finally, Section 6 closes the paper and draws future research.
Section snippets
An overview on the IEEE 802.15.4 Time Slotted Channel Hopping
TSCH is now part of the latest version of the IEEE 802.15.4 standard and represents a key feature of the IEEE 802.15.4e amendment [11], conceived to improve the reliability of wireless links and reduce energy consumption in industrial environments [8], [25], [39], [40], [41].
Algorithms for fast synchronization
Herein, four novel algorithms to speed up the joining phase in a IEEE 802.15.4e network are developed and described. In what follows, they will be referred to as: Random Vertical filling (RV), Enhanced Coordinated Vertical filling (ECV), Random Horizontal filling (RH), and Enhanced Coordinated Horizontal filling (ECH). Both RV and RH schemes are very lightweight and are meant to moderately boost up joining operations. Instead, ECV and ECH algorithms, (relying on a distributed coordination among
Analytical models
As previously discussed in Section 2, a node willing to join the network needs to receive an EB. Unfortunately, when using TSCH, the frequency used for transmitting the EB changes during time according to the Eq. (1). As a consequence, the time required to complete the join procedure depends on the probability that the joining node is listening on the same frequency channel where the EB is being transmitted. In the previous section, four algorithms have been designed to speed up joining
Experimental evaluation
The algorithms described in this work have been implemented in the OpenWSN stack [38]. An extensive experimental campaign was carried out by using TelosB motes. In such experiments, it was used a scheduling structure like the one shown in Fig. 2 with a multi-slotframe of 15 slotframes and each slotframe lasting 101 timeslots.
The topology used for the experiments is pictured in Fig. 9: it is composed by N nodes already synchronized (including the coordinator) that are in radio visibility to each
Conclusion
This contribution deeply explored the joining phase in a TSCH network from theoretical and experimental points of view. The problems arising from this fundamental stage of a TSCH LLN have been firstly stated and four different algorithms have been proposed to lift the limitations of currently available solutions. Then, the four novel joining schemes have been theoretically modeled in order to derive the average joining time, in close form expressions, as a function of the density of nodes, the
Elvis Vogli received the Bachelor’s degree in electronic engineering and Master’s degree in telecommunications engineering from the Politecnico di Bari, Bari, Italy, in 2008 and 2012, respectively, the MBA degree in management of information system from the Institute Universitaire Kurt Bosch, Bramois, Switzerland, in 2013, and is currently working toward the Ph.D. degree in electronics engineering at the Politecnico di Bari. He has been a Visiting Student with the SARA team of LAAS-CNRS,
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Elvis Vogli received the Bachelor’s degree in electronic engineering and Master’s degree in telecommunications engineering from the Politecnico di Bari, Bari, Italy, in 2008 and 2012, respectively, the MBA degree in management of information system from the Institute Universitaire Kurt Bosch, Bramois, Switzerland, in 2013, and is currently working toward the Ph.D. degree in electronics engineering at the Politecnico di Bari. He has been a Visiting Student with the SARA team of LAAS-CNRS, Toulouse, France, focusing on machine-to-machine (M2M) interoperability issues. He received a grant from the Apulia Region, Italy, for attending a professional master from the Institute Universitaire Kurt Bosch. His research interests include wireless sensor networks architectures, information centric networking (ICN), and M2M communications.
Giuseppe Ribezzo received the Bachelor’s degree in telecommunications engineering and Master’s degree in telecommunications engineering from the “Politecnico di Bari”, Italy, in 2011 and 2015. In 2015, he joined the Telematics laboratory at DEI - “Politecnico di Bari” as research engineer within the H2020 Bon voyage project. His main research interests are: industrial IoT protocols and future internet architectures.
Luigi Alfredo Grieco is an Associate Professor in Telecommunications at “Politecnico di Bari”. Formerly he has been Visiting Researcher with INRIA (Sophia Antipolis, France) in 2009 and with LAASCNRS (Toulouse, France) in 2013, working on Internet measurements and M2M systems, respectively. He has authored more than 100 scientific papers published in international journals and conference proceedings of great renown that gained more than 2000 citations. His main research interests include TCP congestion control, quality of service in wireless networks, IoT, and Future Internet. He serves as editor of the IEEE Transactions on Vehicular Technology (for which he has been awarded as top associate editor in 2012) and as Editor in Chief of the Transactions on Emerging Telecommunications Technologies (Wiley). Within the IETF and IRTF, he is actively contributing to the definition of new standard protocols for industrial IoT applications and new standard architectures for tomorrow ICN-IoT systems.
Gennaro Boggia received, with honors, the Dr. Eng. Degree in Electronics Engineering in July 1997 and the Ph.D. degree in Electronics Engineering in March 2001, both from the “Politecnico di Bari”, Italy. Since September 2002, he has been with the Department of Electrical and Information Engineering at the “Politecnico di Bari”, Italy, where he is currently Associate Professor. From May 1999 to December 1999, he was visiting researcher at the “TILab”, TelecomItalia Lab,Italy, where he was involved in the study of the Core Network for the evolution of 3G cellular systems. In 2007, he was visiting researcher at FTW (Vienna), where he was involved in activities on passive and active traffic monitoring in 3G networks. He has authored or co-authored more than 100 papers in international journals or conference proceedings, gaining more than 2000 citations. His research interests span the fields of Wireless Networking, Cellular Communication, Network Security, Information Centric Networking, Internet of Things (IoT), Protocol stacks for industrial applications, Internet measurements, Network Performance Evaluation. Currently, he serves as Associate Technical Editor for the IEEE Communications Magazine, Associate Editor for the Springer Wireless Networks journal, and Executive Editor of the Transactions on Emerging Telecommunications Technologies (Wiley).