A Survey of Limitations and Enhancements of the IPv6 Routing Protocol for Low-power and Lossy Networks: A Focus on Core Operations <s> f) Fuzzy-Based Composition Enhancements <s> Accurate and low-cost sensor localization is a critical requirement for the deployment of wireless sensor networks in a wide variety of applications. In cooperative localization, sensors work together in a peer-to-peer manner to make measurements and then forms a map of the network. Various application requirements influence the design of sensor localization systems. In this article, the authors describe the measurement-based statistical models useful to describe time-of-arrival (TOA), angle-of-arrival (AOA), and received-signal-strength (RSS) measurements in wireless sensor networks. Wideband and ultra-wideband (UWB) measurements, and RF and acoustic media are also discussed. Using the models, the authors have shown the calculation of a Cramer-Rao bound (CRB) on the location estimation precision possible for a given set of measurements. The article briefly surveys a large and growing body of sensor localization algorithms. This article is intended to emphasize the basic statistical signal processing background necessary to understand the state-of-the-art and to make progress in the new and largely open areas of sensor network localization research. <s> BIB001 </s> A Survey of Limitations and Enhancements of the IPv6 Routing Protocol for Low-power and Lossy Networks: A Focus on Core Operations <s> f) Fuzzy-Based Composition Enhancements <s> We consider the problem of estimating link quality in an ad-hoc wireless mesh. We argue that estimating links well requires combining information from the network, link, and physical layers. We propose narrow, protocol-independent interfaces for the layers, which in total provide four bits of information: 1 from the physical layer, 1 from the link layer, and 2 from the network layer. We present a link estimator design with these interfaces that reduces packet delivery costs by up to 44% over current approaches and maintains a 99% delivery ratio over large, multihop testbeds. <s> BIB002 </s> A Survey of Limitations and Enhancements of the IPv6 Routing Protocol for Low-power and Lossy Networks: A Focus on Core Operations <s> f) Fuzzy-Based Composition Enhancements <s> The diversity of applications that current and emerging Wireless Sensor Networks (WSNs) are called to support imposes different requirements on the underlying network with respect to delay and loss, while at the same time the WSN imposes its own intricacies. The satisfaction of these requirements highly depends on the metric upon which the forwarding routes are decided. In this view, the IETF ROLL group has proposed the RPL routing protocol, which can flexibly work on various routing metrics, as long as they hold specific properties. The system implementer/user is free to decide whether to use one or multiple routing metrics, as well as the way these metrics can be combined. In this paper, we provide ways to quantify the routing metrics so that they can be combined in an additive or lexical manner. We use extensive simulation results to evaluate the impact of several routing metrics on the achieved performance. <s> BIB003 </s> A Survey of Limitations and Enhancements of the IPv6 Routing Protocol for Low-power and Lossy Networks: A Focus on Core Operations <s> f) Fuzzy-Based Composition Enhancements <s> Several ways to estimate the position of a Wireless-Sensor-Network (WSN) node have been discussed in the past years. Unlike in outdoor solutions where the Global Positioning System (GPS) could be used in most applications, a general solution for indoor usage has not been found. The few existing indoor localization solutions on the market are highly specialized and rely on infrastructure or on very expensive special designed hardware. Both — infrastructure and expensive hardware — do not fit well into most scenarios where WSNs are commonly used because of the adhoc characteristics and the large amount of nodes of such installations. In this paper we present a solution to get a precise estimation of the distance between two nodes without the needs for special purpose chips or a redesign of already existent nodes. We use radio runtime measurement to calculate the distance between nodes and present algorithms to refine the measurements. A comparison with a professional solution which is available on the market is also presented. <s> BIB004 </s> A Survey of Limitations and Enhancements of the IPv6 Routing Protocol for Low-power and Lossy Networks: A Focus on Core Operations <s> f) Fuzzy-Based Composition Enhancements <s> The design and implementation of healthcare system using sensor network technology has been one of the most active research topics currently. The research on wireless sensor network mostly focuses on energy saving, such as duty cycle scheduling, and path routing, such as routing protocol for low power and lossy networks (RPL) which however only takes into account a single metric, reliability or energy, as routing decision. If RPL only considers the reliability metric, nodes will suffer from uneven energy. If it only considers the energy metric, nodes will suffer from the rise of packet loss ratio. In this paper, we propose an energy-oriented routing mechanism to improve RPL routing protocol by combining the expected transmission count (ETX) and remaining energy metrics. The simulation will be conducted to analyze the performance of the proposed mechanism. <s> BIB005 </s> A Survey of Limitations and Enhancements of the IPv6 Routing Protocol for Low-power and Lossy Networks: A Focus on Core Operations <s> f) Fuzzy-Based Composition Enhancements <s> Low power and lossy networks (LLNs) require efficient routing protocols that should meet the requirements of the critical applications, such as real-time, reliability and high availability. RPL has been recently proposed by the ROLL working group as a tree routing protocol specifically designed for LLNs. It relies on objective functions to construct routes that optimize or constrain a routing metric on the paths. However, the working group did not specify the set of metrics and/or constraints to be used to specify the preferred path, and left it open to implementations. In this paper, we design OF-FL, a novel objective function that combines a set of metrics in order to provide a configurable routing decision based on the fuzzy parameters. OF-FL has the advantage to consider the application requirements in order to select the best paths to the destination. Our evaluation with a large-scale testbed in ContikiOS reveals that OF-FL can achieve remarkable performance of the RPL-based LLNs in comparison with the existing objective functions, and appropriately satisfy the quality of service contract of the different applications. <s> BIB006 </s> A Survey of Limitations and Enhancements of the IPv6 Routing Protocol for Low-power and Lossy Networks: A Focus on Core Operations <s> f) Fuzzy-Based Composition Enhancements <s> Available wireless sensor networks targeting the domain of healthcare enables the development of new applications and services in the context of E-Health. Such networks play an important role in several scenarios of patient monitoring, particularly those where data collection is vital for diagnosis and/or research purposes. However, despite emerging solutions, wearable sensors still depend on users' acceptance. One proposed solution to improve wearability relies on the use of smaller sensing nodes, requiring more energy-efficient networks, due to smaller room available for energy sources. In such context, smaller wireless sensor network nodes are required to work long time periods without human intervention and, at the same time, to provide appropriate levels of reliability and quality of service. Satisfaction of these two goals depends on several key factors, such as the routing protocol, the network topology, and energy efficiency. This paper offers a solution to increase the network lifetime based on a new Energy-Aware Objective Function used to design a Routing Protocol for Low-Power and Lossy Networks. The proposed Objective Function uses the Expected Transmission Count Metric and the Remaining Energy on each sensor node to compute the best paths to route data packets across the network. When compared with state of the art solutions, the proposed method increases the network lifetime by 21% and reduces the peaks of energy consumption by 12%. In this way, wireless sensor network nodes wearability can be improved, making them smaller and lighter, while maintaining the required performance. <s> BIB007 </s> A Survey of Limitations and Enhancements of the IPv6 Routing Protocol for Low-power and Lossy Networks: A Focus on Core Operations <s> f) Fuzzy-Based Composition Enhancements <s> In the recent past, we witnessed to a dramatic growth of networks with a huge number of interconnected wireless nodes exchanging large amounts of information. This has brought to the need of ad-hoc, energy-aware protocols suitable for low-power and lossy networks. Among these, Routing Protocol for Low Power and Lossy Networks (RPL) is surely one of the most interesting ones. It chooses the optimal routes from a source to a destination node, based on specific metrics. In this work we present a RPL compliant composite metric that considers both reliability and energy. Its aim is twofold. First, it realizes energy consumption balancing, so that each node tends to consume the same amount of energy of all the others, thus prolonging the overall network lifetime. Second, it takes into account data reliability along the paths, expressed by the well known Expected Transmission Count (ETX). The effectiveness of the proposed metric is confirmed by some interesting simulation results, that strongly encourage a deeper investigation on this issue. <s> BIB008 </s> A Survey of Limitations and Enhancements of the IPv6 Routing Protocol for Low-power and Lossy Networks: A Focus on Core Operations <s> f) Fuzzy-Based Composition Enhancements <s> Routing Protocol for Low-Power and Lossy Networks (RPL) is the Internet Engineering Task Force (IETF) standard for IPv6 routing in Low-power and Lossy Networks (LLNs). By using the Objective Function (OF) and a collection of routing metrics, it can build a Destination Oriented Directed Acyclic Graph (DODAG). There are two kinds of OFs specified by the IETF, Objective Function Zero (OF0) and Minimum Rank with Hysteresis Objective Function (MRHOF) respectively. The former is based on minimum hop count while the latter is based on Expected Transmission Count (ETX) and is more preferable. However, as the network size increases, both MRHOF and OF0 will introduce long single hop, which may become the routing bottleneck that restricts the entire network. In this paper, the problem is solved by optimizing the MRHOF with a new routing metric, the PER-HOP ETX. Instead of working with additive ETX metrics along a route, PER-HOP ETX distributes the ETX value to each node along the route. This method can achieve better routing performance in large-scale sensor network scenario. A detailed mathematical proof is given in the paper, and simulations were carried out based on the Contiki operating system. Experiments showed that PER-HOP ETX, the proposed optimization outperforms OF0 and ETX in terms of network latency, packet delivery ratio and energy consumption when the network scale becomes large. <s> BIB009 </s> A Survey of Limitations and Enhancements of the IPv6 Routing Protocol for Low-power and Lossy Networks: A Focus on Core Operations <s> f) Fuzzy-Based Composition Enhancements <s> This paper proposes a new framework to improve the power efficiency of the IPv6 routing protocol for low-power and lossy networks (RPL). The new framework makes it possible for the root node of a destination-oriented directed acyclic graph (DODAG root) to determine which new device will join the DODAG and its parent DODAG node, which is the parent node for the new device on the DODAG tree, based on the proposed composite value. This composite value is based on the distance between a new device and a potential parent DODAG node, the number of child DODAG nodes, which are the DODAG nodes directly under the potential parent DODAG node, and the hop-count between the DODAG root and the new device. The device having the lowest composite value to a potential parent DODAG node is chosen by the DODAG root, and the new device joins the DODAG by connecting itself directly under the parent DODAG node. When this framework is applied to RPL, the longevity of the devices on the DODAG increases dramatically, becoming as much as 10 times longer. In addition, by changing the weights of each metric of the composite value, we can adjust the composite value appropriately depending on the features of various wireless sensor networks. <s> BIB010 </s> A Survey of Limitations and Enhancements of the IPv6 Routing Protocol for Low-power and Lossy Networks: A Focus on Core Operations <s> f) Fuzzy-Based Composition Enhancements <s> To design a routing protocol for applications over low-power and lossy networks (LLNs), the IETF ROLL Working Group standardized the IPv6 Routing Protocol for LLNs (RPL), which organizes nodes in a LLN into a tree-like topology called Destination Oriented Directed Acyclic Graph (DODAG). RPL shows good scalability and fast network setup. However, it may suffer from severe unreliability due to the selection of suboptimal routes with low quality links. To optimize the reliability of RPL routes, this paper proposes a stability metric based routing protocol named sRPL for reliable routing and data collection in LLNs. We introduce a new routing metric for RPL called stability index (SI), which exploits stability characteristics of RPL nodes to select more stable routes. In addition, we present a passive and lightweight network layer technique to measure the bi-directional expected transmission count (ETX) for wireless links in LLNs. As a use case of SI, we combine SI metric with ETX metric to make routing decisions. Simulation results show that sRPL can improve packet delivery rate of RPL routing protocol by 20%. <s> BIB011 </s> A Survey of Limitations and Enhancements of the IPv6 Routing Protocol for Low-power and Lossy Networks: A Focus on Core Operations <s> f) Fuzzy-Based Composition Enhancements <s> Minimizing the energy consumption and hence extending the network lifetime is a key requirement when designing an efficient sensor network protocol. QoS-aware routing in Wireless Sensor Network (WSN), aims to take into account other networks performance aspects as minimizing end-to-end delay (as well as jitter), reducing packet loss rate and minimizing the energy consumption of the network during data transmission. These objectives are sometimes conflicting therefore tradeoffs must be made between energy conservation and QoS considerations. The general problem can be reformulated as a Multi-Constrained Optimal Path problem (MCOP) which is known as NP-complete. The latter raises a real challenge as sensor nodes are very limited in resources capabilities. We propose to use fuzzy inference mechanism to seek a good tradeoff between all given metrics and constraints. This paper discusses the implementation of combining several routing metric using fuzzy logic to design a RPL objective function, the routing standard for the Internet of Things. The proposal is integrated on Contiki operating system, the deployment was performed on a real world indoor WSN. Obtained results show improvements compared to the common implementation of the RPL protocol and demonstrate relevance of our contribution. <s> BIB012 </s> A Survey of Limitations and Enhancements of the IPv6 Routing Protocol for Low-power and Lossy Networks: A Focus on Core Operations <s> f) Fuzzy-Based Composition Enhancements <s> Several Link Quality Estimators (LQEs) have been proposed for Wireless Sensor Networks. However, their adequacy to smart grid environments has not been properly investigated. This paper addresses the problem of efficient low-power link quality estimation for smart grid environments. The first part of this paper presents a performance study of representative LQEs, namely ETX, four-bit and F-LQE, in three typical smart grid environments. These LQEs are evaluated in terms of reliability, stability and reactivity, by analyzing their statistical behavior. This study shows that F-LQE is more reliable and more stable than ETX and four-bit. However, it is not the most efficient for smart grid due to the lack of reactivity and also its higher complexity. Hence, the second part of this paper introduces Opt-FLQE, an optimized version of F-LQE that overcomes its limitations. The performance analysis of Opt-FLQE shows that it is more reactive than F-LQE while still being more reliable. <s> BIB013
Several Fuzzy-based OFs for RPL have been also introduced in several studies. For instance, the authors in BIB006 highlighted the problem of relying on a single-metric objective function. They further pointed out that even combining two routing metrics might be insufficient to address the requirements of multiple applications as the performance objectives may vary so widely. In addition, combining just two routing metrics may enhance the network performance of the parameters associated with these, but at the expense of negatively affecting other parameters. For example, considering the ETX and latency metrics may help the RPL network to discover more reliable paths with low delay, but may lead to a battery depletion due to the overuse of some routers BIB006 . Thus, they assert that there is a need to design a holistic objective function that combines multiple routing metrics to optimize all significant parameters simultaneously. To fulfill this goal, they propose a fuzzy logic approach named the Fuzzy-Logic OF (FL-OF) that combines four representative routing metrics namely the hop count, node energy, link quality and end-to-end delay. It is shown that the proposed OF-FL has a tendency to reduce average hop count in comparison with the MRHOF in dense networks. In addition, OF-FL has a much better performance in terms of packet delivery ratio than OF0, and almost the same packet delivery ratio as MRHOF with ETX BIB006 . Furthermore, the results indicated that OF-FL has a better load distribution among nodes leading to a more balanced energy consumption than F0 or MRHOF with ETX. Finally, OF-FL demonstrates the lowest average end-to-end delay for nodes on the edge of network while the delay is comparable with the standardized OFs in other cases. However, OF-FL experiences higher churn compared to MRHOF with ETX BIB006 . Another fuzzy-based approach to combining routing metrics is introduced in BIB012 . The authors used a two-stage fuzzy process to combine three linguistic variables (routing metrics), namely, delay, ETX and energy. In the first stage, the delay and ETX are combined to compute what they call Quality of Service (QoS). In the second stage, the energy is combined with the computed QoS value. The proposed fuzzy-based approach is then evaluated against ETX-RPL using a real testbed network of twenty-eight sensor nodes. The two protocols are compared in terms of packet loss ratio, energy consumption and routing stability (number of preferred parent changes). It is reported BIB012 that the fuzzy-based approach outperforms ETX-RPL in terms of packet loss ratio by up to 20% and slightly enhances end-to-end delay. In addition, the proposed approach is shown to build a topology of more stable routes with an average of 6.63 parents change per hour compared to ETX-RPL with an average of 43.52. A third fuzzy-based routing metric is proposed in BIB013 [101] referred to as Opt-FLQERM. This composite metric considers three link estimation metrics: Average Received Signal Strength Indicator (ARSSI), Smoothed Packet Reception Ratio (SPRR), and Smoothed Required Number of Packet retransmissions (SRNP). These three routing metrics are combined using a fuzzy approach that produces a score from the range [0...100], where 100 is the best quality and 0 is the worst. To select the optimal path, the inverses of the individual link qualities are added and then the path with minimum value is selected. The authors claim that relying on the inverse when selecting the optimal path allows the metric to avoid low-quality links while favoring paths with fewer hops. For evaluation purposes, the proposed routing metric is compared to RPL, ETX-RPL and the four-bit CTP BIB002 metrics using the well-known Cooja simulator in terms of average packet loss, average end-to-end delay, average hop-count and average power consumption. The authors show that their proposal produces the lowest packet loss, the lowest end-to-end delay and the lowest churn (number of parent changes) among the compared metrics. The superiority of Opt-FLQERM over ETX is attributed to the conservative approach used by ETX to estimate link quality: this is based on data traffic, which is only obtained after topology establishment. In contrast, Opt-FLQERM bases its calculation for link qualities on both control and data traffic resulting in an accurate estimation of link quality at the time of topology construction that results in constructing more stable paths BIB013 . Summary and Insights: The study in BIB003 was the first attempt to provide RPL with a way to quantify routing metrics and allow them to be combined lexically or in an additive manner. Although the study presents a good proposal to distribute energy load among nodes, by combining the RE and the hop-count, it does not elaborate on the effect of this combination on the network reliability, a critical performance criterion. It is also unclear whether the study uses the aggregated value of the RE metric or a local optimum value. A major issue with the study in BIB005 is that only up to six nodes are used for the simulation experiments, which may be insufficient to reach the conclusions reported. In addition, the authors did not elaborate on how the composite metric may affect the reliability of the network. The shortcomings of the articles in BIB003 [90] are addressed in BIB007 . First, the author introduces the parameter MIN_ENER to limit the churn in the network due to energy-related parent switches. Second, the study introduces a reliability-related performance evaluation of the composite matric. However, only 25 nodes were used in the simulation experiments, which means that conclusions reached cannot be generalized to larger networks. Although the study in BIB008 claims that the gain in network lifetime is obtained without affecting network reliability, the study does not reports any results regarding the reliability nor does it justify how the authors reach this conclusion. In addition, the authors used their own bespoke simulator for evaluation purposes, which may lack in features compared to the well-known simulators such as Cooja. The study reports setting the Trickle timer interval for emitting DIOs to 1 hour. It seems the authors have configured only one interval in their simulations, which is a confusing deviation from the normal operation of Trickle protocol. In  there is a higher risk of layer 2 fragmentation as DIOs transmitted by nodes running SCAOF need to carry a relatively large poll of parameters in their headers. This represents a serious problem in the LLNs as it increases the probability of errors and packet loss, especially in multipath routing. A major issue with PER-HOP ETX metric proposed in BIB009 is that the monotonicity property of the combined metric is not satisfied so the network might be at the risk of forming loops. The work in BIB010 suffers from the problem that the estimation of a node's positions in real testbed deployments is not a straightforward process and so live physical distance estimations are likely to be either imprecise (e.g. RSSI) or power-hungry (e.g. GPS) BIB001 BIB004 . The frequency of control messages (DIOs, DAOs, and DISs) is used in BIB011 to measure stability of the node and the routing topology but, in some cases, the higher frequency of control messages does not imply higher instability. For instance, a node with a higher number of children will have to transmit a higher number of DAOs than a node with a fewer number. In this scenario, it is clear that the number of children has caused the higher control overhead, and not the instability problem. A more elegant solution is to base the measuring of the instability index on the DIO messages alone. Finally, the fuzzy-based approaches are known to incur greater complexity compared to other approaches, especially when multiple instances exist under the same RPL topology  . For instance, in BIB006 more than four parameters need to be transmitted within the DIO metric container. Thus, there is a higher risk of fragmentation, which incurs more overhead due to the larger size of DIOs  . In BIB012 the stability of routes is claimed to be the reason of the superiority of proposed approach; however, no justification is given to explain why the fuzzy-based approach is more stable. The lack of justification also applies to the slightly improved delay. The work in BIB013 does not clarify how the control traffic messages (DIOs) have been incorporated into the link estimation calculation. Finally, Opt-FLQERM tends to favor shorter paths in terms of hop count, which may result in selecting paths containing low-quality single-hop links.