A Survey of Limitations and Enhancements of the IPv6 Routing Protocol for Low-power and Lossy Networks: A Focus on Core Operations <s> 3) Common Challenges and Pitfalls of OFs Enhancements <s> Mobile ad hoc networks (MANETs) consist of a collection of wireless mobile nodes which dynamically exchange data among themselves without the reliance on a fixed base station or a wired backbone network. MANET nodes are typically distinguished by their limited power, processing, and memory resources as well as high degree of mobility. In such networks, the wireless mobile nodes may dynamically enter the network as well as leave the network. Due to the limited transmission range of wireless network nodes, multiple hops are usually needed for a node to exchange information with any other node in the network. Thus routing is a crucial issue to the design of a MANET. In this paper, we specifically examine the issues of multipath routing in MANETs. Multipath routing allows the establishment of multiple paths between a single source and single destination node. It is typically proposed in order to increase the reliability of data transmission (i.e., fault tolerance) or to provide load balancing. Load balancing is of especial importance in MANETs because of the limited bandwidth between the nodes. We also discuss the application of multipath routing to support application constraints such as reliability, load-balancing, energy-conservation, and Quality-of-Service (QoS). <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> 3) Common Challenges and Pitfalls of OFs Enhancements <s> Several routing schemes have been proposed in the context of mobile ad hoc network. Some of them use multiple paths simultaneously by splitting the information among multitude of paths, as it may help to reduce end-to-end delay and perform load balancing. Multipath routing also diminishes the effects of unreliable wireless links in the constantly changing topology of ad hoc networks to a large extent. Route coupling, caused by the interference during the simultaneous communication through multiple paths between a pair of source and destination, severely limits the performance gained by multipath routing. Using node disjoint multiple paths to avoid coupling is not at all sufficient to improve the routing performance in this context. Route coupling may be reduced to a great extent if zone disjoint or even partially zone disjoint paths are used for data communication. Two paths are said to be zone disjoint if data communication through one path does not interfere with other paths. Large path length (number of hops) also contributes to the performance degradation resulting in high end to end delay. So zone disjoint shortest multipath is the best choice under high traffic condition. However, it is difficult to get zone disjoint or even partially zone disjoint multiple routes using omni-directional antenna. This difficulty may be overcome if directional antenna is used with each mobile node. In this paper, we have done a comparative study on the performance of multipath routing using omni-directional and directional antenna. The result of the simulation study clearly shows that directional antenna improves the performance of multipath routing significantly as compared to that with omni-directional antenna. <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> 3) Common Challenges and Pitfalls of OFs 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> 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> 3) Common Challenges and Pitfalls of OFs 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> 3) Common Challenges and Pitfalls of OFs 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> 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> 3) Common Challenges and Pitfalls of OFs Enhancements <s> A wireless sensor network is a large collection of sensor nodes with limited power supply and constrained computational capability. Due to the restricted communication range and high density of sensor nodes, packet forwarding in sensor networks is usually performed through multi-hop data transmission. Therefore, routing in wireless sensor networks has been considered an important field of research over the past decade. Nowadays, multipath routing approach is widely used in wireless sensor networks to improve network performance through efficient utilization of available network resources. Accordingly, the main aim of this survey is to present the concept of the multipath routing approach and its fundamental challenges, as well as the basic motivations for utilizing this technique in wireless sensor networks. In addition, we present a comprehensive taxonomy on the existing multipath routing protocols, which are especially designed for wireless sensor networks. We highlight the primary motivation behind the development of each protocol category and explain the operation of different protocols in detail, with emphasis on their advantages and disadvantages. Furthermore, this paper compares and summarizes the state-of-the-art multipath routing techniques from the network application point of view. Finally, we identify open issues for further research in the development of multipath routing protocols for wireless sensor networks. <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> 3) Common Challenges and Pitfalls of OFs 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> 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> 3) Common Challenges and Pitfalls of OFs 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> BIB008
Although combining two or more metrics may give an application the capacity to optimize more than one aspect at a time, it may lead to undesirable consequences if not designed efficiently. Several problems have been identified as follow. Firstly, using multiple metrics means that a higher load of information need to be carried in DIO control messages, which in turns increases the risk of layer 2 fragmentation  . Apart from consuming network resources such as energy and bandwidth, fragmentation represents a serious problem in the LLNs especially when multipath routing is used, as two fragments belonging to the same packet may take different paths, increasing the probability of errors and packet loss  . The risk of fragmentation is more evident in fuzzy-based approaches as they feature greater complexity, especially when multiple instances exist in the same RPL topology. Secondly, in weighted composition, it is usually hard to decide on what weights to assign to the component metrics and whether the assigned values should be static or dynamic according to the context (e.g. time, position). Thirdly, some suggested metrics, such as node position, cannot be easily estimated in real environments BIB003 BIB004 . Fourthly, the composite metric may fall into the trap of giving one metric such a high priority that it behaves effectively as if it was itself a single metric. Finally, designing a composite metric that violates the monotonicity property should be avoided as it can lead to loops that harm reliability and waste resources BIB005 . In fact, the monotonicity property should be preserved even with single-based metric OFs. Unrealistic assumptions related to the operations of the protocol itself or its perceived environment is another pitfall that may lead to false conclusions. Such assumptions include one-hop communication range, building network-wide decisions based on optional features in the standard, non-duty-cycle and synchronous MAC protocols. For instance, in BIB008 , the frequency of control messages was used to measure the stability of the node and the routing topology but the higher frequency of control messages does not necessarily imply higher instability and this may mislead the routing decision. The RPL standard is intended to run on LLNs encompassing thousands of sensor nodes; however, a number of the surveyed enhancements were evaluated on networks comprising less than 10 nodes (e.g., only six nodes were used in BIB007 ). The small scale of the test network is inadequate to reach strong conclusions reported or display the advantages of proposed enhancements, as results cannot be generalized to large-scale deployments. Multi-path routing techniques are highly desirable in LLN environments as they have been proven to provide a wide spectrum of benefits such as improving fault-tolerance, enhancing reliability, minimizing congestion, increasing network capacity (bandwidth aggregation) and improving QoS BIB006 . However, multi-path routing techniques do have their own disadvantages that should be considered carefully when designing routing primitives for LLNs BIB006 BIB001 . One of the primary concerns associated is that multi-path approaches introduce greater complexity and overhead. In such techniques, the intermediate nodes are required to maintain the state of multiple routes to a destination; this might be infeasible for memory-constrained LLN devices especially in the case of downward traffic, where a node must store routing entries for all destinations in its sub-DODAG BIB001 . The way the data packets are allocated to multiple paths represents another challenge BIB001 . When fragmentation occurs, fragments of the same packet might be transmitted on different routes raising the need for packet re-ordering. This risk is high if a round-robin traffic allocation is used to distribute traffic among multiple paths based on per-packet granularity BIB001 . Ensuring full efficiency of multipath routing requires the discovery and the maintenance of network-wide node-disjoint paths, which creates an extra overhead and may be infeasible in resource-constrained networks with highly dynamic links and scarce energy resources BIB002 . Moreover, the broadcast nature of the wireless medium may impede the goals of reducing congestion or load balancing due to the route-coupling effect, a phenomenon in the wireless medium that takes place when several paths are located in close proximity causing communication interference and increasing the risk of collisions BIB002 . Although locationaware routing can be used to mitigate the effect of route-coupling problem by constructing non-interfering routes, the high overhead incurred by such techniques in terms of computational and communication complexity makes them unsuitable for the resource-constrained LLN devices BIB006 .