## 1 Introduction
What is more, the influence of the node failure rate was also studied, showing that the performance improvements obtained with the authors' proposals increased when that failure rate was higher. In order to improve the network throughput and reliability, another modification of the airtime link metric calculation method was presented in [9]. One of the contributions of this work was to give more importance to the upstream transmission status (from smart meters to the concentrator), since most data were transmitted in this direction. Besides, a modification of the path selection mechanism was provided to avoid the classical problem of route fluctuation. With this modification, not only the current airtime link metric value, but also its variations were taken into account to select (or not) a new route between two network nodes. ns-3 simulations were presented to show the achieved benefits in terms of packet delivery ratio, end-to-end delay and data retransmission count. The results also highlighted the need for congestion control mechanisms when the network size was increased. Some of the same authors of [9] performed in [10] a study of the HWMP routing protocol, with the goal of identifying its weakness, both from the HWMP protocol itself (route instability and route recovery) and from the integration with smart grid networks (oversimplified calculation of airtime link metric and the need for traffic differentiation). Here, a modification of the airtime link metric computation was also proposed, as well as a proposal for the path selection mechanism. Besides, to get a better performance in terms of packet losses, reserve routes were stored in the network nodes. This idea gave rise also to a reduction in the traffic management traffic needed when a path was broken. Moreover, in order to provide a better quality of service to some applications, a delay-tolerant traffic management method based on the concept of delay-tolerant networking was proposed. The improvements obtained with the application of these new solutions to the protocol, called HWMP-reliability enhancement (HWMP-RE), were checked and shown by means of ns-3 simulations. Grid topologies were considered, from 9--49 nodes, where every node generated traffic (belonging to seven different applications) to two root mesh stations (gateways). HWMP-RE was compared with the basic HWMP and with the previous proposal in [9], showing a better behavior in terms of packet delivery ratio, end-to-end delay, number of PERR/PREQ generations, throughput and reliability. Other proposals based on the modification of the HWMP metric can be found in [16; 17]. In [16], a QoS-aware and load-balance routing scheme was proposed, which was complemented with an EDCA-based adaptive priority adjustment scheme, with the goal of satisfying the QoS requirements of different NAN applications. The modification proposed for the airtime link metric consisted of including the packet size and calculating the frame error rate separately for the different NAN applications. Besides, to avoid congested paths, the queuing delay was also added to the metric. What is more, the dynamic adjustment of the packet priority allowed a better resources utilization under low load conditions and improved the reliability under heavy load conditions. ns-3 simulations were carried out to evaluate the obtained performance, which showed an increase of both the packet delivery ratio and the throughput, as well as a reduction of the average end-to-end delay. The network scenario consisted of a grid topology where the number of nodes varied between nine and 64. On the other hand, the metric modification proposed in [17] (Interference-Aware Expected Transmission Time (IAETT)) was oriented to reduce the impact caused by inter- and intra-flow electromagnetic interferences. Besides, traffic differentiation was also considered. Based on this metric, an interference-aware QoS routing protocol was proposed and evaluated.