## 1 Introduction
Thus, two of the main design objectives of WSN/IoT applications are to reduce energy consumption and prolong the network lifetime. Another important issue for WSNs/IoT applications is how to mitigate the energy hole or hot-spot problem [13], where the nodes located closest to the BS or in the most used paths tend to use up their energy resources prematurely [14]. This leads to a loss of connectivity between a node near to BS and the packets that are sent but not received at BS, which causes a wasteful of energy and wireless resources. Thus, the route selection scheme must consider the residual energy and end-to-end link quality to avoid the energy holes, while at the same time providing load balance and fair distribution of the scarce network resources. Given the characteristics outlined above, a routing protocol for WSN/IoT scenarios must also minimize the signaling overhead, which is responsible for increasing the processing and energy consumption in the nodes as well as increasing the packet loss and delay in the network. Additionally, reliable routes must be selected by using a mechanism to estimate the end-to-end link quality, based on cross-layer information, such as network conditions, energy and hop count. However, the current routing protocols for IoT applications do not take into account these key features, and novel solutions must be created. This article addresses the problems of ensuring reliability, together with energy-efficiency and load balancing in flat-based (homogeneous nodes) WSN/IoT architectures, by proposing an extended version of a routing protocol based on energy and link quality information (REL) [15]. REL aims to overcome the drawbacks that have been discussed earlier and allows the data transmission with low latency, packet loss, and high reliability, as well as a fair distribution of wireless resources, while increasing the network lifetime, for various flat-based IoT applications, such as smart parking, intrusion detection, and monitoring of river flows. REL proposes an end-to-end route selection scheme based on cross-layer information with a minimum overhead. To achieve energy-efficiency, the nodes send their residual energy to neighboring nodes by means of an on-demand piggyback scheme. Additionally, REL uses an event-driven mechanism to provide load balancing and avoid the energy hole problem. This article extends our previous work [15] as follows: _(i)_ it optimizes the process of route selection by means of end-to-end link quality evaluation and energy information; _(ii)_ it employs a new method for smoothing the link quality estimation; and _(iii)_ it conducts an in-depth analysis of the proposed solution for large-scale flat-based networks. It presents new simulation and testbed experiments to show the effects and benefits of REL for IoT systems. The results show that, compared with the performance of other well-known routing protocols, REL increases the network lifetime, enhances service availability, and ensures QoS support for IoT applications, while reducing the packet loss rate and signaling overhead. The remainder of this paper is structured as follows. Section 2 outlines the existing routing protocols and their drawbacks compared with REL. Section 3 describes the proposed routing by the energy and link quality (REL) protocol for IoT applications. Section 4 shows simulation and testbed experiments. Section 5 summarizes the main contributions and results of this article.