## 1 Introduction
The goal is to generate multi-hop large-scale networks, encompassed into the Wireless Mesh Sensor Networks (WMSNs) technology, in which any source device is able to dispatch data to a destination (or several ones) out of its coverage range, by selecting the best path of intermediate nodes between both end nodes. In addition, as a well-known benefit, a WMSN balances the network traffic load with the aim of avoiding, among other concerns, bottleneck nodes or information losses when an intermediate node dies. This fact reveals that a WMSN achieves much better network performance than the traditional WSN-based implementations restricted, often, by the topology (usually, star or tree topologies). However, successfully running LR-WPAN applications that support mesh capability implies a notable waste of energy on communication issues (_i.e._, generation of the mesh network or efficient data transmission/reception among intermediate nodes belonging to a path). This translates in a reduction of the devices' lifetime (time interval since a device is activated for first time until it depletes its batteries), which complicates long-term solutions. To overcome this, devices belonging to a WMSN must be efficiently self-organized in a mesh topology (regardless of the number of devices and transparently to users) and must operate coordinately to carry out efficient communications. This can be reached through the careful implementation of communication software modules into the resource-constrained devices, considering: (1) their energy-constrained premises (devices generally equipped with a short number of chemical batteries, and placed at isolated locations where the use of external power supply is often unfeasible) and (2) their well-known hardware restrictions of memory and processing capabilities. Thereby, straightforward solutions must be designed and developed to offer the best performance for mesh topologies in terms of scalability, interoperability, mobility, self-organization, reliability, robustness, security or connectivity, among others; without jeopardizing the primary objective of low power consumption [5]. In this sense, an appropriate scheduled operation (comprising active and inactive periods -ON/OFF-) would guarantee an extension of the devices' as well as the network's lifetime, being the network's lifetime the time interval since the network starts its operation until one or several nodes deplete their batteries, making the network operation unfeasible. Currently, this type of solutions has not been completely fulfilled by any WMSN proposal or standard, what makes it an open challenge for the scientific and manufacturer communities. In this context our paper has three main contributions. First, we review the current WMSN standards and proposals that facilitate the development of mesh topologies for LR-WPANs. To this end, we have identified the most relevant WMSN requirements with the aim of learning about the pros and cons of each proposal. The goal is to discern what issues prevent the complete penetration of WMSNs in the market. Regarding these proposals, several organizations and industrial alliances, such as IEEE, Zigbee(r) Alliance, IP500(r) Alliance, the Internet Engineering Task Force (IETF), the Highway Addressable Remote Transducer (HART) Communication Foundation (HCF), or the International Society of Automation (ISA) have promoted their own commercial standards but, unfortunately, most of them lack some significant features (e.g., energy saving mechanisms or interoperability with other wireless technologies) or are simply not validated by a thorough and complete study or evaluation,which means that its true performance is in fact unknown. Despite the existence of available studies comparing some of these proposals [4, 6, 7, 8], no research work jointly deals with all of them or focuses particularly on the WMSN field. Therefore, to the best of our knowledge, this is the first study that addresses these concerns, which means a significant step forward for this technological area.