## 1 Introduction  
The IEEE 802.15.4 specification [1] is the most widely adopted point-to-point communication standard for low-rate wireless personal area networks (LR-WPANs). Its success lies in the design of a very straightforward protocol stack comprising the physical (PHY) and medium access control (MAC) layers, both developed for low-cost, low-power and resource-constrained wireless devices. The goal of this standard is to provide an appropriate solution for a few devices/nodes deployed in a small-sized area. Devices acquire data of interest (monitoring one or several physical parameters) and transmit them to their destination (sink) periodically or in event-driven fashion. Under these conditions, this standard exhibits two main limitations: (1) short-range communications with a small bandwidth available (maximum bitrate of 250 Kbps at 2.4 GHz) and (2) lack of multi-hop communications and mesh capability. In particular, mesh capability is understood here as the suitable provision/promotion of a set of features such as scalability, reliability, robustness, security, connectivity or coverage area in wireless mesh topologies, but always having into consideration the principle of energy efficiency [2]. These drawbacks reveal how IEEE 802.15.4 is unable to satisfy current demands for wireless sensor communications aimed at reaching a right solution for long-term monitoring over large areas. Nevertheless, the main commercial Wireless Sensor Network (WSN) manufacturers have employed this recommendation (or some of its functionalities) to develop their own standards and proposals. Those manufacturers that have developed IEEE 802.15.4-based solutions including mesh capability and multi-hop communications will obtain a twofold benefit: on the one hand, a notable improvement with respect to the traditional already deployed WSNs, and, on the other hand, the ability to plan innovative LR-WPAN applications/services on a multiplicity of different market segments, comprising automation and control (home and industrial), environmental surveillance, precision agriculture, traffic monitoring, and health services, among others. All these issues are reflected, for instance, in the trend shown by the industrial automation market in the last few years. According to a recent report published by On World [3], up to 75% of 216 surveyed industrial automation professionals/end-users (many of them, well-known companies/ manufacturers) have already installed or plan to deploy solutions based on WSNs to cope with their needs. In particular, 57% of the end-users are currently using or pilot testing WSN systems. Seventy-five percent of the current WSN end-users indicate they are considering employing the IEEE 802.15.4 standard, comprising mesh capability, for at least some of their deployments, while the 20% of the surveyed professionals reported that they intend to exclusively use mesh solutions for LR-WPANs. Furthermore, the On World study points out that advances on wireless mesh sensor technology over the next few years will assure a strong expansion of this technology in the consumer market. In this sense, the On World study foresees that, in 2016, around 39% of the wireless devices will be used for new applications and services that are uniquely enabled by LR-WPAN mesh networks. Therefore, we observe a clear migration from conventional LR-WPAN networks composed of only a few nodes to large-scale deployments where all devices communicate with each other by means of a standard mesh solution. Thereby, in order to provide traditional LR-WPANs [4] with mesh capability, in the last few years the scientific community has mainly concentrated its efforts on the design and development of upper layers on top of the IEEE 802.15.4 protocol stack.