## 2 Wireless Mesh Sensor Network Requirements  
Providing conventional LR-WPANs with mesh capability is crucial for the commercial expansion of this technology, as well as for generating new applications or niche markets (_i.e._, protection firefighter operations, multimedia services, situational awareness and precision asset location, or security and environmental monitoring, among others). To achieve this goal, the WMSN design phase must consider several significant _requirements_ influencing the performance of WMSNs. They are summarized as follows:  
1. _Energy-management policy_ and _energy-efficient design_: This requirement is a critical issue in the design of WMSNs. Devices are usually fed by AA/AAA chemical batteries, which sharply restricts the life of network nodes in the case of continuous operation. Furthermore, in many occasions, the use of external power supply sources (e.g., solar or wind) is too costly or,simply, unfeasible for the type of application or emplacement. These concerns are even more decisive in large-scale mesh networks because the number of hops between source and destination (multi-hop) may be high, increasing the energy-demands associated to data transmissions/retransmissions, and therefore shortening network lifetime. Energy-management mechanisms alleviate these concerns as they reduce the power consumption of nodes by means of, for instance, the temporary disconnection of specific hardware components such as the CPU or the radio transceiver. Nevertheless, the rest of node's activities (such as routing procedures, addressing scheme, or security) must also be carefully designed to avoid nodes overload, what also increases the energy demands. 2. _Link reliability_: One of the most important factors when designing WMSNs is ensuring the permanent link connectivity between source-destination pairs during the entire operation of the application. In contrast to other topologies (_i.e._, the tree topology) a mesh network guarantees link reliability through the possibility of multiple paths connecting sources and destinations. This, in turn, enables alternative routes in the case of, for instance, a dead intermediate node. Furthermore, this requirement can be improved by the increase in the number of devices in a delimited sensing area, what implies greater path redundancy. This means an improvement in the connectivity for all nodes belonging to the area under consideration because they have a greater number of neighbors in coverage area. 3. _Robustness_: A WMSN must furnish fault tolerance and self-healing features in order to cope with concerns, such as topology changes [e.g., nodes (dis)appearing], aggressive dynamic environmental conditions or radio interferences. In particular, interference may be caused by different phenomena: network devices sharing the same broadcast medium or external signals coming from other wireless technologies operating in the same band, such as Wi-Fi. Therefore, this requirement may be clearly improved by exploiting the path redundancy of the network and by using efficient mechanisms that mitigate or eliminate such effects. 4. _Scalability support_: This requirement pursues the non-degradation of the network performance as the number of devices grows. The goal of WMSNs consists of extending the coverage range of traditional WSNs through networks formed by hundreds or even thousands of battery-powered devices. Thereby, scalability becomes a decisive requirement for large-scale monitoring applications in order to guarantee robustness, link reliability and an efficient consumption of energy. 5. _Interoperability_: A searched feature of WMSN protocols is that they guarantee the compatibility and inter-connection among heterogeneous networks including other wired and wireless technologies. To this end, interoperability must be provided without increasing the complexity of the software and hardware modules.