## 3 Recent Advances on IEEE 802.15.4-Based WMSN Approaches  
This section summarizes the current state of the art focusing on the most relevant approaches providing conventional LR-WPAN with mesh capability. Special attention is paid to the advantages and shortcomings of the different proposals reviewed. Among them, we should highlight the most extended commercial WMSN solutions which are illustrated in Figure 1, namely: Zigbee Pro [11], the IETF group with its solution [12], which encompasses the IPv6 over Low power Wireless Personal Area Networks (6 LoWPAN) [13] and the IPv6 Routing Protocol for Low Power and Lossy Networks (RPL) [14], IP500 [15], WirelessHART [16], and ISA SP100.11a [17]. In addition, the recent IEEE 802.15.4e MAC standard [18] is also discussed, because despite not being strictly a WMSN solution, it can be interpreted as the underlying support for future mesh proposals. Finally, although out of the scope of the above mentioned solutions, the IEEE 802.15.5 standard [19] is a novel WMSN standard that demands attention as well, therefore it is further described in the next Section. Figure 2 shows a general snapshot of the protocol stack referred to the WMSN solutions under consideration. Observing this Figure, all these proposals share a common starting point: they fully or partially implement the recommendations of the IEEE 802.15.4 standard for their respective PHY and MAC layers. Studying deeply each one of these proposals, they can be classified into two main categories: (i) those that only use rules related with the IEEE 802.15.4 PHY layer, and (ii) proposals that employ both, the MAC and PHY layers of this standard. Regarding the first category, WirelessHART and ISA SP100.11a are two specifications which integrate in their implementations some foundations of the IEEE.802.15.4 PHY layer. The second  
Figure 1: Most relevant standards and proposals which provide LR-WPANs with mesh capability. Figure 2: LR-WPAN architecture for the current wireless sensor approaches enabling mesh deployments. category comprises proposals that share the same frame format (thus contributing to their interoperability) but differ in the use or not of scheduling mechanisms at the IEEE 802.15.4 MAC layer. Then, the second category is further subdivided into two groups as follows: _non-beacon mode_ and _beacon mode_ solutions. Following with this sub-classification, the first group, the IEEE 802.15.4 MAC non-beacon mode solutions, is characterized by transmitting messages according to the unslotted Carrier-Sense Multiple Access-Collision Avoidance (CSMA-CA) algorithm [20]. It presents a higher flexibility for the transmission operations of the mesh network but at the expense of deteriorating other network performance figures, such as the message delay in conditions of high network traffic. As illustrated in Figure 2, WMSN solutions such as Zigbee Pro, 6 LoWPAN/RPL, IP500 or IEEE 802.15.5 fit in the non-beacon mode at MAC layer category. Finally, in this group, we have also included a mechanism belonging to the IEEE 802.15.4e standard called Time Slotted Channel Hopping (TSCH) [18], which provides the basis for future WMSN implementations in non-beacon mode. On the other hand, the IEEE 802.15.4 beacon mode category transmits information within strict temporal intervals. In the beacon mode, a LR-WPAN is created and managed by a special device called PAN Coordinator. The PAN Coordinator has a double functionality: (i) it usually acts as destination/sink of the information and (ii) it coordinates the access to the physical medium of the remaining network nodes. The PAN Coordinator periodically transmits _beacon frames_ (specific messages including control information, _i.e._, the operation channel, address identifier, _etc._) informing the network nodes about the beginning of the so-called _superframe_, a time interval divided into a slotted active period -ON- and an inactive period -OFF- (this ON/OFF operation is also known as duty-cycle). Thus, nodes listen to the medium for the reception of these _beacon frames_.