## 3 Recent Advances on IEEE 802.15.4-Based WMSN Approaches
When it occurs, network nodes can transmit data (during the active period) as follows: (i) by using the slotted CSMA-CA algorithm [21] and/or (ii) by reserving dedicated slots in order to guarantee the data delivery at destination fulfilling strict temporal requirements, thus increasing the network reliability. When the active period ends, nodes switch to a low-power state (the sleep state) to save energy. However, the main disadvantage lies in the difficulty to extend the traditional one-hop IEEE 802.15.4 topologies to multi-hop mesh. In this sense, if we assumed a mesh network running under beacon mode, we would require intermediate devices to retransmit the sensed data when sources (usually, end-devices) and sinks are mutually out of coverage. This requires that intermediate nodes perform a similar functionality to that of the PAN Coordinator, that is, they must also dispatch beacon frames (hence acting as _coordinator devices_) in order to coordinate the access to the medium of other intermediate nodes and/or end-devices in their coverage range. In this case, the lack of an appropriate coordination mechanism taking into account the PAN and the _coordinator_ nodes may provoke collisions of beacon frames (and also of data messages) and, therefore, message retransmissions or even losses, deteriorating the network performance [22]. Currently, to the best of the authors' knowledge, there is no WMSN implementation using the beacon mode of the IEEE 802.15.4 standard. However, we should mention the IEEE 802.15.4e standard, a recent MAC solution that incorporates a mechanism denoted as _Deterministic and Synchronous Multi-channel Extension_ (DSME) [18], which solves the coordination of the nodes in a mesh network by means of the beacon mode scheme. The goal of this mechanism is to set the premises for designing future WMSN proposals over IEEE 802.15.4 PHY/MAC in beacon mode. WMSN Solutions Developed on Top of the IEEE 802.15.4 PHY Layer: WirelessHART(r) and ISA(tm) SP100.11a  
WirelessHART [16] and ISA SP100.11a [6; 17] are two WMSN standards which have been released by the HART Communication Foundation (HCF) and International Society of Automation (ISA) groups, respectively, and which use only some functions of the IEEE 802.15.4 PHY layer. Both solutions are intended for the industrial sector, in particular, for control and automation processes. In the case of LR-WPAN, WirelessHART protocol stack [6; 16; 23; 24] implements some features of the IEEE 802.15.4 PHY layer, providing its own implementation for the rest of this and other upper layers (link, network, transport and application). WirelessHART employs a modulation based on a combination of Direct-Sequence Spread Spectrum -DSSS-, and Frequency-Hopping Spread Spectrum -FHSS- at the PHY layer in order to assure efficient data transmission and to be more robust against interferences. Furthermore, a Time-Division Multiple Access (TDMA) scheme [25] strictly controls the access of the messages generated by neighbors in coverage to the medium, offering deterministic latencies and minimizing the number of collisions. This TDMA mechanism allows WirelessHART devices to switch, in non-active periods, to the sleep mode (OFF) in order to extend the node's lifetime. Regarding the upper layers of WirelessHART, they include the proprietary HART specification (last version named HART 7). In particular, as regards the network layer, WirelessHART uses a central node, denoted as Network Manager (NM), intended to create the network and to establish and maintain all the source-destination paths. To this end, the Network Manager needs to discover the current network topology. Additionally, continuous information is collected by this node to detect any change occurred in the different network paths.