## 3 Recent Advances on IEEE 802.15.4-Based WMSN Approaches
### WMSN Solutions Developed on Top of the IEEE 802.15.4 PHY and MAC (Non-beacon Mode) Layers
#### 3.2.4 IEEE 802.15.4e: The TSCH Protocol
Secondly, the standard does not provide any mechanism for the efficient assignmentof slots to nodes placed in the same coverage area, and for the initial channel configuration; both features aim at mitigating possible message collisions. In TSCH, this aspect has been considered to be developed by the upper layers. In this context, works in [40] and [41] deal with this concern, presenting a centralized and a distributed algorithm, respectively. In [40], a central node with more capabilities than the remaining network nodes has a complete knowledge of the network topology, which guarantees a quick configuration of the slot and channel employed by each network node. However, the work in [40] penalizes the network overhead and energy consumption due to network reconfiguration tasks whenever a topology change occurs. Concerning the study in [41], it offers a good performance for dynamic scenarios in which there are mobile nodes. However, for static scenarios where nodes remain always at the same location, this solution overloads the network with additional control traffic because devices must periodically transmit information about the slots in use and the channel status. Finally, another significant drawback of TSCH is the inefficient use of the spectrum, which has a negatively impact on robustness and link reliability. The reason is that the IEEE 802.15.4 channels are selected in TSCH without taking into consideration the interference level caused by other wireless technologies as Wi-Fi [42]. WMSN Solutions Developed on Top of the IEEE 802.15.4 PHY and MAC (Beacon Mode) Layers: IEEE 802.15.4e (DSME)  
In this category, two scheduling techniques were suggested to solve the collision of beacon frames dispatched by different coordinators (intermediate nodes) or the PAN sharing the same coverage range. The first technique schedules all beacon transmissions within a dedicated period denoted as _Beacon Only Period_ (BOP) [43]. In this period, each coordinator device selects a free time-slot which will be employed to transmit its respective beacon frame. Then, once the BOP period is finished, all coordinators (and their associated devices) sharing the same physical medium perform their communications in accordance with the traditional _superframe structure_ as defined by the IEEE 802.15.4 standard. However, this approach needs significant modifications in the IEEE 802.15.4 standard to include the BOP period, what restricts its interoperability with the rest of WMSN solutions supported by this same standard. In addition, this approach leads to another important concern. Since simultaneous data transmissions from different coordinators are not scheduled, messages may collide, thus degrading network performance. On the other hand, the second technique, known as _Superframe Duration Scheduling_ (SDS), aims at scheduling all the _superframes_ dispatched in the same coverage area within a specific interval called _multi-superframe structure_. It consists of a set of consecutive and non-overlapping _superframes_, each one of them from a different coordinator device. So, active periods of each coordinator are sufficiently separated in time, which will, a priori, avoid any data/beacon collision. Note that the _multi-superframe structure_ is similar to a TDMA scheduling where each slot contains a _superframe_. The research in [44] shows that in SDS the number of collisions is significantly lower than the one caused by using BOP approaches. This fact notably improves network performance in terms of throughput (number of data messages successfully delivered at the sink). This is one of the reasons why this technique has been the one adopted by the recent standard IEEE 802.15.4e-2012 [18]. Concerning the beacon mechanism, IEEE 802.15.4e-2012 proposes a scheme, called DSME, based on the ideas behind SDS. In DSME, each coordinator periodically transmits a specific beacon frame to inform the nodes in coverage about the exact time instant that the _multi-superframe structure_ starts.