## 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
When a new device with the role of coordinator wants to join the network, it listens to the physical medium waiting for the reception of this beacon frame. Then, once the starting time of the _multi-superframe_ is known, the new coordinator searches for an empty slot within the _multi-superframe_ to transmit its own beacons and _superframes_. In addition, to further reduce collisions from devices transmitting in the same coverage area, DSME provides two possible solutions, on the one hand, the channel hopping technique described in the previous subsection and, on the other hand, the channel adaptation method based on the concept that nodes always remain in the same channel as long as the interference level experienced is kept within an acceptable range [18,45]. In accordance with DSME, the main problem of this approach arises when network topology changes (e.g., device failures, new nodes willing to join the network or mobility issues) occur in large-scale deployments. Under these conditions and considering that the _multi-superframe structure_ is long enough to contain all coordinator's _superframes_ in coverage area, the overhead caused by the re-scheduling of the beacon frames and/or the selection of a non-conflicting frequency channel may be quite significant. This fact increases the energy consumption of each device, as well as resource utilization in terms of computing and memory. Another important weakness appears in random topologies when the number of coordinator devices and their location in some sensing areas is a priori unknown by the end-user. In these cases, the time dedicated by the DSME protocol and the energy demanded by the devices may be notably high, especially during the network initialization and configuration phases. On the other hand, a high network density (number of devices per area unit) improves, for instance, routing issues because there are more alternate paths between sources and destinations, which alleviates the impact of the dead intermediate nodes. However, it can negatively affect the delay (time interval since a sender waits for transmitting an information block until this block is finally received by a one-hop neighbor) [46]. Due to the inherent TDMA nature of the _multi-superframe structure_, a coordinator and its associated devices only operate in their corresponding _superframe_, which is in a unique slot belonging to the _multi-superframe_. Then, if the amount of information cannot be dispatched in a unique _superframe_, the set formed by the coordinator and its associated nodes must wait for the next _multi-superframe structure_ to continue the transmission of data. Regarding high density networks, _multi-superframe structures_ are formed by a greater number of coordinators in coverage, which implies a longer duration of these time structures. Under these conditions, the delay associated to the data transmissions that occupy more than one _superframe_ increases as a function of the number of _multi-superframes_ employed. Consequently, this fact negatively affects delay. Finally, it should be mentioned that few recent investigations are devoted to studying the network performance using DSME [45,47]. Both works examine diverse metrics of interest such as throughput or energy consumption comparing DSME with the traditional slotted CSMA-CA of the IEEE 802.15.4 beacon-enabled mode under different conditions. However, the topologies used (mainly, the star one) are not consistent with the concept of large-scale WMSNs and, therefore, the results have a questionable validity when such a big network is analyzed.