## 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  
As a particular proposal in this group of solutions, we should also mention the TSCH protocol, which is founded on the design of an _ad-hoc_ TDMA-based schedule over the IEEE 802.15.4 non-beacon mode. TSCH is included in the IEEE 802.15.4e specification; a MAC mechanism aimed at amending the last revision of IEEE 802.15.4 standard (released in 2011 [38] to include in a same document the IEEE 802.15.4-2006 together with the PHY amendments published in 2007 and 2009) to offer support to the industrial market segment. IEEE 802.15.4e is not a complete WMSN solution because it does not have upper layers on-top of the MAC level which would provide, for instance, mesh routing capabilities. Nevertheless, this standard furnishes mechanisms such as TSCH and DSME (described in the next subsection), in order to offer support for future WMSNs. One of the main advantages of TSCH is the use of the technique denoted as channel hopping, which consists of switching dynamically among the 16 available channels of IEEE 802.15.4 PHY layer at running time [18]. The goal of this technique is to mitigate interferences of devices belonging to the WMSN transmitting in the same coverage area and the effect of path loss fading, what in turn facilitates simultaneous transmissions in the vicinity. This is advantageous in relation to the traditional IEEE 802.15.4 standard because it only allows us to select different frequency channels at the network initialization phase in order to avoid interferences with other existing WPANs. Furthermore, this frequency-diversity scheme along with its TDMA-based technique may be very useful for the WirelessHART and ISA SP100.11a proposals, since the scheduling and modulation they implement are fully compatible with TSCH. Therefore, WirelessHART and ISA SP100.11a proposals could improve their interoperability in the industrial field by employing the TSCH mechanism. In TSCH, a set of nodes in coverage range are synchronized [39] by means of a periodic time-slotted structure known as _slotframe_. Each slot belonging to the _slotframe_ is long enough to allow the communications between two nodes, in particular, the transmission of one data message and its corresponding acknowledgement. Going into more detail, the operation of TSCH is based on the fact that devices reserve one slot for receiving messages within the _slotframe_ structure. It implies that, before transmitting a message, a sender must learn about the following two issues: (1) the beginning of the receiver's reception slot; and (2) the channel employed by the receiver [18]. In particular, a different channel known by sender and receiver is selected on every new slot. To this end, both nodes employ a pseudo-random number which, with a high probability, resolves the same channel. Additionally, the design of TSCH allows different senders to dispatch information to a specific receiver in the same channel and slot. Then, in order to solve possible collisions within the slot, the standard employs the slotted CSMA-CA algorithm. Another interesting advantage of TSCH protocol is that devices can be members of different _slotframes_. In this case, a device may reserve a reception slot within a _slotframe_ of length \(n\) slots to communicate with a particular group of neighbors and, in turn, the device under consideration reserves a second reception slot within a different _slotframe_ of length \(p\) slots (\(n\
eq p\)) in order to accomplish data transmission with another group of neighbors. Unfortunately, TSCH presents also several drawbacks. First, it requires extra energy and hardware resources on: (i) senders to appropriately carry out the aforementioned two issues that guarantee the right information transaction with receivers and (ii) senders and receivers in order to perform fast channel switching tasks.