## 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
#### 3.2.2 IETF: 6 LoWPAN and ROLL  
The Internet Engineering Task Force (IETF) and, in particular, its three work groups: 6 LoWPAN [13], Routing Over Low-power and Lossy networks (ROLL) [14] and Constrained RESTful Environments (CoRE) [33] play an important role in the contribution to finding a suitable WMSN solution. The goal pursued is the adaptation of the IPv6 Internet protocol onto the IEEE 802.15.4 standard MAC (in its non-beacon mode) and PHY layers [12]. The work in [12] shows how CoRE, 6 LoWPAN and ROLL groups work on a complete solution to offer Internet connection to WMSN devices mainly at home, building and industrial scenarios. In particular, CoRE defines different aspects of the IP-based application and transport layers, delegating the addressing, header compression and fragmentation tasks to the 6 LoWPAN association and the routing issues to the ROLL group. Likewise, 6 LoWPAN and ROLL develop an efficient solution to implement the network layer capabilities into the PPAN devices. The 6 LoWPAN deals with this issue integrating IPv6 datagrams within the frame format of the IEEE 802.15.4 standard. To this aim, 6 LoWPAN employs header compression and fragmentation techniques to reduce the 1,280 bytes of the IPv6 maximum transmission unit (MTU) to the available payload of an IEEE 802.15.4 frame (maximum 118 bytes at PHY layer) [34]. However, implementing these techniques implies in fact a high computational cost (CPU, memory and energy consumption) for the resource-constrained devices, which is observed as a weakness of 6 LoWPAN [35]. Regarding the addressing scheme, each network node is identified by a unique 128-bit IEEE logical address (IPv6 address). To generate this IPv6 address, 6 LoWPAN offers the developer two procedures: (i) the inclusion of an IPv6 prefix to the 64-bit IEEE MAC address of each node, thus extending the header size and (ii) the addition of a Dynamic Host Configuration Protocol version 6 (DHCPv6) server in the network, which in turn involves an extra cost. In both cases, the use of IPv6 addresses implies a clear disadvantage in terms of increasing memory and processing resources when mapping (generating and handling the routing tables) every neighbor node to its respective IPv6 address. On the other hand, ROLL is in charge of developing routing tasks and solving the multi-hop communications among network devices through the RPL routing protocol [12,36]. In particular, RPL enables large-scale deployments, but unfortunately, communications are based on tree-based routing. This means that every node only communicates with its parent and/or children nodes, what, on the one hand, _reduces the link reliability and robustness of the network_ when, for instance, a parent is dead; and, on the other hand, it is prone to bottlenecks (no alternate paths different to tree routes). Another RPL disadvantage is related to the need to be further tested and evaluated on large-scale networks. This is an issue about which, as far as we know, no research work has been published yet. Finally, both 6 LoWPAN and ROLL specifications _lack energy saving mechanisms_, which is a critical weak aspect for long-term WMSN deployments. In order to overcome it, the authors of works in [12,34] propose an asynchronous duty-cycle scheme that periodically listens to the physical medium to determine if there is any ongoing transmission (technique known as _low-power listening_). In particular, nodes are woken up at periodic intervals to sense the medium; they activate the radio to hear for any communication activity. This activity is started by a sender dispatching a message called _preamble_, which contains control information about, for example, the type of transmission (unicast or multicast), addressing information or the specific receiving node/s of the data. On the other hand, all nodes placed in the sender's coverage area must be active to receive the _preamble_.