## 6 IEEE 802.15.5 Discussion and Future Trends  
Among all the approaches addressed in this work, we should highlight the IEEE 802.15.5 because it is the standard that appropriately satisfies a greater number of WMSN requirements for low-rate, low-power and resource-constrained devices. This means that the IEEE 802.15.5 considers all those features that turned the IEEE 802.15.4 standard into the most widely adopted WSN solution for one-hop topologies but extending its scope to multi-hop networks and offering mesh capability. Furthermore, since IEEE 802.15.5 includes the IEEE 802.15.4 MAC/PHY layers, it is fully compatible with other proposals that also comprise the IEEE 802.15.4 standard in their protocol stack. As a strong point, IEEE 802.15.5 offers two energy saving mechanisms, ASES, which tackles the traditional sensing WSN applications, and SES, aimed at delaying sensitive applications. This feature, along with its good scalability, assures long-term operation in large-scale mesh networks. Furthermore, this standard also provides additional services (non-mandatory functions), which can be implemented without jeopardizing the processing and memory capabilities of low-cost resource-constrained devices. However, the IEEE 802.15.5 standard presents the following open issues, which reveal future challenges to be explored by the scientific community:  
* There are few research works (mostly published in the open literature recently) [2,8,48,49,50,51,54,55,68] focused on this standard. Studying them carefully, we can conclude that one of the current weaknesses of IEEE 802.15.5 is the lack of a complete performance evaluation of its main operation and additional functionalities, such as the mobility support or the reliable broadcast transmission.
* To the best of our knowledge, no open-access software implementation and real deployments are available yet to the scientific and professional community. To deal with this negative aspect, software modules should be developed to promote this LR-WPAN mesh solution and to achieve its fast technological transference and penetration in the consumer market.
* Unfortunately, the IEEE 802.15.5 standard is unable to solve some relevant concerns inherited from the broadcast nature of wireless communications. In particular, the main drawback is referred to well-known issues such as the hidden and exposed terminal problems [69]. These problems negatively affect the network performance, causing message losses, increase in latency and energy consumption, poor bandwidth utilization, _etc._ Therefore, more research should be conducted to mitigate or eliminate these problems.
* Even though SES energy saving mechanism is intended for delay-sensitive applications, multimedia services [70] are not supported, a priori, by the standard. This implies considering a new research line, because applications with high traffic demand must be adapted to the WMSN area, paying special attention to significant LR-WPAN concerns as the resource-constrained devices and the available low bandwidth. To this end, SES must be appropriately complemented with new software modules to enable this type of services.
* Finally, aspects such as end-to-end reliability or security are not specified in the current version of the standard [19], what may constitute an interesting topic of research for future work.