## 4 The IEEE 802.15.5 WPAN Mesh Standard  
The IEEE 802.15.5 WPAN mesh standard [2, 19, 48, 49, 50] is aimed at providing traditional WPANs with mesh capability. Even though proposals described in Section 3 can achieve the same performance as IEEE 802.15.5 in some specific aspects (robustness, energy-consumption, scalability, _etc._), IEEE 802.15.5 satisfies the highest number of WMSN requirements, as shown in Table 1. One of its main benefits is that this standard copes with a wide range of possible applications and this implies, for its better understanding, its division into two different parts denoted as _Low-Rate (LR) WPAN mesh_ and _High-Rate (HR) WPAN mesh_. Our attention is focused on the low-rate part which was conceived as a complete mesh support to the IEEE 802.15.4-2006 standard. Thereby, different features such as scalability, robustness, complete autonomy or link reliability were included in the IEEE 802.15.5 recommendation; on-top of the physical (PHY) and MAC (in its non-beacon mode) layers of the IEEE 802.15.4 standard, thus maintaining interoperability. This was possible through the design of new and straightforward primitives that constitute the basis for the initialization, establishment, maintenance and operation of a mesh network. In addition, this standard offers other significant functionalities, such as reliable broadcast and multicast transmission [2], mobility support [51], and a traceroute function, which are useful for those applications that may require them. Nevertheless, the most remarkable functionalities comprised in this recommendation are the synchronous and asynchronous energy saving mechanisms intended to increase network lifetime. As it will be seen along this section, the greater strengths of IEEE 802.15.5 in comparison with the rest of technologies reviewed in this paper are simplicity, low power consumption and bigger improvement of mesh capability, providing an efficient solution for an unlimited number of applications including environmental monitoring, precision agriculture, home building and industrial automation and control, among others. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Feature** & **Zigbee* & **6 LoWPAN [13]** & **IPS00* & **IEEE802.15.4 era** & **WirelessHART* & **ISA*m** & **IEEE** \\  & **Pro [11]** & **RPL [14]** & **solution [15]** & **[18]** & **[16]** & **SP100.11a [17]** & **802.15.57*** [19]** \\ \hline  & Home, Building and Industrial automation and control; Precision agriculture; Environmental surveillance; _etc._ & Home, Building and Industrial automation and control, smart cities & Home, Building and Industrial and automation and control & Industrial automation and control & Industrial automation and control & Industrial automation and control & Industrial automation and control & Industrial automation and control & Industrial automation and control & Industrial automation and control \\ \hline
**802.15.4** & 2003 & 2003 (RFCs 4919 and 4944) & 2006 & 2011 & 2006 & 2006 & 2006 \\ \hline  & & & & IEEE 802.15.4 & & & \\  & & & & PHY along with & & & \\  & & & & channel hopping & & & \\  & & & & (TSCH) & IEEE 802.15.4 & & \\
**Physical Layer** & IEEE 802.15.4 PHY & IEEE 802.15.4 PHY & PHY (only 433, & IEEE 802.15.4 & & \\  & PHY & 868 and 915 & & & DSSS/FHSS & DSSS/FHSS & PHY \\  & & & MHz bands) & & channel hopping or channel adaptation & modulation & \\  & & & & (DSME) & & & \\ \hline
**Power** & No (Adressed in future spec.) & No (Adressed in future spec.) & No. Only end devices & Yes & Yes & Yes & Yes \\ \hline
**MAC layer** & Non-beacon mode & Non-beacon mode & Non-beacon mode & & & \\  & & IEEE 802.15.4 MAC & IEEE 802.15.4 MAC & & & & \\  & & & & & MAC: TSCH & Specific. Based on & \\  & & & & & Bacon mode & TDMA and & 802.15.4 MAC \\  & & & & & IEEE 802.15.4 & & \\  & & & & & MAC: DSME & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Main features of current wireless mesh sensor network approaches.