## 4 Review of the Routing Protocols  
The IEEE 802.15.4 standard defines the operational functionalities of LR-WPANs [124]. As it supports peer-to-peer and star topologies, it is considered a popular standard for low-power sensor monitoring applications for healthcare, factories and disaster areas, etc. [125; 126; 127; 128]. A detailed comparison analysis regarding hop selection is provided in [129]; it concludes that multi-hop communication leads towards better network performance in terms of energy consumption, with a minor decrement in the packet delivery ratio; this can be ignored because less energy consumption will increase the network lifetime in WBASNs. The discussion in [130] indicates that a route using several short hops provides better energy consumption and network lifetime than routes with long hops. The research discussed \begin{table}
\begin{tabular}{p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt}} \hline
**Protocol** & **Year** & **Standard** & **Access scheme** & **Shortcomings** & **QoS** \\ \hline PA-MAC [116] & 2016 & IEEE 802.15.4 IEEE 802.15.6 & Hybrid & It requires hardware modification, which is a difficult task for existing standards & P, E, C existing standards \\ \hline AT-MAC [117] & 2016 & IEEE 802.15.4 Hybrid & The proposed mechanism focuses on reliability for WBASN medical applications, whereas a trade-off discussion between reliability, delay and energy usage is missing & R \\ \hline CoR-MAC [118] & 2016 & IEEE 802.15.4 IEEE 802.15.6 & Hybrid & For the implementation of such a mechanism, strong synchronisation required between reservation mechanisms, which require more processing power and memory & D \\ \hline C-MAC+ [110] & 2017 & IEEE 802.15.6 Hybrid & Hybrid & A strong a-synchronisation mechanism is required to avoid collision by incorporating a duty cycle mechanism. An extensive modification is required to implement C-MAC in existing standards & D, E \\ \hline Interference mitigation model [119] & 2018 & IEEE 802.15.6 & CSMA/CA & Required more resources in terms of energy and memory due to queue management & M, T \\ \hline TCP-CSMA/CA [120] & 2019 & IEEE 802.15.4 & Slotted CSMA/CA & Implementation requires more energy consumption and could add more delays for not-prioritised traffic & P, D \\ \hline TA-MAC [121] & 2019 & IEEE 802.15.4 Hybrid & The proposed traffic-based priority mechanism works well; however, inclined average delay values for the other traffic types are noticed & P \\ \hline DCSS [122] & 2019 & IEEE 802.15.6 Hybrid & The proposed dynamic channel selection mechanism selects a good channel to avoid interference; however, for that, it needs information from the physical layer, which will require more time and resources & I, T \\ \hline PBDT [123] & 2019 & IEEE 802.15.6 Hybrid & Hybrid & Posture-based data transmission helps to identify the posture based on RSSI values; however, the proposed mechanism is complex and maybe not be suitable for sensors with delay-sensitive data & I, M \\ \hline \end{tabular}
\end{table} 
Table 8: _Cont._various reasons to prove that routing strategies with the involvement of long hops are more successful in terms of power efficiency. For single-hop access communication, it was concluded that the node with more distance from the base station consumes high energy and, ultimately, less network lifetime. The work in [131] shows that single-hop communication may not provide reliable communication for WBASNs, whereas, for multi-hop communication, they define separate routes with minimum delays and high overall. WBASN routing protocols have many challenges, including limited resources, energy efficiency, time-bounded delay (<50-250 ms for medical applications), traffic priorities and the unreliability factor of low-power wireless links. The routing protocols for WBASNs can be categorised into four broader categories, as shown in Figure 8.  
Multi-hop routing protocols for WBASNs are mostly classified into four categories: QoS-based, cluster-based, cross-layer-based and link-quality-based.