## 4 Review of the Routing Protocols
### Cross-Layer-Based Routing Protocol Comparison  
Cross-layer routing protocols combine the challenges of the network layer with other layers. Even though these protocols have low energy consumption, high throughput and fixed end-to-end delay, they cannot supply high performance in scenarios with high path loss and body motion. Table 10 provides a comparison among cross-layer routing protocols; the comparison parameters include congestion avoidance, mobility, scalability, energy efficiency and the used methodologies.  
\begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline \multirow{2}{*}{**Protocols**} & \multicolumn{2}{c}{**Comparison Parameters**} \\ \cline{2-3}  & **QoS Focus** & **Methodology** \\ \hline \multirow{2}{*}{QPRR [56]} & \multirow{2}{*}{Reliability} & \(\bullet\) & Link reliability using EWMA \\  & & \(\bullet\) & Use of RSSI for localisation \\  & & \(\bullet\) & Numerical modelling \\ \hline \multirow{2}{*}{QPRD [132]} & \multirow{2}{*}{Delay} & \(\bullet\) & Queuing and channel delay using EWMA \\  & & \(\bullet\) & Use of RSSI for localisation \\ \hline \multirow{6}{*}{DMQoS [133]} & \multirow{2}{*}{Delay, reliability, priority traffic} & \(\bullet\) & Lexicographic optimisation for energy-aware forwarding, Greedy approach for reliability \\  & & \(\bullet\) & Queuing delay using EWMA, transmission delay using weighted average transmission delay (WATD) \\  & & \(\bullet\) & Link reliability using windowed mean EWMA (WMEWMA) \\ \hline \multirow{2}{*}{LOCALMOR [134]} & Latency, energy reliability, priority traffic, residual & \(\bullet\) & Power efficiency module uses a min–max approach [PERSON]’s formula for queuing delay \\ \hline \multirow{2}{*}{RL-QRP [135]} & \multirow{2}{*}{Packet delivery, delay, congestion} & \(\bullet\) & Q-value implementations \\ \hline \multirow{6}{*}{EN-NEAT [136]} & \multirow{6}{*}{Energy, packet delivery} & \(\bullet\) & EN-NEAT utilises multi-hop communication to reduce energy depletion and maximises network longevity. Firstly, avoid the transmission of normal data. Secondly, compare the sensed information, and if there is a variation, a transmission occurs; otherwise, no transmission occurs. Lastly, a minimum cost function was proposed to carefully choose the parent or forwarder node that has the highest residual energy and the shortest distance to sink. \\ \hline \multirow{6}{*}{Temperature-aware routing [137]} & \multirow{2}{*}{Energy, packet delivery, Delay} & \(\bullet\) & In the proposed work, a secondary base station is selected; this helps to reduce the temperature of the neighbour nodes, as these neighbour nodes will not be a part of the new data routes. At this time, the sensor node will transmit only the priority packets to the secondary base stations. \\ \hline \multirow{2}{*}{TARA [138]} & \multirow{2}{*}{Energy, priority and throughput} & \(\bullet\) & A thermal-aware routing algorithm is proposed to reduce the number of transmissions from hot-spot nodes or the nodes bearing more traffic by assigning the priorities \\ \hline \hline \end{tabular}
\end{table}
Table 9: Comparative analysis of QoS-based protocols.  
The aforementioned cross-layer routing protocols show the following aspects:  
* Energy consumption, end-to-end delay and throughput are the main considerations.
* Most of them agree to a tree-based approach to improve energy consumption.
* Time division mechanisms are also used to provide channel guarantees.
* Transmission power should be adopted according to the distance.