## 4 Comparative Discussion
### Major Features  
In this subsection, the temperature-aware routing protocols are reviewed in terms of major features and, then, compared with each other qualitatively. The comparison results are summarized in Table 1.  
TARA is the first routing protocol that introduced temperature as a routing metric. The first advantage of TARA compared to the SHR (shortest hop routing) protocol is that it achieves a better performance in terms of reducing the maximum and average temperature rise. The SHR protocol only cares about how to send the packet to the destination node using as short a route as possible without considering any other factors. While it achieves a low packet delivery delay, it suffers from high temperature rise, since the same nodes will always be used for a particular route. On the other hand, the TARA protocol is concerned with the temperature of the nodes and uses the withdrawal strategy; it will avoid hotspot nodes by sending back the packet to the previous node and tries another available route. This mechanism will reduce the average temperature rise of the network and limit the maximum temperature to the predefined \begin{table}
\begin{tabular}{c c c c c c c} \hline
**Routing Protocol** & **Routing Approach** & **Application Area** & **Packet Discarding Mechanism** & **Routing Decision** & **Addressing Scheme** & **Link Model** \\ \hline TARA & table-driven & _in vivo_ nodes & not available & per-hop basis & global-ID & bi-directional \\ LTR & table-driven & _in vivo_ nodes & available & per-hop basis & global-ID & bi-directional \\ ALTR & table-driven & _in vivo_ nodes & not available & per-hop basis & global-ID & bi-directional \\ LTRT & table-driven & _in vivo_ nodes & not available & end-to-end basis & global-ID & bi-directional \\ HPR & table-driven & _in vivo_ nodes & available & per-hop basis & global-ID & bi-directional \\ TSHR & table-driven & _in vivo_ nodes & not available & per-hop basis & global-ID & bi-directional \\ RAIN & table-driven & _in vivo_ nodes & available & per-hop basis & local-ID & bi-directional \\ \hline \end{tabular}
\end{table} 
Table 1: Comparisons of temperature-aware routing protocols. HPR, Hotspot Preventing Routing; TSHR, Thermal-Aware Shortest Hop Routing; RAIN, Routing Algorithm for Network of Homogeneous and ID-less Biomedical Sensor Nodes. threshold value. Nevertheless, as the trade-off, the withdrawal strategy introduces high delay, because the packet will be detoured arbitrarily using another route as long as there is an available next hop node. The second advantage of TARA is reducing the traffc congestion in the network by means of load balancing. TARA calculates the temperature of neighboring nodes by observing the nodes' activities, that is, how many packets are transmitted and received by the nodes. It means that the more the number of packets that are transmitted and received by a node, its temperature will rise. Therefore, while TARA tries to distribute the temperature in the network, at the same time, the load of the network is also distributed, thus resulting in less traffic congestion. The packet delivery delay plays an important role in routing protocols in WBSNs. The drawback of TARA, which introduces delay, due to the withdrawal strategy, is trying to be solved by the LTR protocol. Unlike TARA, if the next hop node is the destination node, the packet is forwarded immediately, with no buffering mechanism. There is also no withdrawal strategy in LTR, which means there is no packet sent back to the previous node, and a list of recently visited nodes is also maintained to prevent routing loops. Moreover, from the beginning, LTR tries to send the packet via the coolest neighbor. However, there is a packet discard mechanism to prevent a packet roaming around too long in the network. The ALTR protocol is an adaptive form of LTR. The adaptive form, here, means that the general mechanism is the same as LTR, but a different threshold, called MAX HOPS ADAPTIVE, is introduced.