## 4 Classification of Routing Protocols
### Temperature-Aware Routing Protocols
#### 4.2.9 Comparative Study of Temperature-Aware Routing Protocols  
All the aforementioned temperature-aware routing protocols ultimately intend to reduce the temperature rise of implanted bio-medical sensor nodes. A comparison in terms of temperature rise, packet discarding mechanism, addresses scheme, delay, packet delivery ratio and energy consumption with other state-of-the-art schemes is given in Table 2.  
The first routing protocol that addresses the overheating issue of implanted bio-medical sensor nodes in WBSNs is TARA [28]. It performs better in terms of reducing the temperature rise, and load balancing as compared to SHR, but its withdrawal strategy increases the number of transmissions, energy consumption, and end-to-end delay. The delay issue due to withdrawal strategy in TARA is addressed in LTR [55] and ALTR [55]. As compared to TARA, both LTR and ALTR exhibit robustness at different packets arrival rates, and perform well in terms of temperature rise, energy consumption, packet delivery ratio, and end-to-end delay. Compared to LTR, ALTR incurs low end-to-end delay and high packet delivery ratio. The disadvantages of LTR and ALRT are that it is not guaranteed that the packets are transmitted in the correct direction towards the destination, as they are only looking at temperature rise of bio-medical sensor nodes. Furthermore, in ALTR, if the hop count exceeds the threshold value, the packets might be transmitted through hotspots, resulting in temperature rises. To address these disadvantages of LTR and ALTR, LTRT [56] focuses on end-to-end route temperature and this result in less temperature rise and delay as compared to LTR and ALTR. To control the temperature rise, HPR [29] prevents the formation of hotspots by sending the data packets to the coolest nodes. Furthermore, the threshold temperature of any sensor node is computed based on temperature of its own along with its neighbors, which makes it different from other temperature aware routing protocols [15]. As compared to TARA [28], HPR performs better to reduce temperature rise, delay and packet loss ratio. The authors of TSHR have modified the HPR [29] by introducing a fixed threshold along with the dynamic threshold. TSHR [57] performs well in terms of node's temperature rise, the packet delivery delay (especially at high packet arrival rates) and packet loss ratio then TARA [28] and LTR [55]. While as compared to HPR, TSHR has lower packet drop ratio, higher packet delivery delay and almost same temperature rise. TSHR along with TARA and ALTR have no packet drop mechanism. \begin{table}
\begin{tabular}{l l l l l l l l} \hline \hline \multirow{2}{*}{**Protocol**} & \multicolumn{6}{c}{**Characteristics**} \\ \cline{2-9}  & **Goal** & **Temp. Rise** & **Discarding Mechanism** & **Address Scheme** & **Delay** & **PDR** & **Energy Cons.** \\ \hline TARA & To reduce the possibility of (2005) & Overheating & Verg & No & Global & Very & Very & Very \\ LTR & To reduce the temp rise and (2006) & High & Yes & Global & High & Low & High \\ ALTR & To reduce the temp rise, (2006) & energy consumption and & Low & No & Global & Medium & High & High \\  & end-to-end delay & & & & & & & \\ LTRT & To find routes with (2007) & Very & Yes & Global & Low & Very & Low \\ HPR & To prevent the formation of (2008) & hotspots and reduce & Verg & Global & Low & High & High \\  & end-to-end average delay & & & & & & \\ RAIN & To reduce average temp (2008) & Verg & Local & Low & High & Low \\ TSHR & To reduce temp rise, energy (2009) & Verg & No & Global & Medium & Very & Low \\ M-ATTEMPT & To reduce temp rise, energy (2013) & Low & Yes & Global & Low & High & Low \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Temperature-aware routing protocols. By comparing TARA [28], LTR [55], ALTR [55], LTRT [56], HPR [29] and TSHR [57] in terms energy consumption, packet delivery delay, packet delivery ratio and temperature rise, we conclude the following: LTRT [56] outperforms the others in terms of energy consumption while TARA [28] is the worst.