## 4 Classification of Routing-Related Surveys
### Highly Focused Areas
#### 4.1.3 Energy-Efficiency Based Routing Surveys  
Energy-efficient routing surveys consider those routing protocols which try to conserve energy for enhancing network lifetime. On the basis of reviewed literature, energy-efficient based routing surveys may be classified into following four categories i.e.,  
1. In intelligence based categorization, energy-aware swarm intelligence based routing protocols are considered. 2. In structure based categorization, energy-efficient data-centric, flat, hierarchical and location-aware routing protocols are examined. 3. The operation based categorization discusses energy-efficient data-delivery-model-based, QoS-based and multipath-based protocols. 4. The partial energy-efficient categorization includes those surveys whose cornerstone is not routing, however they still discuss some aspects/protocols of energy-efficient routing in WSNs such as location/connection-driven based and data acquisition based routing and so on. As far as the surveys with complete emphasis on routing are concerned, the authors in [93] have discussed energy-aware swarm intelligence based approaches particularly Ant Colony Optimization (ACO) techniques and classified energy-sensitive routing techniques at different layers of communication protocol stack. In [39; 95; 105; 107], the authors have discussed both structural and operational based classification of energy-aware routing protocols in WSNs. While [98] discusses energy-efficient hierarchical structure based routing protocols in WSNs. The authors in [42; 50; 63; 67; 83; 132], and [38; 106] have mainly discussed energy efficiency in structural, and operational based routing protocols respectively. The authors in [38] have discussed design challenges of multimedia routing techniques and limitations of non-multimedia based routing protocols. Whereas in [106], the authors have discussed those green routing protocols that focus on achieving a balance of energy efficiency and QoS in WMSNs. In [105], energy-efficient homogeneous and heterogeneous routing protocols under the static and mobile categorization are discussed. Additionally, the application of each discussed routing protocol is highlighted that may provide guidance regarding designing a routing protocol for a particular scenario. The authors in [107] have discussed various energy harvesting methods in WSNs while in [83], the authors have put forth main causes of energy wastage in WSNs. The design issues and objectives are put forth in [39; 93; 98], and [67; 95] respectively. The authors in [38; 42; 83; 93; 105; 107] have outlined future research challenges. Moreover, Table 6 \begin{table}
\begin{tabular}{c l l} \hline \hline \multicolumn{1}{c}{**Survey Type**} & \multicolumn{2}{c}{**Categorization and Design Requirements Based Analysis of Classical Routing Surveys**} \\ \hline \multirow{3}{*}{**Structure based**} & _Hg_ & [35] \\ \cline{2-3}  & _Md_ & [36; 46; 47; 52; 53; 61; 66; 68] \\ \cline{2-3}  & _Lw_ & [51; 62; 79; 88; 104] \\ \hline \multirow{3}{*}{**Operation based**} & _Hg_ & [35] \\ \cline{2-3}  & _Md_ & [36; 46; 47; 52; 53; 61; 66] \\ \cline{2-3}  & _Lw_ & [62; 88] \\ \hline \multirow{3}{*}{**Partial based**} & _Hg_ & [126] \\ \cline{2-3}  & _Md_ & [115; 121; 124] \\ \hline \hline \multicolumn{3}{l}{Hg\({}^{*}\) = High Count, Md\({}^{*}\) = Medium Count and Lw\({}^{*}\) = Low Count. Note: Various categories of count} \\ \cline{1-1} \cline{2-3}  & correspond to Count Number Index (CNI) in Tables 2 and 3. \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Design analysis of classical routing surveys in WSNs. shows that this category includes only 2 high count surveys, 8 medium count surveys and 4 low count surveys. The surveys having partial emphasis on routing include [128; 133; 136; 139; 135; 136]. Among them, ref. [128; 135] discuss location-driven and connectivity-driven routing protocols in more or less the similar manner. While both [136; 139] discuss some energy-efficient routing protocols in WSNs.