## 4 Classification of Routing-Related Surveys
### Highly Focused Areas
#### 4.1.2 Classical Routing Surveys  
The classical routing surveys consider those routing protocols which are general in nature and do not concentrate on any specific routing domain of WSNs. Such surveys generally perform three types of routing categorizations in WSNs as described below:  
* In network structure based categorization, the classification metric is data centric, flat, hierarchical or location-aware.
* In network operation based categorization, the classification metric is network-flow-based, communication-model-based, QoS-aware and multipath-based.
* In partial based categorization, such classical surveys are included which although are not entirely focused on routing, however they discuss routing in a limited extent e.g., they may discuss WSNs protocols at each layer of communication protocol stack including network layer.  
As far as the surveys with complete emphasis on routing are concerned, the authors in [35; 36; 46; 47; 52; 53; 61; 62; 66; 88] have mainly classified routing protocols into both structural and operational based protocols while [51; 54; 68; 79] have classified them mainly using structural based categorization. The authors in [46; 52; 53; 54; 61; 62; 68; 79; 88] have outlined design issues that should be considered while devising new protocols for WSNs. In [46; 52; 54], the authors have discussed various application areas of WSNs. Performance trade-offs and security challenges of data-aggregation routing protocols are outlined in [47]. The design challenges and future research directions are discussed in [35; 36; 46; 47; 51; 62; 66; 68]. Additionally, it is evident from the Table 5 that only \(1\) routing survey is of high count, \(9\) routing surveys are of medium count and remaining \(5\) surveys are of low count.  
The surveys having partial emphasis on routing include [115; 126], and [121; 124] which discuss WSNs, and WMSNs related protocols respectively. The surveys in [115; 121; 124] discuss various techniques at each layer of communication protocol stack including the network layer whereas [126] elaborates network and cross layer routing protocols in WSNs. The authors in [115; 126], and [121; 124] have outlined design issues that should be considered while devising new protocols for WSNs, and WMSNs respectively. In [115; 121; 124; 126], the authors have discussed various application areas of WSNs, and WMSNs respectively. The design challenges and future research directions are discussed in [115; 121; 124; 126].  
\begin{table}
\begin{tabular}{c l c} \hline \hline \multicolumn{1}{c}{**Survey Type**} & \multicolumn{2}{c}{**Categorization and Design Requirements Based Analysis of Hierarchical Routing Surveys**} \\ \hline \multirow{3}{*}{**Typical based**} & _Hg_ & [74; 86] \\ \cline{2-3}  & _Md_ & [56; 60; 73; 75] \\ \cline{2-3}  & _Lw_ & [69; 70; 85] \\ \hline \multirow{3}{*}{**Atypical based**} & _Hg_ & [99] \\ \cline{2-3}  & _Md_ & [87; 97] \\ \hline \multirow{3}{*}{**Security based**} & _Md_ & [72; 78] \\ \cline{2-3}  & _Md_ & [98] \\ \cline{2-3}  & _Lw_ & [42; 50] \\ \hline \multicolumn{1}{c}{**Mobile based**} & _Md_ & [109] \\ \hline \hline \multicolumn{1}{c}{Hg* = High Count, Md* = Medium Count and Lw* = Low Count. Note: Various categories of count} \\ \cline{1-1} \multicolumn{1}{c}{correspond to Count Number Index (CNI) in Table 2.} \\ \hline \hline \end{tabular}
\end{table}
Table 4: Design Analysis of Hierarchical Routing Surveys in WSNs.