## 4 Classification of Routing-Related Surveys
### Highly Focused Areas
#### 4.1.1 Hierarchical Routing Surveys  
The hierarchical routing surveys focus on cluster based routing techniques which may exhibit high efficiency for large sized networks and are scalable in nature. They are considered energy efficient and may accommodate heavy traffic load, cover large sensing area and have the ability to perform data fusion/aggregation. These surveys may be mainly classified into succeeding four categories i.e.,  
* In typical based categorization, the hierarchical protocols are classified on the basis of clustering procedure (such as Cluster Head (CH) election techniques, even-distribution, convergence time, clustering methodology and objectives), clustering properties (such as cluster count, size, balance, hop-count, stability, re-clustering support and inter/intra cluster topology and routing) and CH capabilities (such as type, role and mobility support).
* In atypical based categorization, the hierarchical routing protocols belonging to block, grid, chain and tree based topologies may be discussed.
* In security based categorization, security aspects of cluster-based protocols are evaluated on the basis of requirement of security goals, selection of security mechanism and prevention of security threats.
* In energy-efficient based categorization, those routing surveys are discussed which focus on energy efficiency in cluster based routing protocols.
* In mobile hierarchical based categorization, those hierarchical routing protocols are discussed where either sensor or sink node or both are mobile. Such routing protocols may exhibit single-sink or multi-sink approach.  
The authors in [56; 60; 69; 70; 73; 74; 75; 76; 77; 78; 86] have mainly classified hierarchical routing protocols into typical based categorization. In [87; 97; 99], the authors have classified hierarchical routing protocols using atypical based categorization. The authors in [72; 78], and [42; 50; 98] have mainly classified hierarchical routing protocols that put forth security, and energy-efficiency based categorization respectively. The mobile hierarchical oriented categorization is exhibited by [109]. In [75], the authors have outlined a more detailed classification in terms of CH selection, cluster formation, data aggregation and communication. Additionally, the issues relevant to various phases of taxonomy are also figured out. In [99], the authors have discussed and evaluated various atypical hierarchical routing approaches in WSNs. Additionally, the application scenario of discussed protocol are outlined too. The authors in [78] devise a metric that may determine fitness of different secure hierarchical protocols to suitable applications. In [74], the authors have discussed objectives of clustering while [73] delineates routing issues and discusses both clustered and non-clustered homogeneous/heterogeneous routing protocols. In [109], the authors have discussed and compared the characteristics of various mobility-oriented hierarchical routing protocols in WSNs. The review also highlights the applications of discussed protocols that may provide guidelines for protocol development in the corresponding fields in future. The authors in [69; 98] discuss the design challenges of hierarchical-based and general-based routing protocols. In [85], the efficiency of various hierarchical routing protocols is compared on the basis of numerous performance oriented parameters using Network Simulator 2 (NS-2) [145]. Cluster-oriented issue are delineated in [60] whereas general, and security-based issues are outlined in [97], and [72; 78] respectively. Future research directions are discussed in [42; 56; 60; 69; 70; 74; 75; 86; 87; 97; 99; 109]. Besides that Table 4 clearly exhibits that this taxonomy comprises of only 3 high count, 10 medium count and 5 low count surveys. Additionally, all the surveys under consideration in this category have complete emphasis on routing.