## 7 Summary and Conclusions
\begin{table}
\begin{tabular}{|c|c|c|c|c|c|c|c|c|c|c|} \hline
**Clustering Routing Protocols** & **ACE** & **BCDCP** & **PEGASIS** & **TEEN** & **APTEEN** & **TTDD** & **CCS** & **HGMR** \\ \hline \multirow{6}{*}{Cluster characteristics} & Variability of cluster count & variable & variable & variable & fixed & variable & variable & variable & variable \\ \cline{2-10}  & Uniformity of cluster sizes & even & even & even & even & even & even & uneven & even \\ \cline{2-10}  & Intra-cluster routing & single-hop & single-hop & multiple-hop & simple-hop & single-hop & single-hop & multiple-hop & single-hop \\ \cline{2-10}  & Inter-cluster routing & single-hop & single-hop & multiple-hop & single-hop & multiple-hop & multiple-hop & multiple-hop & multiple-hop \\ \hline \multirow{6}{*}{Cluster characteristics} & Existence & cluster-head based & cluster-head based & no-cluster-head based & cluster-head based & cluster-head based & cluster-head based & cluster-head based & cluster-head based \\ \cline{2-10}  & Difference of capabilities & homogeneous & homogeneous & N/A & homogeneous & homogeneous & heterogeneous & homogeneous & homogeneous \\ \cline{2-10}  & Mobility & stationary & stationary & N/A & stationary & stationary & stationary & stationary & stationary \\ \cline{2-10}  & Role & relay aggregation & relay aggregation & N/A & relay aggregation & relay aggregation & relay aggregation & relay aggregation \\ \hline \multirow{6}{*}{Clustering process} & Control manners & distributed & centralized & distributed & distributed & distributed & distributed & distributed & distributed \\ \cline{2-10}  & Execution nature & iterative & iterative & probabilistic & probabilistic & probabilistic & probabilistic & probabilistic & probabilistic & probabilistic \\ \cline{2-10}  & Convergence time & constant & constant & constant & constant & constant & constant & constant & constant \\ \cline{2-10}  & Parameters for CH election & adaptive & adaptive & adaptive & adaptive & adaptive & adaptive & adaptive & adaptive \\ \cline{2-10}  & Proactivity & proactive & proactive & proactive & reactive & proactive & proactive & proactive & proactive \\ \cline{2-10}  & Objectives & load balancing & load balancing & load balancing & reactive scenes & proactive scenes & scenes of multiple & lifetime extension & lifetime extension \\ \cline{2-10}  & \multirow{2}{*}{Algorithm stages} & \multirow{2}{*}{Algorithm stages} & \multirow{2}{*}{cluster construction} & \multirow{2}{*}{data transmission} & \multirow{2}{*}{data transmission} & \multirow{2}{*}{data transmission} & \multirow{2}{*}{data transmission} & \multirow{2}{*}{data transmission} & \multirow{2}{*}{data transmission} & \multirow{2}{*}{data transmission} \\ \cline{2-10}  & & & & & & & & & & \\ \hline \end{tabular}
\end{table} 
Table 2: _Cont._To conclude, we want to sketch some future directions for the field. Firstly, further research would be needed to address QoS problem of clustering routing, which mainly exists in real-time applications, such as battle-target tracking, emergent-event monitoring, and _etc_. Recently there is very little research focuses on handling QoS requirements in the resource-constrained WSN environment. Moreover, further studies are necessary to settle the problem of node mobility, including the sink and sensor nodes. There are a lot of node-mobility applications, such as battle scenarios, thus how to handle the overhead of node mobility and topology changes must be further taken into account. Finally, with the increase of the network scale in WSNs, more redundant information is created and a certain degree of redundancy may be desirable for increasing reliability of the network. Thus, a trade-off between redundancy reduction and redundancy utilization is still an open question. We hope that this will encourage protocol researchers and designers to take into account the various characteristics of the clustering routing methods when designing an effective clustering routing protocol, and that more work would be done in the future directions for the field.