## 3 Comparison among the Routing Protocols and Open Issues
\begin{table}
\begin{tabular}{|c|c|c|c|c|c|c|c|c|c|c|} \hline  & **Classification** & **Message** & **Memory** & **Localization** & **Data** & **Clustering** & **Intra-Cluster** & **Cluster Head Election** & **Multi-path** & **routing** \\ \hline DECROP [24] & control overhead & \(O(n)^{2}\) & Low & NO & YES & proactive & multi-hop & node’s degree & NO \\  & reduction & & \(O(n)^{7}\) & & & & & & & \\ \hline ONCP [25, 26] & control overhead & \(O(n)\) & \(L_{\text{Low}}\) & NO & NO & reactive & single hop & residual energy & NO \\  & reduction & & \(O(n)^{2}\) & & & & & & & \\ \hline
2L-OFFIS [28] & control overhead & \(O(n)^{2}\) & Low & YES & YES & proactive & multi-hop & random & NO \\  & reduction & & \(O(n)^{10}\) & & & & & & & \\ \hline ARPES [30] & energy consumption & \(O(n)^{2}\) & Low & NO & YES & reactive & single hop & residual energy, & NO \\  & mitigation & & \(O(n)^{1}\) & & & & & & & information quantity \\ \hline DGMA [32] & energy consumption & \(O(n)\) & Low & YES & YES & reactive & multi-hop & event severity & NO \\  & mitigation & & \(O(n)^{1.5}\) & & & & & & & \\ \hline DMSTRP [34] & energy consumption & \(O(n_{\text{high}})^{3}\) & Low & YES & proactive & multi-hop & random & NO \\  & mitigation & & \(O(n)^{3}\) & & & & & & & \\ \hline JCOR [56] & energy consumption & \(O\left(\mathcal{H}\right)^{2}\) & Low & YES & NO & reactive & single hop & source-based & NO \\  & mitigation & & \(O(n)^{1}\) & & & & & & & \\ \hline HGMR [37] & energy consumption & \(O\left(\mathcal{H}\right)^{3}\) & Medium & YES & NO & proactive & multi-hop & encoding overhead & NO \\  & mitigation & & \(O(n\pm n)^{4}\) & & & & & & & \\ \hline GESC [40] & energy balance & \(O(n^{\pm}m)\) & Low & NO & YES & reactive & single hop & node importance, & NO \\  & & \(O(n)^{1}\) & & & & & & & residual energy & NO \\ \hline DCSSC [41] & energy balance & \(O(n)^{2}\) & Low & NO & YES & proactive & multi-hop & residual energy & NO \\  & & \(O(n)^{10}\) & & & & & & & & \\ \hline MELEACH-L [42] & energy balance & \(O(n_{\text{high}})^{3}\) & Low & YES & YES & proactive & single hop & residual energy & NO \\  & & \(O(n)^{1}\) & & & & & & & & \\ \hline ASN [44] & energy balance & \(O(n)^{2}\) & Low & NO & YES & proactive & single hop & the number of required & NO \\  & & \(O(n)^{3}\) & & & & & & & CH & \\ \hline MuMIR [45] & energy balance & \(O(n)^{2}\) & Low & NO & YES & proactive & multi-hop & random & YES \\ \hline \end{tabular}
\end{table} 
Table 1: Comparison of different routing protocols. According to the discussion of the routing protocols for large-scale WSNs in Section 2, it can be concluded that the flooding is usually used for route discovery, route maintenance and topology update in most of the routing protocols mentioned. In large-scale WSNs, this flooding causes such excessive message collisions that the network efficiency is reduced. However, the flooding has obvious advantages over the location-based unicast/multicast in complexity and economic cost without additional equipment such as GPS. Therefore, research on flooding technique is necessary. For instance, an efficient flooding scheme using 1-hop neighbor information in an _ad hoc_ network was proposed in [48]. In this scheme, one-hop neighbor information can be obtained by exchanging the HELLO messages in the MAC layer. By choosing the minimum forwarding nodes, redundant flooding messages are reduced. Additionally the connected dominating set (CDS) [49] technique can be also utilized for reducing the redundant flooding messages. Because blind flooding problem also exists in a large-scale wireless sensor network, these efficient flooding schemes are worthy of implementation. In a large-scale WSN, the deployment of the sensor nodes is dense, and the topology of the network communication is self-organizing and dynamical. Contrary to a wired network, a wireless sensor network does not have a practical backbone structure, and thus the overall nodes in the network must be responsible for routing processes and maintenance of the routing information.