## 4 Conclusion and Open Issues  
WSN has many unique characteristics, different from traditional networks. It also introduced much challenges to design suitable routing algorithms, which has attracted a lot of attention in the recent years. Different from the survey on routing algorithms, we have summarized recent research results on position-based routing in WSN. It is classified into four main categories: flooding-based, curve-based, grid-based, and ant-based intelligent. Their performance is also compared based on some popular metrics. Though all these algorithms are based on the position information of sensors, how to use the position information is different. In flooding-based routing, the position information is used to limit the flooding area by constructing different flooding area. Compared with other algorithms, flooding-based routing algorithms are very easy to realize. Curve-based routing algorithm is one kind of novel routing strategy. Position information is used to construct the routing curve and forward the data. Curve-based routing combine source based routing with Cartesian forwarding. The source initiates the algorithms, selects the forwarding curve and encodes the parameter, but without specifying all the intermediate nodes. Each sensor takes greedy action to forward the packet based on local information. \begin{table}
\begin{tabular}{|p{56.9 pt}|p{56.9 pt}|p{56.9 pt}|p{56.9 pt}|p{56.9 pt}|} \hline  & Flood-based & Curve-based & Grid-based & Ant algorithm-based \\ \hline Energy consumption & High & Low & Low & Middle \\ \hline Negotiation-based & No & No & Yes & Yes \\ \hline Complexity & Low & Low & Middle & High \\ \hline Reliability & High & Middle & Middle & Middle \\ \hline Scalability & Low & Good & Good & Middle \\ \hline Multipath & Yes & Yes & No & Possible \\ \hline Data aggregation & No & No & Possible & No \\ \hline Query-based & No & No & No & Yes \\ \hline \end{tabular}
\end{table} 
Table 3: Comparison among four kinds of position-based routing algorithms. Gird-based routing algorithms are based on the grid structure. The key problem is how to use the position information to construct and maintain the grid. In different algorithms, the grid is constructed by different sensors (for example, source, sink). Once the grid is constructed, data forwarding can use suitable methods, such as geographical greedy forwarding, curve-based forwarding. Ant-based intelligent routing algorithm is originated from the phenomena that ants always find the optimal path when finding food. The algorithms are similar to the ant's colony intelligent behavior via cooperation. Through several ants to the sink, the algorithms can automatically form the optimal path based on the position information. Although extensive efforts have been exerted so far on the position-based routing problem in WSN, there are still many challenges because of the unique characteristics of WSN. Future trends in routing focus on different directions and all share the common objective of prolonging network lifetime, handling the overhead of mobility and topology changes in an energy-constrained environment. In the last section, some of these directions related with position information are summarized as follows:  
1. The redundancy detection mechanism. In WSN, node is typically dense deployment. It is unnecessary that all these sensors simultaneously work. How to control the number of working sensors is valuable. Especially, the strategy combining the coverage algorithms with routing technology is hopeful. Another interesting idea is exploiting effective sensor-density control algorithms. 2. The secure routing problem. Security is very difficult to assure because the unsymmetrical encryption strategy is unsuitable for the energy requirement. Only the symmetrical encryption strategy can be used, which also make key assignment and time synchronization difficult. Most of current routing algorithms aim to optimize the energy consumption.