# Data Access Based on a Guide Map of the Underwater Wireless Sensor Network
## 3 System Model and Problem Description
### Problem Description  
An important purpose of UWSN is to make the user (such as the observation node) obtain the required data as fast as possible, and to prolong the lifetime of the network. In DAGM, there are three stages from the sensor nodes generating data to the user getting data, that are: generation and storage of data, diffusion and synchronization of metadata, and query and receipt of data. In DAGM, to make the user query and receive data of different granularity and quality as fast as possible, at first, when sensor nodes generate data, the UWSN will not only store data, but it will also generate metadata and diffuse it to the center ring. Secondly, in the user query data, the UWSN not only can send the query sentence to the nearest center ring node, but it can also transmit the data with a data transmission routing strategy. This can not only reduce the delay in data access, but also save network energy and prolong the network lifetime. When it comes to the time performance of network, it contains two parts. One part is the time of data storage and the metadata diffusion, and the other is the interval from user sent out the data query sentences to who receives the specific application data. Similarly, the network energy consumption also includes two parts. One is the energy consumption of data storage and metadata diffusion, and the other is energy consumption of the data query and specific application data transmission. The data is generated after the sensor nodes monitor the environment or detect the target, and then it is transmitted to the storage node and the metadata generated for it. Then, metadata is sent to center ring node that is the nearest to the storage node and is diffused and synchronized at the other nodes within the center ring, so the data map be formed. It is only in this period where the user cannot find the data in the network when it sends out the query sentence to the network. However, the probability is very low. Not considering this case, one important problem of UWSN is ensuring a minimum  
Figure 1: Data access based on a guide map (DAGM) mechanism. time interval from the user sending a data query sentence to it receiving specific application data. Another important problem is ensuring the network energy consumption is as little as possible. Due to the communication energy consumption for data transmission being much higher than the energy consumption for data processing into the node, the time of data processing in node is much less than the time of data transmission. Hence, the delay in data query mainly includes the delay in the query sentence and specific application data transmission. At the same time, the total energy consumption of the network mainly includes energy consumption on transmission of data storage, diffusion and synchronization of metadata, as well as query sentence and specific application data. Based on the above analysis, the core problem of the DAGM is to determine the location of the center ring nodes based on the minimum delay from the sent query sentence to received specific application data by the user, and minimizing the energy consumption of the network. The data transmission in the network is completed through multiple hops. Assuming the number of hops on sending query sentence and on querying results returning is same in each node, \(k\) denotes the center ring node; \(i\) presents other nodes exception the center ring node; and \(D(k,i)\) shows the number of hops from node \(i\) to the center ring node \(k\) that is the nearest from \(i\). Thus, the distance sum (the hops sum, that is, the path length) from all nodes to their nearest center ring nodes is, \(D(k)=\sum_{i=1}^{n\times n-m}D(k,i)\), where \(m\) represents the number of center ring nodes.