# Data Access Based on a Guide Map of the Underwater Wireless Sensor Network
## 3 System Model and Problem Description  
### System Model  
We assume \(n\)\(\times\)\(n\) sensor nodes are uniform or randomly distributed in a square area where the length of a side is \(L\), as shown in Figure 1. The observation node is the data query user, randomly located within the network, and it can query different granularity and quality data in accordance with certain query conditions. In this paper, the square area of the UWSN does not limit the model's generalization, because when a network adopts a circle shape region, the circumscribed square of this circle can be considered as the network region, and when the sensor nodes are randomly deployed, according our previous work [42], every sensor node can be can be considered to deploying in regular square lattice that the network region is divided regularly. Therefore, the model and method of this paper can be applied to other network models, and the specific assumptions are as follows:  
1. The nodes are divided into three types: sensor nodes, storage nodes and center ring nodes. Each node can obtain its own location information (through the global positioning system (GPS) or locating protocol). The network is connected, and any two nodes can communicate through the multiple hops, assuming that the node has a small mobile range but it does not affect the communication and undermine the connectivity of network. 2. The network works in a two-dimensional plane. The nodes are deployed according to square lattice, with the network scope divided into regular square lattice. Each lattice has an ID number,and in terms of the nodes located in it, when each lattice consists of multiple sensor nodes, only one node is working, while the others are sleeping. Each node knows the situation of the deployment, such as the scope of network, neighbor nodes, and so on. 3. Each node can be considered as a data query user. They can send a data query sentence to the nearest center ring node at any time, and will not leave the location out of the communication range before the end of data query. Based on the above assumptions, the basic ideas of DAGM are:  
1. To establish the center ring structure. In the process of deployment, the scope of network is well divided into \(n\times n\) square lattices, and sensor nodes are deployed at the inside of the square lattice. Then, the center ring structure is established as composed by nodes according to the shortest average user query path in the network for storing the metadata. 2. To store data in the network. When the sensor nodes generate data by monitoring the environment and detecting the target, the lattice ID number and location of the storage node is determined by the geographic harsh table (GHT). At the same time, the data will be transmitted to the storage node by multiple hops. Then, after the data stored, the storage node generates metadata that include the content abstract and storage location abstract for it. 3. To establish a data map for the network. When there is metadata generated in any of the storage nodes, it will be sent to a center ring node which is the nearest from it. This center ring node will store the metadata and become a metadata set. At the same time, metadata will be diffused and synchronized to other center ring nodes in a double direction, so that the data map of the network will be formed because every center ring node has stored the metadata of the network. 4. To query data from center ring node that is the nearest from it. When each node queries data, it will send the query sentence to the nearest center ring node from it. After the center ring node receives the query sentence, it will try to find whether data is exiting the network from the data content abstract that is stored in map data. If the data does not satisfy the query sentence, \"_query failed_\" is informed to the user. If there is data that the user requires, data transmission routing will be generated according to the storage location abstract described by the metadata.