## 1 Introduction  
Wireless Sensor Networks (WSNs) have focused the interest of both the industry and academia in the last decade. A wireless sensor is a cheap node with wireless communication capabilities that is able to measure some features of its environment, but has limited storage and battery capacity. When many of these nodes collaborate together to sense some physical phenomenon of interest across a region, transmit it by relay nodes and process it in order to get a common goal, a Wireless Sensor Network is obtained. The main characteristic of WSNs is that their nodes are battery-powered, thus being limited in energy as well as in processing and storage (e.g., memory limited). These limitations have differentiated WSN as an entire research area inside wireless ad hoc networks. Therefore, new protocols and algorithms have been proposed for WSN at all levels of the network stack from physical to application layer [1]. In particular, in [2] we find the main MAC protocols proposals for WSN, whereas the most important routing proposals for WSN are presented in [3, 4]. WSN-specific transport protocols are summarized in [5]. Finally, some examples of WSNs applications are described in [6]. Furthermore, the interest of the industry on WSN has pushed the standardization of light-weight and low power communications technologies that are suitable for sensor nodes. For instance, IEEE specified the 802.15.4 ([[http://www.ieee802.org/15/pub/TG4.html](http://www.ieee802.org/15/pub/TG4.html)]([http://www.ieee802.org/15/pub/TG4.html](http://www.ieee802.org/15/pub/TG4.html))) standard that covers the physical and MAC layers. On top of 802.15.4, the Zigbee Alliance ([[http://www.zigbee.org/](http://www.zigbee.org/)]([http://www.zigbee.org/](http://www.zigbee.org/))) has defined a protocol stack covering routing and application layers for WSNs. One of the most interesting WSN's research lines is the so called Data-Centric paradigm. The main idea underlying Data-Centric proposals is that for many WSN applications what is important is not who generates the data, but the nature of the data the WSN gathers (also where and when it was obtained could be important). For instance, a node in a sensornet could be interested in establish communication paths only with nodes that measure a given data type (e.g., temperature). Under this definition some works like Direct Diffusion [7, 8] defined a Data-Centric routing, where routing tables are created in function of nodes interest in particular data types. Therefore, a node forwards a particular message to a neighbour if it is interested in the data contained in the message or it is the next hop towards an interested node. Besides routing, storage is another widely studied topic in WSN research because the obtained data need to be stored in some way before processing it. There are two straightforward approaches: (i) Local storage, that is, the node measuring the physical phenomenon stores the data. Then, some protocol should be defined in order to allow potential consumers of those data to find and access the nodes where they are stored. (ii) External storage, which means that standard WSN has a central node (sink, base station, etc) that manages the whole network and acts as an access point for external users. Therefore, when a sensor measures a physical event it could push it to the sink that then stores it. However, a third approach could be used if storage is mixed together with the Data-Centric paradigm. In this approach, a rendezvous node within the sensornet is defined in order to store the information. This alternative storage mechanism is considered a Data-Centric approach when the rendezvous node is selected in function of the data (or event) type instead of a predefined one as in the sink case. With Data Centric Storage (DCS), when a sensor measures a physical event for a particular data type, it pushes the information towards the rendezvous node in charge of that data type.