## 2 Previous Work
### Trust and Security in WSNs
Then, they divide the general security issues into seven categories, namely: cryptography, key management, attack detections and preventions, secure routing, secure location security, secure data fusion, and others. In this division, they summarize the different techniques used and point out their pros and cons. The authors in [13] assure that trust is an important factor for WSNs security schemes and provide a detailed study on trust mechanisms and attacks and countermeasures. They provide a categorization of all trust-related attacks in WSNs. They also analyze the different trust schemes and illustrate the differences and challenges of each. In their work, they provide an extensive literature survey of trust mechanisms used in WSNs and they present open research directions. In [14] the authors focus on the energy limitations of the sensor nodes and propose a centralized energy-efficient routing protocol for WSNs to reduce energy consumption and thus, increase network lifetime. Their protocol, called Base-Station Controlled Dynamic Clustering Protocol (BCDCP), increases network lifetime and average energy savings by evenly spreading the energy dissipation among all nodes. Although this work does not target trust or security, it is of interest since they highlight the energy saving benefits of a centralized routing scheme, on which we focus our work. All of the surveyed previous work focuses on distributed methods to calculate trust and reputation and to provide security for the wireless sensor network. However, it is more logical to utilize the centralized approach in a WSN and make use of the more powerful BS to perform these calculations and lessen the burden of the power-consuming reputation inquiries and computations on the sensor nodes. Several surveys discussed comprehensively the different routing techniques in WSNs [2; 7; 15; 16; 17; 18; 19; 20; 21] presented the most popular research in this field. Table 1 presents a comparison between the most popular routing techniques in WSNs based on common attributes. One thing that can be directly noticed is that out of the whole list, only CENTERA utilizes the centralized approach, which is in harmony with the recommendation of the Open Networking Foundation (ONF), Software Defined Networking (SDN) to use centralization of network intelligence as one of the new norms for networks [22]. To the best of our knowledge and till the time of this writing, CENTERA is the first centralized routing protocol in WSN that incorporates security and trust criteria in the core of the routing decision engine. The different parameters included in Table 1 are explained as follows:  
* Classification: Routing protocols are classified based on the network structure into flat-based routing containing nodes with equal roles; hierarchical-based routing containing nodes with different functions; and location-based routing where routes depends on nodes' positions. * Mobility: Sensor nodes may be considered as stationary or mobile. * Position Awareness: A sensor node is aware of its spatial position. * Power Usage: Indicates the power consumption of the whole protocol. * Negotiation-based: Such protocols initiates negotiation messages prior to data messages in an attempt to eliminate the transmission of duplicate or redundant information. * Data Aggregation: The option to combine data from different nodes based on some function. * Localization: The ability to estimate the location of each node. * Complexity: Given the limited-nature of sensor nodes, applied protocols must be simple. * Scalability: The protocol must cope with the huge number of typically available sensor nodes. * Multipath: In an attempt to improve network reliability, some routing protocols utilize multiple paths to route packets at the expense of additional overhead. * Query-based: In such routing protocols, the destination node queries the network for its required data, and the node with such data replies.