## 8 Challenges and Future Work  
In WSN virtualization there are many open problems, open issues and questions still to be answered. Middleware design for the virtual sensor network has to deal with the challenges like: guaranteeing connectivity, choosing the optimal frequency, localization, data aggregation, security, load balancing, radio selection, clustering and inter cluster coordination. To achieve all the mentioned properties we need a comprehensive middleware design for WSN virtualization. To achieve this goal middleware architecture has to face several non-trivial algorithmic challenges. In WSN virtualization, multiple applications need to run; each application has its own requirements as reliability, robustness, fault tolerance, high throughput and QoS level. The applications like elderly care, military, smart home, smart city and traffic controlling are the examples of the applications that need more reliable, stable and robust networks. Due to the limited resources of the sensor nodes and the networks, it is highly probable that the performance of multiple running applications cannot be satisfied simultaneously. For network virtualization, middleware should provide a general runtime environment to the application developer. Furthermore, it should also guarantee the correct functionality of concurrent applications running on the multiple networks consisting of resource constrained devices. Therefore, the middleware should manage multithreads efficiently to reduce the probability of error and failure. \begin{table}
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Middleware is required to efficiently tradeoff among the several QoS dimensions of a particular application as well as multiple applications. Therefore, a mechanism is to be derived such that it adopts the policy of tradeoffs between different QoS parameters and available resources. Software design for network virtualization should collectively fulfill the application needs. One of the key challenges of middleware design is to make energy-aware protocols and policies that should gather the power information from the multiple physical networks and make virtual networks according to the power parameters. However, contrary to the power awareness QoS need not to be compromised. Therefore, an optimized metric for QoS parameters, applications running and available resources must be part of the middleware design. Furthermore, mobility of sensing devices imposes extra challenges to the network virtualization. It poses a number of challenges like QoS provision, clustering and localization. There must be a standard to be followed while dealing with service and resource discovery module. The service provider should get the parametric values of the resources and services from distinct heterogeneous networks in a standard format. This would lead to efficient resource and service allocation. Moreover, the role of multi radios is quite significant in the future network designs. The research work carried out on using multiple radios is very limited, and this area is still open for the researchers and need significant improvement. Another important issue in the middleware design is data aggregation; middleware for network virtualization should provide mechanisms to merge and synthesize the data. Network virtualization creates virtual links between nodes. The speed of the virtual link should not be too slow as compared to that of a native link. The programming model for virtualization middleware should be based on multi-threading techniques so as to speed up the process. In order to meet the demands of each application the middleware should be able to fulfill application requirements and provide services in a seamless manner. On the other hand the complexities of networks, devices and platforms are also to be managed in such a way that the network longevity is increased and the resources do not exhaust early.