## 1 Introduction  
For the future Internet of Things (IoT) environment, the effective integration of various types of networks will be an important consideration. In addition to the co-existence of wired and wireless links, domains may involve sensor nodes and actuator nodes from multiple vendors. Moreover, applications running over such mixed networks will make different demands of the network, such as locational information or other node-specific information. To realize the IoT environment, wireless sensor networks (WSNs) will be a crucial technology to allow collecting and acting on environmental information. WSNs will be integrated into future communication infrastructure [1]. Here, however, networks are considered to be constructed independently to provide service within a local area. For such heterogeneous WSNs to be consolidated into infrastructure and to share physical sensor substrates across multiple IoT applications, virtualization of WSNs is one solution. In virtualization methods, the functionality of a WSN is split into two parts: the physical infrastructure of sensors and the applications, which rely on the aggregated resources. The main advantage of virtualizingWSNs is that this provides the ability to realize shared infrastructure that can satisfy various service demands [2; 3]. For example, sensor substrates deployed for fire-detection or fire-tracking applications can be shared between homeowners and city administration [4]. In extant task-oriented WSNs, redundant sensor nodes are deployed in the shared domain, and the duplicated nodes are chosen according to application. This is done because the required granularity or service demands of the distinct users are different, but such redundant deployment is inefficient. Virtualization can eliminate tight coupling between applications and resources, which enables multi-application use of existing sensor substrates. Moreover, virtualization of WSNs provides a new business model, sensor-as-a-service, for both infrastructure owners and service providers [5]. Virtualization of WSNs has been studied by many researchers. The Federated Secure Sensor Network Laboratory (FRESnel) [6] and Virtualized dlstrohts of smart Objects (VITRO) [7] projects, in particular, are focused on the virtualization of WSNs. FRESnel aims to federate large-scale WSNs and enable the simultaneous running of multiple applications. VITRO aims to provide dynamic cooperation among sensor nodes by dividing WSNs into physical sensor substrates and applications. In previous work on the virtualization of WSNs, virtualization at two levels has been studied: node and network [4]. Node-level virtualization methods enable a single node to concurrently process multiple applications. Approaches to node-level virtualization can be divided into three classes according to the element that provides concurrency: a virtual machine [8], the operating system [9] or middleware [10]. Under network-level virtualization, the virtual network for running a single application consists of a subset of sensor nodes. Network-level virtualization results in efficient use of resources because the nodes not used by one application can be used by applications in other virtual networks. Approaches to network-level virtualization can be divided into two classes: overlay-based solutions [11] and cluster-based solutions [12]. Within this system of classification, we aim for overlay-based network-level virtualization. Many other researchers have proposed overlay-based approaches to network-level virtualization [3; 11]. Those approaches focus mainly on providing a framework that allows the sharing of physical sensor resources. Although improved manageability has been mentioned as a reason for virtualizing networks, the method of constructing a virtual wireless sensor network (VWSN) topology for each application in an overlay layer has not been discussed enough. Moreover, within the IoT environment, changes to traffic patterns, variation in traffic demand and the addition or removal of virtual nodes can all occur.