## 3 Middleware for WSN Virtualization
### Middleware for Multi Radio Support  
**MiLAN** presents well defined APIs through which the application tells its desire to use high or low data rate protocol. It also has an abstraction layer that decouples the application layer from the network that issues orders to discover the available components and configure the network. The network specific interfaces convert Milan commands to specific network protocols. MiLAN supports Bluetooth and WiFi. **VITRO** provides a framework that integrates multiple sensor networks of any type, using heterogeneous radios. Virtualization techniques in the middleware are used to virtualize the whole network such that it hides the low level details and different networks look like a single network. **SensEye** is a multi-tier sensor network of cameras using heterogeneous camera devices in each layer. All tier 2 nodes are equipped with multi radios, _i.e._, 802.11 and 900 MHz to communicate with other tier devices. The project concentrates more on bringing efficiency using heterogeneous sensors rather than the utilization of radios. **I-living** provides a middleware framework on a cloud based concept of utilizing all the radios used in the home environment. The small devices using Bluetooth and Zigbee, and smart devices connected through the 802.11. Middleware API provides standard application services. An assisted living hub is a specialized device capable of connecting to 802.11, 802.15.4, Bluetooth, UWB, infrared independent devices. Supports TCP/IP stack at the hub. **GSN**[28, 70] also deals with the heterogeneity of the nodes and provides virtual sensor that hides the differences and details of the nodes by the use of a wrapper developed in TinyOS on every sensor node. It could hide any kind of data generator including 802.11b. **MEMOSEN**[90] architecture takes into account the heterogeneous and mobile sensor and sinks into account. Furthermore, the architecture is composed of three layers that combine the cellular network terminals with wireless sensor networks, thus modeling the sensors and mobility. It is a hybrid model with mobile terminals using dual radios. The framework of the middleware supporting multiple radios in the Machine to Machine (M2M) communication and managing multiple applications is presented in [81,84,91]. M2M framework in [81] works by creating an overlay network on top of the physical network to achieve high network performance. The overlay network is managed by the home gateway that collects the data and transmits it to the base station. Furthermore, Middleware based on virtual machines for M2M is developed using Linux platform in [92] to provide ease of programming to the developer in energy efficient way; the radio used is 802.15.4. The system architecture proposed by Starsinic in [91] is a home gateway based architecture supporting 802.15.4, 802.11, Bluetooth and Ethernet managing all the home devices on a single platform. Table 1 provides the summary of the software architecture designs used in the middleware for WSN virtualization. Furthermore, it shows whether the middleware design supports the main components of WSN virtualization, _i.e._, sensor virtualization, network virtualization, multi radio support and heterogeneity of sensor nodes.