## 2. TYPES of Virtualization
### Multi Radio Support  
The communication part is the one consuming maximum energy in a sensor network [34, 35]. In order to achieve IoE low power transceiver and microprocessor system designs are critical [36]. Wireless radio choice is one of the most critical choices in the design parameters of WSN. The wireless radio is responsible for communicating among the nodes, and between the nodes, and the gateway. Traffic load in WSNs is usually low and therefore the radio does not need to be active all the time.  
Most of the time the sensor's radio is in sleep mode. The radio includes idle listening mode and the actual data transmission/reception mode [35]. Radios are switched on only for a particular time when data transmission or reception is required otherwise they are kept off in order to save the energy.  
In network virtualization the choice of selecting the radio is dictated by the application types and needs. The data signal transmitted from one place can face many hazards like reflection, attenuation, path loss and fading that can cause errors in the sensor's actual readings. Network virtualization allows multiple radios with different channels, bandwidth and properties to provide a wide range of options [37].  
The most commonly used frequency band in WSN is the industrial, scientific and medical [38, 39] radio bands. In ISM band there are a number of channels and radio choices e.g., 802.15.4 and 802.11. Furthermore, 802.15.4 is a low power radio that can offer adequate data rate up to 250 Kbps. However, on the other hand 802.11b consumes more power, but provides high data rate up to 11 Mbps [40].  
There are a number of bottlenecks and tradeoffs in using more radios in a network. The difference between the energy consumption of two radios is that the energy per bit of 802.15.4 radio is 979 nJ/bit, and for 802.11b it is 112 nJ/bit, that is approximately 9 times higher than 802.11b, but when packets are formed and sent the energy consumption 802.15.4 radio is always less than the 802.11 radio [41, 42]. Moreover, 802.11b have a long startup time as compared to 802.15.4 that further consumes more energy. It is an energy efficient approach to transmit a large number of bytes in a single packet so that the overhead of constructing the packet and then sending are minimized. The startup time for 802.11 is to be reduced, to take full advantages. One hop of 802.11b equals 2-3 hope range of 802.15.4.  
Radios supporting high bandwidth are energy efficient only if large numbers of bytes are to be transmitted. Processors of the sensors have to play their role in using multiple radios. WiFi nodes can operate for a longer period of time with a strict energy budgeting as in as 802.11g that can operate in a power saving mode, keeping the wireless node in sleep for the most of the operating time [43]. The low power processor can't support the high bandwidth as the processor is slow. However, on the other hand, if more powerful processor is used, it consumes more energy even for the low power radio that can be operated on lower power processors [38, 41]. Combination of radios and processors is vital for energy efficient, higher bandwidth transmissions [41].  
Network virtualization, middleware ensures to take the maximum advantage from multiple radios, and provide QoS for the application and users.