# Energy Efficiency in Short and Wide-Area IoT Technologies--A Survey
## 2 IoT Technologies
### Low-Power Short-Area Networks (LPSANs)
#### 2.1.3 Bluetooth and BLE  
Bluetooth is one of the most widespread technologies for data transmission over short distances, and can also be adopted to IoT scenarios, such as medical data transmissions and real-time location systems (RTLS), among others [39].  
Bluetooth uses short-wavelength UHF radio waves in the 2.4 GHz ISM band, and its physical layer is based on specifications defined in the IEEE 802.15.1 standard [40]. In the initial design, Bluetooth used Gaussian Frequency Shift Keying (GFSK) modulation, leading to a so-called Basic Rate (BR) mode, with a bit rate of 1 Mbps; the introduction of Bluetooth 2.0+Enhanced Data Rate (EDR), led to data rates up to 3 Mbps, respectively. Bluetooth adopts Frequency Hopping (FH), with 1600 hops per second, over about 80 different carriers spaced of 1 MHz to guarantee coexistence with other technologies operating in the ISM band. Time Division Duplexing (TDD) is adopted within a Bluetooth network with data transmission in uplink and downlink directions taking place alternatively in time [41; 42].  
A Bluetooth network, referred to as piconet, consists of one master and up to seven slave devices. Within a piconet, the master selects the hopping sequence and the transmission times to be used by slave devices. Multiple piconets can be coordinated to form a so-called scatternet, as shown in Figure 3.  
Bluetooth devices can operate in one of four modes [41]:  
* Active mode: the regular connected mode, where the device is transmitting or receiving data;
* Sniff mode: the slave device listens only to specified slots for messages that are meant for it;
* Hold mode: the device does not transmit data for a long time;
* Park mode: the device is temporarily deactivated, in order to allow for its active member address to be re-assigned.  
The design and deployment of Bluetooth Low Energy (BLE) has been initiated as part of the Bluetooth 4.0 Core specification [43]. BLE inherits all the main features and functionalities from Bluetooth, such as operating with the same pairing, authentication and encryption functionalities, as well as the master-slave model and the use of Frequency Hopping for coexistence. Unlike Bluetooth, BLE devices operate in sleep mode and awake only when a connection is initiated. Therefore, BLE offers a power consumption in the order of 5-10% compared to that of the original Bluetooth [44; 45]. BLE operates in the 2.4 GHz spectrum like Bluetooth, but it divides the available spectrum into 40 channels, with a 2 MHz spacing. Out of these channels, three are the so-called Advertising Channels, used for device discovery, connection establishment, and broadcast transmissions; the remaining channels are Data Channels, used for bidirectional data exchange [46; 47]. Similarly to Bluetooth, GFSK is used for signal modulation and access, achieving a transmission rate of about 1 Mbps [48].  
Bluetooth and BLE are in general used for different purposes; Bluetooth is more suitable and works best with applications requiring a relatively high throughput, while BLE is tailored on IoT applications requiring periodic data transmission, for example, building automation and lighting.