# Energy Efficiency in Short and Wide-Area IoT Technologies--A Survey
## 4 Classification of Energy Efficiency Methods
### Mechanisms for RFID Active Tags
#### 4.1.3 Bluetooth and BLE Bluetooth
In the ACB-tree, the root node is the CH while all the other nodes are leaf nodes. The formation process proposed in EMTS consists in three phases:
* _Neighbor Discovery_, that makes possible the connectivity of the generated scatternet;
* _Piconet Formation_, during which the devices are grouped into piconets, and the ones with the highest residual energy are selected to be masters, with the purpose of creating as few piconets as possible. The master devices perform a PAGE operation to their neighbors to build the piconet. All the other devices perform a PAGE SCAN, and become slaves of the master that contacted them first. Each piconet is then formed of five selected slaves, with two free slots for future connections with other piconets;
* _ACB-Tree Growing_, that makes possible the connection between piconets into a grow-aware ACB-tree, using ACB-Tree combinations [129]. The reorganization techniqueis instead divided in two phases:
* _Local Reorganization_: This phase is done to relieve the workload of the energy-depleting devices, that could be pure slaves, slave/slave bridges, or masters. When the energy level of a node drops below a pre-defined thresholds, this node will be substituted by one of her children nodes;
* _Device Reorganization_: This phase is separated in two sub-phases: 1. _Device Arrival_, during which the slaves of each leaf nodes periodically perform an INQUIRY SCAN procedure to discover new devices. Any new device that has performed an INQUIRY procedure to enter the existing scatternet is absorbed in the scatternet;
2. _Device Departure_, during which a slave leaves the scatternet, without affecting the overall scatternet composition. EMTS analysis was performed via the Blueware 1.0 simulator [133], built on top of Network Simulator v2 (NS-2) [134]. Results showed a significant scatternet lifetime increase, thanks to a beneficial shortening of the scatternet diameter. **Resource Allocation techniques. ** Two methods for energy saving were proposed in [130], where the network lifetime was adopted as performance indicator. Both methods make use of the RSP option proposed in [125], coupled with the ability to control the transmitted power of devices, as provided in the Bluetooth standard [40]. The first method assumes that the master node is responsible for all packet transmissions to/from the slaves, so the piconet masters drain their batteries faster than slaves. The RSP option allows each device to be a piconet master, thus being able to balance the energy consumption across devices. The current master periodically monitors both its own energy and the energy of its slaves. If its own energy is less than a fraction \(X(0<X<1)\) of the maximum available energy among all slaves, the RSP option is activated, and the slave having the maximum residual energy becomes the new master. A disadvantage of this method is that a continuous RSP activation may degrade the overall system performance. Simulation results showed that when the value of \(X\) increases, the number of master/slave switches also increases, leading to network throughput degradation. The second method, referred to as Distance Based Power Control (DBPC), takes root from the Bluetooth specification on the transmission power control. Three power classes are defined, each one with power ranges from 2 to 8 dBm. Assuming that the distance \(d\) between a master and a slave in known at both sides, DBPC chooses the transmission power of both master and slave, evaluating the power loss due to their distance, which is proportional to \(d^{\eta}\)[131]. DBPC can significantly improve the network lifetime, with an effect that however decreases as packet arrival rate increases, due to the higher load in the devices and thus higher energy consumption. Adaptive Share Polling (ASP) was proposed in [132], in order to reduce the power consumption of Bluetooth networks, where the traffic is composed by short packets transmitted with constant bit rate.