Bluetooth Low Energy is a power-conserving variant of Bluetooth personal area network (PAN) technology, designed for use by Internet-connected machines and appliances. Also marketed as Bluetooth Smart, Bluetooth LE was introduced in the Bluetooth 4.0 specification as an alternative to Bluetooth Classic.
Bluetooth Low Energy, also known as “Bluetooth Smart,” was developed by the Bluetooth Special Interest Group. It has a relatively shorter range and consumes lower energy as compared to competing protocols. The BLE protocol stack is similar to the stack used in classic Bluetooth technology. It has two parts: controller and host. The physical and link layer are implemented in the controller. The controller is typically a SOC (System on Chip) with a radio. The functionalities of upper layers are included in the host . BLE is not compatible with classic Bluetooth. Let us look at the differences between classic Bluetooth and BLE [63, 64].
The main difference is that BLE does not support data streaming. Instead, it supports quick transfer of small packets of data (packet size is small) with a data rate of 1 Mbps.
There are two types of devices in BLE: master and slave. The master acts as a central device that can connect to various slaves. Let us consider an IoT scenario where a phone or PC serve as the master and mobile devices such as a thermostat, fitness tracker, smart watch, or any monitoring device act as slaves. In such cases, slaves must be very power efficient. Therefore, to save energy, slaves are by default in sleep mode and wake up periodically to receive packets from the master.
In classic Bluetooth, the connection is on all the time even if no data transfer is going on. Additionally, it supports 79 data channels (1 MHz channel bandwidth) and a data rate of 1 million symbols/s, whereas, BLE supports 40 channels with 2 MHz channel bandwidth (double of classic Bluetooth) and 1 million symbols/s data rate. BLE supports low duty cycle requirements as its packet size is small and the time taken to transmit the smallest packet is as small as 80 s. The BLE protocol stack supports IP based communication also. An experiment conducted by Siekkinen et al.  recorded the number of bytes transferred per Joule to show that BLE consumes far less energy as compared to competing protocols such as Zigbee. The energy efficiency of BLE is 2.5 times better than Zigbee.