Bluetooth Low Energy, formally known as Bluetooth 4.0, is an extension of the classic BT technology. BLE was developed as an ultra-low, short range (3 to 10 meters; see note 1) wireless protocol for energy-stingy applications and disposable devices.
The nice (and smart) thing about BLE is that it inherits all the functionality, standards, and interoperability that its parent, BT classic has. That means developing for the BLE platform has a lot of technology that can be ported. It is just designed to use less energy. BLE is designed to be exactly the opposite of its bigger, faster brothers, 1.2, 2.0 + EDR (the enhanced data rate version), and 3.0 +HS (the high speed version).
Peeling back the layers and looking at the technology finds that the design re-uses much of the existing framework for BT radios, including the 2.4 GHz ISM band. That means it is backward-compatible with all existing BT designs. Following is a laundry list of specifications:
- OTA data rate: 1 Mbps.
- Modulation scheme: GFSK (see note 2) with frequency hopping.
- Channels: 40.
- Max output power: +10 dBm.
- Nominal output power: 0 dBm.
- Major operational modes: advertising, scanning, master device and slave device.
- Packet data transfers: 8 to 27 octets per packet.
- Latency: As low as 3 ms, 6 ms is typical.
- Robustness: 24-bit CRC per packet.
- Built-in security: AES-128, with counter mode, CBC-MAC, and application layer user defined.
- Topology: 32-bit scatternet, one-to-one and one-to-many.
BLE improves on classic BT in a number of areas that relate to power footprints. BLE boasts energy efficiencies of 20 times that of Classical BT, yet reduces power consumption by an order of magnitude in some places. It does this by using a number of schemes and a very simple link layer that is capable of a fast connect cycle. Typically, a BLE device spend only 1% of its time awake, (lowest sleep mode can will typically consume < 1.0 microA) and when it does wake, the idle current mode is a few tens of microA. When in data transmission mode, that peak current rises to 15 mA, maximum.
rom the onset, BLE was designed with simplicity in mind. In order to achieve the lofty goals of minimal power consumption, BLE design focuses on applications and devices that need to transfer small quantities of data, over relatively short distances. That was accomplished with a new protocol stack that can quickly construct simple links. The process sets up an ultra-fast connection, followed by a short bursts of minute data packets, and an ultra-fast disconnect. This resembles a pulsed data transmission scheme and works extremely well for quick communication of data snippets such as “the temperature in the refrigerator has changed more than two degrees, or the room is no longer occupied.”
There are three main elements that are used to accomplish this. They are an intelligent host controller, and an adjustable duty cycle and message length. As far as the controller goes, being smart means it has the ability to monitor activity and respond only to activity that requires action specific to the host. That mean the host can remain in sleep mode much of the time.
The other two primary low-energy components are the adjustable duty cycle and message length. The duty cycle can be adjusted down to as low as 0.1%. What that does is present app and device developers a minimal target to aim for to give them maximum run time, should that be their goal. The adjustable message variable gives the app the options of packaging messages in longer or shorter packages for efficiency. The reasoning is that longer single messages are more energy efficient than multiple short messages, mainly due to setup and tear down overhead, but other system parameters have an effect, as well.
Another key parameter for efficiency is robustness. Frequency hopping is used because it is relatively immune to interference, thereby reducing redundancy requirements. This is particularly useful in multi-wireless environments, such as the home, or public wireless hotspots where multiple protocols exits (Wi-Fi, ZigBee, cellular, etc.).
The more cycles that wireless device has, the more latency becomes a factor. Simply put, latency is created when the activity needs to add extra processing to the signal. This can be due to link budgets, signal or component instability, or signal strength. BLE addresses that by its simple architecture, which uses small packets and simple protocols.
BLE is designed for small-packet transmissions. Typically that includes between 8 and 27 octets (see note 3) per data package. Additionally, connections implement sniff sub-rating (see note 4), which provides very low duty cycles.
Perhaps the greatest doctrine of BLE is its simplicity. The layered GATT architecture (see Figure 2), simplifies creating and implementing profiles. Because ease of implementation was part of the initial design considerations, applications and embedded devices can be quickly fitted to the BLE architecture.