The Bluetooth SIG announced Bluetooth low energy features with version 4.0 of the Bluetooth specification in 2009. It was later re-branded as Bluetooth Smart, and has just been rebranded again in 2016 (see footnote 1 above). Consequently the terms Bluetooth Smart, Bluetooth Low Energy, BLE, and now Bluetooth low energy technology are often used interchangeably.
Bluetooth low energy technology is a radical departure from what is known as Bluetooth Basic Rate / Enhanced Data Rate (Bluetooth BR/EDR) or Classic Bluetooth, introduced in the late 1990s and used today in handsets, speakers, earphones, car kits, etc. The SIG’s goal with Bluetooth low energy technology was to define a new version of Bluetooth which could operate for years on a coin cell battery and was better suited for sending small bits of data on an infrequent basis.
Bluetooth BR/EDR is still the protocol of choice for voice or streaming music, but Bluetooth low energy technology is better suited to wireless sensor and control applications. Bluetooth low energy technology also reduced data latency to only 10% that of Bluetooth BR/EDR and introduced the ability for Bluetooth to broadcast data.
Like Bluetooth BR/EDR, Bluetooth low energy technology utilizes the 2.4 GHz ISM band and a frequency hopping technique to spread its RF energy between multiple channels. But in a departure from Bluetooth BR/EDR, it uses 40 2MHz-wide channels instead of 79 1MHz-wide channels. Consequently the two versions are fundamentally incompatible over the air. Devices that can support both Bluetooth BR/EDR and Bluetooth low energy technology have been called “Bluetooth Smart Ready” up until March of 2016. Most handsets, tablets, and so on fall into this category.
Three of Bluetooth low energy technology’s 40 channels (37, 38, and 39) are reserved for broadcasting “advertising packets” that contain information about the broadcasting node’s capabilities. These advertising packets are strategically located on frequencies between the three 2.4 GHz Wi-Fi channels to avoid interference from Wi-Fi.
Bluetooth Generic Attribute (GATT) Profile
The SIG also greatly simplified the application profiles for Bluetooth low energy technology, which now uses the GATT (Generic Attribute) profile, a structured list that defines the services, characteristics and attributes of a given application. GATT profiles can use either SIG-defined services or custom services defined by the OEM. Each GATT profile’s service is distinguished by a Universally Unique Identifier (UUID) which is either 16 bits long for SIG-adopted services or 128 bits long for custom services defined by the developer.
A beacon can include multiple services. When a service needs to be advertised, the service UUID is broadcast in the device’s advertising packet. Subsequently, when a Bluetooth scanner receives an advertising packet, the UUID is registered by the operating system to a specific application, which takes follow-on actions.