As described in the Physical Layer, Bluetooth® Low Energy (BLE) uses 40 channels. They are divided into two types and a frequency hopping scheme is implemented for robust operation.
There are 2 types of Link Layer channels: advertising channels and data channels:
Advertising Channel Usage
- Device Discovery
- Connection Establishment
- Broadcast Transmissions
Data Channel Usage
- Bidirectional communication between connected devices
- Adaptive frequency hopping used for subsequent connection events
(Adaptive) Frequency Hopping
When in a data connection, a frequency hopping algorithm is used to cycle through the 37 data channels:(1)
Where fn+1 is the frequency (channel) to use on the next connection event, and hop is a value that can range from 5-16, and is set when the connection is created. It is added onto the last frequency modulo 37.
The following diagram depicts 3 active BLE connections, showing the frequency hopping sequence (frequency hopping on Link 3 is outlined for clarity):
The diagram shows that frequency hopping provides a robust method for maintaining a connection in the presence of interference or many other devices in radio range.
Adaptive Frequency Hopping
This mechanism is used by the Link Layer to remap a given packet from a known bad channel to a known good channel so that interference from other devices (i.e. Wi-Fi®) is reduced.
Suppose, for example that a BLE device is in the same area as several Wi-Fi® networks on channels 1, 6, and 11. The BLE device would mark channels 0-8, 11-20, 24-32 as bad channels. This means that when the two devices are communicating, they would cycle through the channels and remap these channels to a set of good channels as shown: