Bluetooth® Low Energy Channels
As described in the physical layer, Bluetooth® Low Energy (BLE) uses 40 channels, divided into two types. A frequency hopping scheme is implemented for robust operation.
Channels
There are two 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:
fn+1=(fn+hop)mod37
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 module 37.
The following diagram depicts three 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 the radio range.
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, and 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 in the accompanying image.
