Isochronous Channels

Isochronous Channels

LC3 codec and Auracast." data-category="LE Audio">LE Audio">Isochronous Channels are a BLE 5.2 transport feature that provides time-bounded, deterministic data delivery. Unlike GATT operations that use asynchronous ACL data channels, isochronous channels guarantee that audio samples or sensor readings arrive within strict timing windows -- essential for real-time audio streaming in LE Audio.

Architecture

The isochronous transport layer sits alongside (not on top of) the standard asynchronous L2CAP channel. Two modes are defined:

• CIS (Connected Isochronous Stream): Point-to-point streams within an existing BLE connection, used for unicast audio such as phone-to-earbud streaming
• BIS (Broadcast Isochronous Stream): Connectionless broadcast streams for one-to-many delivery, used by Auracast public audio

Streams are organized into groups. A CIG (Connected Isochronous Group) contains one or more CIS streams sharing timing parameters -- for example, left and right earbud streams in a stereo pair. A BIG (Broadcast Isochronous Group) contains one or more BIS streams sharing a common broadcast schedule.

Timing and Scheduling

Each isochronous channel operates on a fixed interval (ISO Interval), typically 7.5 ms or 10 ms for audio. The BLE controller schedules isochronous events alongside regular Connection Events, managing coexistence between audio and data traffic. Flush timeout (FT) parameters control how long the controller retries lost packets before flushing them -- a tradeoff between reliability and latency.

Hardware Requirements

Isochronous channels require Bluetooth 5.2-compliant silicon. The controller must support ISO scheduling, which adds complexity to the link layer firmware. Chips like the nRF5340, nRF54H20, and ESP32-C6 include isochronous channel support. Developers should verify isochronous support in their SDK -- not all BLE 5.2 chips implement the full ISO feature set.