CIS

Connected Isochronous Stream (CIS)

CIS is the unicast audio transport mechanism within BLE Isochronous Channels. It provides point-to-point, time-bounded data delivery between two connected BLE devices -- typically a phone streaming audio to a wireless earbud or hearing aid.

How CIS Works

A CIS operates within an existing BLE Connection Event between a Central and Peripheral. The Central (host device) creates a CIG (Connected Isochronous Group) and establishes one or more CIS streams within it. Each CIS has its own timing parameters: ISO Interval, burst number (BN), and flush timeout (FT).

Unlike the best-effort delivery of regular GATT notifications, CIS provides bounded latency. The controller guarantees a maximum transport latency for each packet, and if retransmissions exceed the flush timeout window, the packet is discarded rather than blocking subsequent frames. This behavior is critical for audio, where late packets are useless.

Bidirectional Capability

CIS supports both unidirectional and bidirectional data flow. For music playback, the Central streams audio downstream to the Peripheral. For hands-free calling, both directions are active simultaneously. The upstream path also supports microphone audio from hearing aids back to the phone.

CIS vs BIS

Implementation Notes

When implementing CIS on a BLE SoC, the link layer firmware must schedule CIS events at precise intervals alongside regular ACL traffic. Most modern BLE 5.2 chips handle this in hardware. The ASCS service on the peripheral exposes Audio Stream Endpoints (ASE) that the central configures to set up CIS streams with the desired LC3 codec parameters and QoS settings.