Connection Interval

Connection Interval

The connection interval (CI) defines the time between two consecutive connection events in an active BLE link. It is one of the most important parameters in BLE system design because it directly controls the trade-off between latency, throughput, and power consumption.

Value Range and Granularity

The connection interval is specified in units of 1.25 ms, with an allowed range from 7.5 ms (6 units) to 4000 ms (3200 units). During connection establishment, the central device proposes a minimum and maximum interval, and the two devices negotiate a value within that range. Common values include 7.5 ms for gaming controllers, 30 ms for interactive wearables, and 1000 ms or more for environmental sensors that report infrequently.

Power vs. Latency Trade-off

A shorter connection interval means the radio wakes up more frequently. At 7.5 ms CI, the radio is active roughly 133 times per second, delivering low latency but consuming significantly more power. At 1000 ms CI, the radio wakes only once per second, extending battery life by orders of magnitude but introducing up to 1 second of latency for data delivery.

The optimal connection interval depends on the application. A real-time gaming peripheral needs sub-10 ms CI. A temperature sensor reporting every 5 minutes can use a 2000--4000 ms CI and sleep between events. A heart rate monitor typically uses 500--1000 ms CI as a balance between data freshness and battery life.

Dynamic Adjustment

BLE 5.3 introduced Connection Subrating, which allows devices to dynamically switch between different effective connection intervals without renegotiating the base parameters. A keyboard, for example, can operate at a fast CI while the user is typing and automatically switch to a slow CI during idle periods -- all within the same connection, without the overhead of a full parameter update procedure.

Interaction with Slave Latency

Slave latency works in conjunction with CI to further reduce power consumption. Even if the CI is set to 30 ms, a peripheral with slave latency of 10 can skip up to 10 consecutive connection events, effectively polling at 330 ms intervals when it has no data to send. This gives the peripheral the best of both worlds: low latency when data is available, and low power when idle.