2.5 Broadcast Interval

A beacon achieves low-power consumption by residing in sleep most of the operating time, only waking up briefly to broadcast data. The time between these broadcast events are referred to as advertising interval, which is illustrated in Figure 4. For non-connectable beacons, the interval cannot be smaller than 100 ms. For connectable beacons, it cannot be smaller than 20 ms. To this interval, a 0-10 ms pseudo-random delay is added to ensure that beacons can coexist, even if they might start broadcasting at the same time.

Figure 4. Advertising Interval

The advertising interval is chosen based on requirements of power consumption vs latency. Higher interval allows more time in sleep mode but increases the time it might take for an observer to obtain a broadcasted packet.

An observer typically uses a scanning with a duty cycles less than 100% to conserve energy or allow other wireless protocols a time slot. One good example is Smartphones, which in most cases has a one chip solution for Bluetooth and Wi-Fi®. If an audio headset is connected via Bluetooth and Wi-Fi has a connection to a local access point, Bluetooth low energy scanning will probably have a very low-duty cycle. The time slots are essentially split between the 2.4 GHz protocols on the device.

An observer can be either scanning in passive or active mode. If active mode is used and a beacon supports it, a Scan Request will be sent, which the beacon must respond to with a Scan Response. The request itself is an empty packet (no data allowed), whereas, the response is typically static information such as name or model of device. It is fully proprietary so it could also be calibration data for sensors or any other useful information. When an observer is scanning in passive mode, it will not send a Scan Request.