Bluetooth Low Energy (BLE) is part of the Bluetooth Core Specification maintained by the Bluetooth SIG. It first appeared with Bluetooth 4.0 in 2010; Bluetooth 5.0 (2016), 5.1 (2019) and subsequent revisions extended range, throughput and direction-finding capabilities. BLE operates in the 2.4 GHz ISM band over 40 channels (3 advertising and 37 data).

Proximity-based sequential dispatch

A common pattern in field device management is to carry a preconfigured set of commands or programs in a mobile application and physically move through the site. When the operator enters BLE range (~10–30 m, depending on the RSSI threshold) the application discovers the target device, automatically sends the next queued task and moves on to the following device. Each connection also performs time synchronisation (RTC update from the phone's NTP/GNSS-derived clock), which is critical for offline devices that must keep their schedule accurate. The advantages of the approach are: no need for one-by-one manual connections, multi-device commissioning during a single linear walk-through, and no dependence on an internet connection.

Architecture and protocol layers

The BLE protocol stack consists primarily of the following layers:

  • PHY — physical layer; LE 1M, LE 2M and long-range LE Coded (S=2/S=8) modes.
  • Link Layer — connection, advertising, scanning and channel hopping.
  • L2CAP — packet segmentation/reassembly and logical channels.
  • ATT/GATT — attribute-based data model with services and characteristics (UUIDs).
  • SM — pairing and key management (LE Secure Connections, ECDH P-256).

Advertising and scanning

BLE devices announce their presence to the surrounding area using advertising packets. The advertising interval is configurable from 20 ms to 10.24 s and is chosen according to the battery target. A scanner listens in passive or active mode; in active mode it requests additional data (scan response) via SCAN_REQ. Advertising packets carry up to 31 bytes (legacy) or 255 bytes (extended advertising, BLE 5.0) of payload.

RSSI and proximity estimation

Received Signal Strength Indicator (RSSI, in dBm) is used for proximity estimation. Typical values: −40 dBm (very close, <1 m), −70 dBm (medium, ~5 m), −90 dBm (weak, >15 m). Under field conditions, metallic surfaces, foliage, the human body and multipath reflections can cause RSSI to fluctuate by 6–10 dB; distance estimation therefore requires filtering (e.g. Kalman or moving averages). For more precise localisation, BLE 5.1 direction finding (AoA/AoD) is recommended.

GATT operations and multi-device workflows

GATT operations include read, write, write without response, notify and indicate. A typical field-configuration sequence is: scan → connect → service discovery → characteristic write/read → disconnect. A single GATT operation completes within 50–200 ms; the total per-device connection time can be reduced to 2–5 seconds.

In IoT deployments a sequential task queue (sequential commissioning) approach is common. The mobile application keeps a predefined command list in memory; for each device that comes within range during scanning, the corresponding task is triggered automatically. Among devices simultaneously in range, an RSSI-based priority is applied; failed operations are retried with exponential backoff.

Practical note: connection parameters (Connection Interval 7.5 ms–4 s, Slave Latency, Supervision Timeout) directly affect both battery consumption and responsiveness. A short interval is preferred for configuration sessions; for measurement and reporting sessions, the interval is widened to preserve battery life.

Mass provisioning

For projects with hundreds of devices, manually pairing each unit is impractical. Three industry approaches are common:

  • Sequential queue — configuration is pushed to each device as the operator walks the site.
  • BLE Mesh (Bluetooth SIG, 2017) — broadcast-based configuration using a shared network key.
  • Out-of-band provisioning — DevEUI and keys transferred via QR code or NFC, with confirmation over BLE.

Security notes

BLE 4.2 introduced LE Secure Connections, which provides ECDH-based key exchange and AES-CCM encryption. The older LE Legacy Pairing scheme is weak against passive eavesdropping and should not be used in production. In field scenarios, additional good practice includes nonce-based one-time command signatures and rotating session keys at the application layer.

OTA updates

For firmware updates over BLE (OTA-DFU), silicon vendors such as Nordic, Silicon Labs and TI provide standardised profiles. A typical 256 KB binary can be transferred in 30–60 seconds over BLE 5.0 with the LE 2M PHY; resume support after a dropped connection is generally implemented.