Field Guide · protocol

Also known as: BLE, Bluetooth LE, Bluetooth Smart, Bluetooth Low Energy

Bluetooth Low Energy (BLE) is the low-power personal-area radio introduced in Bluetooth 4.0, using GFSK modulation across 40 channels in the 2.4 GHz band and reserving three channels for advertising and discovery.1 It is a distinct air interface from Bluetooth Classic, optimised for devices that wake, send a short burst, and sleep — the backbone of many IoT sensors, beacons, and wearables.

2.402 → 2.480 GHz 3 advertising channels (solid) + 37 data channels (light)
BLE reserves three advertising channels, spaced to dodge the common Wi-Fi channels, with 37 hopping data channels in between.

Overview

BLE splits the 2.4 GHz band into 40 channels of 2 MHz each. Three of them — numbers 37, 38, and 39, deliberately placed at the band edges and centre to sidestep the busiest Wi-Fi channels — carry advertising packets that let devices announce themselves, broadcast beacons, or begin a connection. The remaining 37 data channels are used inside an established connection, hopping between them for interference resilience. The base PHY is GFSK at 1 Mbit/s; Bluetooth 5 added a 2 Mbit/s mode and a long-range “Coded PHY” that trades rate for sensitivity.

Technical characteristics

Property Value
Band 2.402–2.480 GHz ISM
Channels 40 × 2 MHz (3 advertising + 37 data)
Modulation GFSK
Bit rate 1 Mbit/s (LE 1M); 2 Mbit/s (LE 2M)
Long range LE Coded PHY (S=2, S=8)
Connection hop Adaptive over 37 data channels

The advertising channels make BLE traffic unusually observable: a device that only ever advertises transmits short, unencrypted packets on three known frequencies, which is why BLE beacons and proximity trackers are straightforward to survey.

History

The Low Energy layer was designed as “Wibree” by Nokia, then folded into Bluetooth 4.0 (2010) by the Bluetooth SIG. Bluetooth 5 (2016) doubled the data rate and added long-range and higher-throughput advertising options; later releases added direction-finding and LE Audio.

Deployment

BLE is pervasive in fitness trackers, smart-home sensors, medical devices, retail beacons, and phone accessories. It coexists with, and often complements, ANT+ in sports sensors and competes with proprietary sub-GHz links elsewhere in the IoT space.

Decoding it with GopherTrunk

GopherTrunk does not decode BLE. Although the three advertising channels are fixed and their packets are short, following BLE is a wideband 2.4 GHz task handled by purpose-built tools (dedicated sniffers, or SDRs with GFSK demodulators), not by GopherTrunk’s narrowband land-mobile trunking decoders. BLE is relevant here only as ambient 2.4 GHz activity.

Sources

  1. Bluetooth Low Energy — Wikipedia, on the BLE air interface, its 40-channel plan, advertising channels, GFSK PHYs, and history. ↩

See also