1One technology, two modes
Bluetooth® wireless technology has two modes: Bluetooth® Classic (BR/EDR), the original mode, and Bluetooth® Low Energy, added later to cut power consumption dramatically. Both use the 2.4 GHz band, but their radio formats and connection procedures differ, so they cannot communicate with each other directly.
- A Classic-only device and a Low Energy-only device will not connect.
- Chips that support both modes, as most smartphones do, are called dual-mode. Chips that support only one are single-mode.
- Before designing a wireless link, first confirm which mode the other device uses.
2At a glance
| Classic (BR/EDR) | Low Energy | |
|---|---|---|
| Best at | Keeping a link open and sending data continuously (audio, serial data) | Sending small amounts of data occasionally and sleeping the rest of the time |
| Power | Draws power continuously while connected | Very low. Can be designed to run for months or years on a coin cell |
| Radio data rate | 1 Mbps (2 or 3 Mbps with EDR) | 1 Mbps or 2 Mbps, plus long-range coded modes (125/500 kbps) |
| Channels | 79 × 1 MHz | 40 × 2 MHz (3 of them for advertising) |
| Time to connect | Seconds for discovery and connection | Milliseconds to a few hundred milliseconds |
| Sending without a connection | No | Yes. Data can be broadcast in advertising packets alone (beacons) |
| Data model | Profiles for each use (serial, audio, keyboard…) | Services and characteristics (GATT), defined by you |
| Smartphone apps | Restricted on some operating systems (section 6) | Directly accessible from apps on most smartphones |
The rates above are over-the-air rates. Actual application throughput is much lower and depends heavily on connection settings and the other device.
3Classic
Classic sets up a link between two devices and keeps it open. It suits data that has to flow without interruption.
- Profiles: Each use has a defined profile — SPP to replace a wired serial (UART) link, A2DP for music, HFP for calls, HID for keyboards and mice. Devices from different makers connect directly when they support the same profile.
- Serial replacement: An existing serial cable can be replaced with a wireless link with little or no change to the equipment's software, which is why Classic has long been used for industrial equipment, printers and instruments.
- Topology: One device leads the link and can talk to up to seven devices at once.
In practice
If you are converting existing serial equipment to wireless and the other side is a PC or an existing terminal, Classic with SPP is the fastest route. If the other side is a smartphone app, check the restrictions in section 6 first.
4Low Energy
Low Energy sleeps most of the time and wakes only briefly to exchange data. It is widely used in sensors, wearables and devices configured from a smartphone app.
- Advertising: The device sends a short "I'm here" packet at regular intervals. Another device can connect in response, or simply read the data inside the packet without connecting. Devices that broadcast this way are called beacons.
- Connection interval: Once connected, the two sides wake at a fixed interval (7.5 ms to 4 s) to exchange data. A short interval means faster response; a long one means longer battery life.
- GATT (services and characteristics): Data is grouped into services, and each value inside is a characteristic — for example, a "temperature" service with "current temperature" and "measurement period" characteristics. The other device can read and write characteristics and be notified when they change. Defining your own services is straightforward, which makes app integration easy.
- Audio: Low Energy was not originally designed for audio, but recent versions of the specification add audio over Low Energy (LE Audio). Check support on the other device before relying on it.
In practice
On a battery-powered device, battery life is set less by the chip than by the advertising period and connection interval. Decide early how much data must be sent and how often, then calculate the current budget against the battery capacity.
5Which one to choose
| If your product… | Choose |
|---|---|
| Streams audio (speakers, headsets) | Classic (consider LE Audio once the other device's support is confirmed) |
| Converts existing serial equipment to wireless, talking to a PC or dedicated terminal | Classic (SPP) |
| Is a battery-powered sensor or tag that must last a long time | Low Energy |
| Is configured or monitored from a smartphone app | Low Energy |
| Broadcasts position or status to many devices without connecting | Low Energy (beacon) |
| Must work with existing Classic equipment and with smartphone apps | Dual-mode |
In practice
Choose the mode by looking at the other device, not your own. If the other side is not fixed and a smartphone app is even a possibility, make Low Energy the default and add Classic only for the functions that truly need it.
6What to check during development
- Smartphone operating systems: On iOS, a regular app can only talk to a device over Classic serial (SPP) through a separate accessory certification program. If smartphone integration is the goal, designing for Low Energy from the start is the safer choice. Connection behavior can also vary by OS version and handset model, so test on several phones.
- Other radios in the same band: Wi-Fi uses the same 2.4 GHz band. If one device contains both radios, antenna placement and coexistence must be designed together.
- Antenna and enclosure: Metal housings, batteries and the human body shorten range considerably. Decide the antenna position on the board together with the enclosure material.
- Security: Both modes have pairing procedures for authentication and encryption. Devices without a display or buttons need their pairing method decided early.
- Standards to consider before launch: Selling a wireless product requires radio certification in each market. Displaying this technology's name or logo on a product also requires membership of the specification body and completion of its qualification process for each product. Both take time and money, so put them in the schedule early.
In practice
Experience developing wireless devices shows that most problems come not from the protocol but from compatibility with the other device, the antenna and power. Schedule tests with the actual phones and terminals, in the actual enclosure, at the prototype stage.