Bluetooth and personal area networks
Your earbuds, watch and keyboard talk to your phone with no cables and no Wi-Fi router. Watch a new watch advertise, get found and paired, then see how Bluetooth hops between radio channels so a busy room does not stop the music.
5 quick questions
Pick an answer to see why. Your score saves with your progress.
01Which channels does a new Bluetooth Low Energy device advertise on?
BLE keeps three advertising channels, 37 (2402 MHz), 38 (2426 MHz) and 39 (2480 MHz), so a scanner only has to listen in three places.
02In the figure, which device is the central?
The phone starts each connection and sets its timing. The earbuds, watch and keyboard are peripherals.
03Why does Bluetooth keep hopping from channel to channel?
The two sides hop in the same order. A noisy channel only hurts the packets that land on it, and adaptive hopping can skip busy channels altogether.
04What does pairing produce?
With LE Secure Connections both sides exchange public keys and compute the same secret, which never travels. The stored key (bonding) lets them reconnect later.
05Classic Bluetooth earbuds hop 1600 times a second. How long does each hop last?
1 s / 1600 = 625 µs, one Classic time slot.
Glossary
- PAN
- Personal area network: a few devices around one person, linked over a short distance, for example by Bluetooth.
- Central
- The device that starts a connection and sets its timing, usually the phone or computer.
- Peripheral
- A device that a central connects to, such as earbuds, a watch or a keyboard.
- BLE
- Bluetooth Low Energy: 40 channels of 2 MHz, short bursts, very low power. Used by watches, keyboards and sensors.
- Classic Bluetooth (BR/EDR)
- The original Bluetooth: 79 channels of 1 MHz and a steady stream. Often used for audio.
- Advertising
- Short broadcast packets on channels 37, 38 and 39 that say a device is there and can be connected to.
- Frequency hopping
- Switching radio channels many times a second in an order both sides know.
- Pairing / bonding
- Making a shared secret key (pairing) and storing it to reconnect later (bonding).
Notes and sources
- Device addresses use the RFC 7042 documentation block 00:00:5e:00:53:xx. Phones often advertise a random private address that changes every few minutes instead.
- The watch name, the 6-digit code 314159, the hop increment and the Classic hop order are illustrative; the real Classic order comes from the central's address and clock.
- "About 10 m" is typical for class 2 devices (2.5 mW); walls, bodies and the antenna change it a lot.
- The figure is slowed down: a real LE link may meet every few milliseconds and a Classic link hops every 625 µs.
Quiz Unlocks after the last step.