Bluetooth Low Energy fundamentals: how it saves power and reaches the internet
Bluetooth Low Energy (LE) is the branch of Bluetooth built for devices that say little and must run for a long time. Earlier versions of Bluetooth added features and larger packets. LE went the other way: it removed whatever a sensor does not need, so that a beacon or a heart-rate monitor can run for months or years on a coin cell.
Where LE sits in the Bluetooth family
Bluetooth 4.0 and the releases after it were a sharp break from the specifications before them. Two later releases matter most for LE.
Bluetooth 4.2, released in December 2014, added:
- LE Secure Connections, with data packet length extension
- link layer privacy, with extended scanner filter policies
- the Internet Protocol Support Profile (IPSP), which makes LE devices ready for IPv6
Bluetooth 5.0 added:
- a 2 Mbit/s physical layer for LE
- LE Long Range
- LE advertising extensions
- high duty cycle non-connectable advertising
- LE channel selection algorithm #2
- the slot availability mask
Bluetooth 5.0 offers four times the range and twice the speed of 4.2.
The standard belongs to the Bluetooth Special Interest Group (SIG), a privately held, not-for-profit trade group. As of February 2018 it had 33,462 member companies and organizations, and membership was free. Every product must pass qualification before it is sold, which is why two devices that carry the Bluetooth logo can be expected to work together. Bluetooth uses the 2.4 GHz ISM (industrial, scientific and medical) band, which needs no licence.
Three kinds of device
A Bluetooth device is one of three kinds:
- BR/EDR. Basic Rate/Enhanced Data Rate devices, which have no LE support.
- Single-mode LE. Devices that support only LE: heart-rate monitors, thermostats, fitness equipment, watches, key fobs. They are expected to last months or years on a coin cell.
- Dual-mode (BR/EDR/LE). Devices that support both, such as phones, tablets and PCs. They can talk to BR/EDR devices and single-mode LE devices at the same time, and their power budget is looser because their batteries are bigger or recharged often.
The SIG’s two trademarks followed the same split. “Bluetooth Smart” marked single-mode LE sensors that collect and send specific data, built on the GATT architecture. “Bluetooth Smart Ready” marked dual-mode devices that can receive from both classic Bluetooth and Bluetooth Smart devices, and that let the user add support for a new Smart device by installing software.
Backward compatibility is a stated concern of the SIG, and it holds with one exception: a BR/EDR device and a single-mode LE device cannot talk to each other.
What LE removed to save power
An LE device is expected to send data only now and then, and to be switched off the rest of the time. Its uses fit that pattern: locating and alerting, proximity detection, sensors, fitness and healthcare wearables, mobile payments. None of them needs constant data transfer or high throughput.
Fast connections. LE uses 3 advertising channels to set up a connection, where BR/EDR scans 32. A connection is made in a few milliseconds, so it is simpler to reconnect each time there is data to send than to keep a link open. Battery use drops sharply as a result.
Less functionality. LE dropped several things BR/EDR does:
- A device no longer has to transmit and receive. A fitness watch can send its readings to a phone without ever listening for instructions.
- There are no voice channels. LE is not meant for devices that stay connected, such as headphones, so SCO and eSCO are gone.
- There are no scatternets. A device is in one piconet at a time, which simplifies the link layer’s state machine. Little is lost: a BR/EDR piconet holds up to seven slaves, and LE sets no such limit.
- There is no switching of the master and slave roles.
- There is no constant checking of the connection. BR/EDR devices keep sending POLL and NULL packets to see whether the other side is still there. LE ends the link and makes it again when needed.
- There are no sniff and park modes. They were power-saving modes for BR/EDR; LE does not need them.
Shorter packets. An LE packet is at most 27 bytes, against up to 1,021 for BR/EDR. A short packet takes less time and energy to send, and the radio does not need to be recalibrated as it would after a long stretch at high power. The length extension in 4.2 made packets about ten times larger while keeping the low power consumption.
A smaller memory footprint. Dynamic memory has to be refreshed constantly; ROM and flash do not. Shorter packets, shorter headers, simpler protocols and uniform packet formats reduce the dynamic memory an LE chip needs, and with it the silicon area.
Other work moved down the stack. Resolving the address of a remote device, for example, is done in the lower layers, so the upper layers stay idle.
How an LE device reaches the internet
A sensor sends short, intermittent signals. Something has to carry them to the internet. There are three ways.
Gateways. Phones and tablets can act as the bridge. For when none is nearby, the SIG defined standard RESTful APIs for GAP (Generic Access Profile) and GATT (Generic Attribute Profile). A gateway is a device that implements them.
HTTP Proxy Service (HPS). A router that runs HPS relays a device’s signals to a cloud server. A temperature gauge, for example, sends its packets to an HTTP client, which posts the data to an HTTP server.
IPv6. IPSP, over low-power wireless personal area networks, lets an LE device send and receive IP packets. A light bulb can take its instructions from a Bluetooth router controlled by a phone.
Privacy
People carry LE devices all day: phones, running shoes, fitness bands. The SIG added features so that these cannot be used to track the person wearing them.
With random addresses, each message a device sends comes from a different address, in a pseudo-random pattern. A paired device holds an identity resolving key (IRK) that turns those addresses back into the real one, so it knows the messages come from the same device. An eavesdropper sees only a string of unrelated addresses.
Mesh
Before mesh, Bluetooth devices could be arranged in two ways:
- Connection topology. Devices form a piconet around one master. The slaves cannot talk to one another.
- Broadcast topology. A device in advertising mode transmits to any device within range that is in scanning mode.
Neither meets what an industrial network asks for: coverage of the whole space with no blind spots, packets that all arrive, low power in hardware and software, and a layout that can change without rebuilding the site. Simple broadcast loses packets and leaves blind spots. A piconet is a star, and when its master fails, every device attached to it is cut off.
In a mesh, devices pass packets on to each other until the data reaches a gateway, which pushes it to a server. Each device that relays extends the reach of the network, by 40 to 100 feet depending on the hardware, so fewer gateways are needed. That matters in factories and offices, where range shifts with building materials, Wi-Fi interference and the movement of people and objects.
CSR Mesh was an early design. A device in a CSR mesh plays three roles at once:
- Broadcaster. It typically sends each packet three times. Network usage goes up and packet loss comes close to zero.
- Observer. While broadcasting it also listens, and relays any packet that arrives in the right format.
- Advertiser. It can still form a master-slave connection, which is how a new device is brought into the mesh.
Needing a master-slave connection to add a device is clumsy. Later, open mesh designs dropped it. In one design from 2017, a network can run in either of two configurations. In an individual mesh, devices send only their own packets, which keeps network load low. In a collaborative mesh, devices also retransmit their neighbours’ packets. Both lose fewer packets than the older topologies and cover more ground. Because the packet format is open, anyone can build on it.
A short glossary of classic Bluetooth
LE is easier to read about with the vocabulary of BR/EDR in hand.
- Piconet. The smallest unit of Bluetooth communication: one master and up to seven slaves, which synchronize their clock and frequency-hopping pattern to the master’s.
- Scatternet. Two or more piconets joined by a shared device, typically a slave that takes part in both in turn.
- Frequency hopping. Bluetooth hops among 79 channels, 1 MHz apart from 2402 MHz, 1,600 times a second, in a pseudo-random pattern, so that nearby radios do not block it.
- Adaptive frequency hopping (AFH). The master marks channels with interference as unused and removes them from the pattern. AFH can reduce the channels in use to 20.
- Baseband controller (link controller). Manages the physical channels and links, selects the hopping frequency, forms piconets and scatternets, formats packets, runs inquiry and paging, encrypts data and manages power.
- ACL. Asynchronous connection-oriented transport: a packet-switched link that carries user data, control signals and broadcast traffic between a master and its slaves.
- SCO and eSCO. Synchronous connection-oriented transports: circuit-switched links for continuous data such as voice. eSCO allows corrupted packets to be sent again and supports higher data rates; it is what routes a call through a headset.
- Link control packets. ID carries an access code before a connection exists. POLL is the master asking a slave for its status. NULL acknowledges a transmission and is a slave’s answer when it has nothing to send. FHS synchronizes hop frequencies while a piconet forms. DM1 carries control and data packets.
- Access codes. Every packet begins with one. The device access code is used while devices try to connect, the channel access code marks every packet inside a piconet, and the inquiry access code is used while scanning for nearby devices.
- HCI. The host controller interface, defined over four transports: UART, USB, Secure Digital and three-wire UART.
- L2CAP. Sits above the baseband layer and lets the protocols above it send packets of up to 64 KB over ACL links.
- Inquiry and paging. Inquiry discovers nearby devices. Paging connects to one whose address is known, and the device that pages becomes the master.
- Park. A slave that does not need to take part in a full piconet can park: it saves power, stays synchronized, and wakes regularly to check whether it is wanted.
- Link manager. Sets up and controls links. Two devices’ link managers talk through the Link Manager Protocol (LMP).
- Pairing. Two devices associate with each other by exchanging a passkey or PIN.
Adapted in October 2026 from four articles first published on the Momentaj blog in 2018. It describes the technology as it stood then.