Bluetooth Low Energy (BLE) is a Bluetooth technology for exchanging relatively small amounts of data with low-power devices such as sensors, fitness trackers, and controllers. It can broadcast data without a connection or establish a bidirectional connection for reading, writing, and receiving device data.
BLE is not simply a slower version of Bluetooth Classic: it uses different discovery procedures, device roles, and application protocols. The sections below explain how advertising, connections, GATT, power use, and common phone APIs work.
What does BLE mean?
BLE stands for Bluetooth Low Energy. It operates in the 2.4 GHz ISM band and is intended for short bursts of relatively small data rather than continuous, high-throughput communication. Android’s BLE overview describes its use for low-power data exchanges.
A temperature sensor, for example, may wake up, transmit a reading, and return to sleep. A fitness tracker may send heart-rate measurements periodically. Spending less time transmitting and listening can help extend battery life.
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BLE is commonly used for:
- Heart-rate monitors and fitness trackers
- Wireless thermometers and environmental sensors
- Smart locks, lights, and other home-automation devices
- Proximity tags and beacons
- Game controllers, keyboards, and styluses
- Medical and industrial monitoring equipment
BLE is not automatically low-power in every situation. Continuous scanning, frequent notifications, short connection intervals, high transmit power, and large data transfers can use substantial battery power.
How BLE communicates
BLE has two main communication modes, as described in the Bluetooth SIG’s Bluetooth LE Primer.
| Mode | How it works | Typical use |
|---|---|---|
| Advertising or broadcasting | A device transmits short packets without establishing a connection. Ordinary advertising packets are not acknowledged at the advertising transport level. | Beacons, sensor broadcasts, presence detection |
| LE ACL connection | Two devices establish a bidirectional connection and exchange data through scheduled connection events. | Reading a sensor, changing a device setting, receiving notifications |
An advertising device can broadcast without accepting connections; a beacon is a common example. Advertising is therefore not necessarily “waiting to be connected.”
Because ordinary advertising packets are not acknowledged, advertising is not reliable transport. A scanner can miss a packet because of radio interference, collisions, scan timing, receiver duty cycling, or because the advertiser was not transmitting when the scanner was listening. Applications that need dependable delivery normally use a connection or repeat broadcast data.
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BLE uses the 2.4 GHz band. Connected communication uses adaptive frequency hopping across 37 data channels. Legacy BLE advertising uses primary advertising channels 37, 38, and 39; these are separate from the 37 data channels. These channel counts are documented in the Bluetooth SIG’s Bluetooth LE Primer.
Advertising is not perfectly periodic. The Bluetooth LE Primer describes a pseudo-random delay of 0 to 10 milliseconds added to the configured advertising interval. This variation helps reduce repeated collisions when devices advertise at similar intervals.
Central and Peripheral roles
The Central and Peripheral roles describe how a BLE connection is established:
- Central: scans for advertisements and initiates connections.
- Peripheral: advertises and accepts connection requests.
A phone commonly acts as the Central while a sensor acts as the Peripheral. A device may support both roles, and the same physical device may act as a Central in one connection and a Peripheral in another. Two devices that support only the Peripheral role cannot connect to each other; the same is true of two devices that support only the Central role. See Android’s BLE overview.
GATT client and GATT server roles
BLE has another pair of roles that are often confused with Central and Peripheral:
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- GATT client: requests data, reads and writes values, and subscribes to updates.
- GATT server: exposes data and responds to client operations.
GATT stands for Generic Attribute Profile. A phone is often both the Central and the GATT client, while a sensor is often both the Peripheral and the GATT server. These pairings are common, but they are not definitions: Central/Peripheral roles govern link establishment, while GATT client/server roles govern access to application data after the link exists. Android’s BLE overview distinguishes these roles.
What happens when a BLE device connects?
- The Peripheral advertises. It sends advertising packets that may contain a name, service UUIDs, manufacturer data, or other small pieces of information.
- The Central scans. It listens on advertising channels and reports packets it receives to the application.
- The Central decides whether to connect. The advertisement must be connectable, and the device must pass any filters or connection rules.
- The Central sends a connection request. The advertiser becomes the Peripheral in the connection, and the initiator becomes the Central.
- The client discovers GATT services and characteristics. It learns which data the server exposes and which operations each item supports.
- The application exchanges data. It can read values, write values, receive notifications, or receive indications.
A successful scan does not guarantee a successful connection. The device may be broadcasting a non-connectable advertisement, already connected to another Central, using directed advertising for a different device, or applying an advertising filter policy. The Central may also reject it because of platform filters, unsupported features, or application logic. The discovery and connection sequence is described in the Bluetooth SIG’s Bluetooth LE Primer.
BLE advertising packets and the 31-byte limit
Legacy BLE advertising uses packet formats including these types, listed in the Bluetooth SIG’s Bluetooth LE Primer:
| PDU type | Purpose |
|---|---|
| ADV_IND | Undirected, connectable, and scannable advertising |
| ADV_DIRECT_IND | Directed, connectable advertising |
| ADV_NONCONN_IND | Undirected, non-connectable, non-scannable advertising |
| ADV_SCAN_IND | Undirected, scannable, non-connectable advertising |
| SCAN_REQ and SCAN_RSP | Request and provide additional scan-response data |
| CONNECT_IND | Legacy connection request |
The familiar 31-byte advertising limit applies to the advertising data area of a legacy advertisement, not the total radio packet structure or every modern BLE advertising mode.
Bluetooth 5 introduced extended advertising. The Bluetooth SIG’s Bluetooth LE Primer describes extended advertising packets of up to 255 octets and chained extended advertising carrying approximately 1,650 bytes. Most data moves to secondary channels 0 through 36, while the primary advertising packet includes an AuxPtr reference to the additional data.
Extended advertising is not guaranteed to work everywhere. The Bluetooth controller, operating system, phone model, and API must all expose and support the required features. A device marketed as supporting BLE does not automatically support every extended-advertising capability.
GATT, ATT, services, characteristics, and descriptors
BLE application data is organized using GATT. GATT is built on the Attribute Protocol (ATT), which transports attribute data over the connection. Android’s BLE overview describes this relationship.
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| Element | What it contains or does |
|---|---|
| Service | A collection of related characteristics |
| Characteristic | A value and a set of supported properties |
| Descriptor | Metadata or configuration associated with a characteristic |
A characteristic may support properties such as:
- Read: the client requests the current value.
- Write: the client sends a value to the server.
- Notify: the server sends updates without requiring a response for each update.
- Indicate: the server sends an update that the client acknowledges.
The Client Characteristic Configuration Descriptor (CCCD) is commonly used when a client enables notifications or indications. Discovering a characteristic does not necessarily enable updates; the client may first need to write the appropriate configuration value.
UUIDs identify services, characteristics, and descriptors. Standard Bluetooth SIG services commonly use assigned 16-bit identifiers represented within the Bluetooth base UUID. Custom application data generally uses a 128-bit UUID. A Bluetooth profile is an application specification, such as a standard way to represent heart-rate data; it is not the same as GATT itself. One BLE device can expose several services and implement more than one profile.
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Connection intervals, latency, and supervision timeout
Once connected, BLE communication is arranged into recurring connection events. The connection interval controls how often the radio may communicate for that link. Peripheral latency can allow a Peripheral to skip some events and sleep when it has nothing to send.
These settings create a trade-off:
- Shorter intervals usually improve responsiveness but increase radio activity and power use.
- Longer intervals reduce power use but can increase the time before data is delivered.
- Frequent notifications and large transfers keep the radio active and can reduce battery life.
The supervision timeout specifies how long the link can go without successfully received Link Layer data before it is considered lost. A connection can fail even when both devices are still physically close. Interference, a blocked radio path, a sleeping controller, or an overloaded system can prevent enough successful traffic from reaching the timeout threshold.
Bluetooth SIG’s supported Bluetooth functionality information describes the optional Shorter Connection Intervals feature, which can reduce the minimum LE connection interval from 7.5 milliseconds to 375 microseconds. This is an optional capability, not a universal change to every BLE connection.
Using BLE on Android
Android apps scan with BluetoothLeScanner. Android’s BLE scanning documentation shows these calls:
bluetoothLeScanner.startScan(leScanCallback)
bluetoothLeScanner.stopScan(leScanCallback)
Android’s documented example uses a 10-second scan timeout:
private val SCAN_PERIOD: Long = 10000
Stop scanning as soon as the required device is found. Android advises setting a time limit because scanning is battery-intensive. To connect to a discovered GATT server, an app commonly uses the call shown in Android’s GATT connection documentation:
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bluetoothGatt = device.connectGatt(this, false, bluetoothGattCallback)
Before scanning, check that Bluetooth is enabled. If it is disabled, getBluetoothLeScanner() can return null. Android’s documented BLE scanner is for BLE devices; it cannot scan for Bluetooth LE and Bluetooth Classic devices simultaneously through those scanner APIs.
Android permissions
For an app targeting Android 12, API level 31, or later, Android’s Bluetooth permissions documentation lists these relevant manifest permissions:
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- android.permission.BLUETOOTH_SCAN
- android.permission.BLUETOOTH_ADVERTISE
- android.permission.BLUETOOTH_CONNECT
These are runtime permissions presented to users under Nearby devices. Request only the permissions needed for the app’s operations.
For Android 10 and lower, legacy BLUETOOTH and BLUETOOTH_ADMIN permissions should be limited with maxSdkVersion set to 30. ACCESS_FINE_LOCATION is still required when an app uses scan results to derive the user’s physical location. If it does not derive location, it may declare BLUETOOTH_SCAN with the neverForLocation flag; Android warns that this can filter some BLE beacons from scan results.
For Android 10 and 11-targeting applications, location permissions and, in some background-discovery scenarios, background-location handling may also be required. Android’s Companion Device Manager can provide a device-selection and pairing flow on Android 8.0 and later without requiring location permission for that selection flow. See Android’s permission guidance for details.
Using BLE on iPhone, iPad, and Mac
Apple’s Core Bluetooth framework abstracts much of the low-level BLE stack. It provides APIs for discovering peripherals, making connections, discovering services and characteristics, and reading, writing, or subscribing to values. See Apple’s Core Bluetooth overview.
On iOS, BLE work is affected by app state. By default, an app cannot perform normal BLE work while suspended or in the background. An app that needs background BLE activity must declare the appropriate Core Bluetooth background execution mode, and background behavior still differs from foreground behavior.
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Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.BLE addresses are not permanent identities
A common programming mistake is treating a BLE MAC address as a permanent device identifier. BLE devices can use private, rotating addresses for privacy, so the address a scanner sees may change. This behavior is specified in the Bluetooth SIG’s Low Energy Link Layer Specification.
Applications should instead use an application-level identifier, a stable identity exposed through GATT where appropriate, or bonded-device identity handling. Device names are also unreliable: a name may be missing from the advertisement, truncated, duplicated, changed, or supplied only in a scan response.
Security considerations
BLE does not always require pairing. A beacon can broadcast without pairing, and a device can permit unsecured operations when its application does not require authentication.
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Pairing and bonding are relevant when a device needs authenticated access, encryption, or a remembered relationship between devices. The exact security method depends on the hardware, operating system, and application requirements.
Bluetooth security does not replace application security. Android warns in its BLE overview that data communicated through BLE pairing can be accessible to apps on the user’s device. Sensitive applications should consider application-layer authentication, authorization, integrity checks, and encryption rather than trusting the Bluetooth link alone.
Why a BLE device may not appear or connect
When troubleshooting BLE, separate discovery, connection, and GATT problems:
- Not visible: the device may not be advertising, may advertise infrequently, may be out of range, or may be filtered by permissions or scanner behavior.
- Visible but not connectable: the advertisement may be non-connectable, directed at another device, or associated with a device that already has a connection.
- Connected but no useful data: the client may be looking for the wrong service UUID, using an incorrect characteristic, or attempting an operation not listed in the characteristic properties.
- Connection drops: interference, range, controller limits, application overload, or the supervision timeout may be involved.
- Notifications never arrive: the client may not have enabled the characteristic’s CCCD, or the characteristic may support read/write but not notify or indicate.
Also check whether the phone supports the advertising mode and PHY being used. “Bluetooth 5” on both devices does not guarantee longer range, higher speed, or extended-advertising support. Actual behavior depends on the controller, PHY, antenna, transmit power, data-length support, operating system, and exposed APIs.
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Is BLE the same as Bluetooth Classic?
No. BLE and Bluetooth Classic use different link-layer procedures and application protocols. BLE is optimized for low-power, intermittent data exchange, while Bluetooth Classic is commonly used for continuous higher-throughput connections such as audio.
Does BLE require pairing?
No. Advertising and some connection-based operations can work without pairing. Pairing or bonding is needed only when the application requires particular security or identity behavior.
Is BLE one-way?
Only advertising is one-way and connectionless. An LE ACL connection is bidirectional, allowing a client to read, write, and receive notifications or indications.
What is the maximum BLE advertising payload?
Legacy advertising provides up to 31 octets of advertising data. The Bluetooth SIG’s Bluetooth LE Primer describes extended advertising packets up to 255 octets and chained advertising carrying approximately 1,650 bytes. Support varies by device and software.
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Why can I see a BLE device but not connect to it?
It may be using a non-connectable advertisement, already have another connection, use directed advertising, apply a filter policy, or be incompatible with the Central or application. Being visible does not mean that a device accepts every connection request.
Does Bluetooth 5 guarantee better BLE range and speed?
No. Bluetooth version numbers indicate supported features, not guaranteed results. Range and speed depend on the PHY, antenna, transmit power, controller, data length, radio environment, and peer-device support.
Can I use a BLE MAC address as a permanent identifier?
Usually not. Privacy features can rotate device addresses. Use an application-level identifier, a suitable GATT identity, or bonded-device identity management instead.
The Bottom Line
BLE is a low-power wireless transport built around advertising, optional connections, and GATT data exchange. A device may broadcast without connecting, or a Central may connect to a Peripheral and use GATT as a client to access services and characteristics. Understanding these separate layers—advertising, link roles, GATT roles, permissions, timing, and security—helps prevent common BLE design and troubleshooting mistakes.
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