Bluetooth Low Energy (Bluetooth LE, BLE, formerly marketed as Bluetooth Smart) is a wireless personal area network technology designed and marketed by the Bluetooth Special Interest Group aimed at novel applications in the healthcare, fitness, beacons, security, and home entertainment industries. Compared to Classic Bluetooth, Bluetooth Low Energy is intended to provide considerably reduced power consumption and cost while maintaining a similar communication range.
Mobile operating systems including iOS, Android, Windows Phone and BlackBerry, as well as macOS, Linux, Windows 8 and Windows 10, natively support Bluetooth Low Energy. The Bluetooth SIG predicts that by 2018 more than 90 percent of Bluetooth-enabled smartphones will support Bluetooth Low Energy.
Bluetooth Low Energy is not backward-compatible with the previous (often called "Classic") Bluetooth protocol. The Bluetooth 4.0 specification permits devices to implement either or both of the LE and Classic systems.
In 2011, the Bluetooth Special Interest Group (SIG) announced the Bluetooth Smart logo so as to clarify compatibility between the new low energy devices and other Bluetooth devices.
- Bluetooth Smart Ready indicates a dual-mode device compatible with both Classic and low energy peripherals.
- Bluetooth Smart indicates a low energy-only device which requires either a Smart Ready or another Smart device in order to function.
With the May 2016 Bluetooth SIG branding information, the Bluetooth SIG began phasing out the Bluetooth Smart and Bluetooth Smart Ready logos and word marks and has reverted to using the Bluetooth logo and word mark. The logo uses a new blue color.
The Bluetooth SIG identifies a number of markets for low energy technology, particularly in the smart home, health, sport and fitness sectors. Cited advantages include:
- low power requirements, operating for "months or years" on a button cell
- small size and low cost
- compatibility with a large installed base of mobile phones, tablets and computers
In 2001, researchers at Nokia determined various scenarios contemporary wireless technologies did not address. The company began developing a wireless technology adapted from the Bluetooth standard which would provide lower power usage and cost while minimizing its differences from Bluetooth technology. The results were published in 2004 using the name Bluetooth Low End Extension.
After further development with partners in particular Logitech and within EU FP6 project MIMOSA, actively promoted and supported also by STMicroelectronics since its early stage, technology was released to the public in October 2006 with the brand name Wibree. After negotiations with Bluetooth SIG members, an agreement was reached in June 2007 to include Wibree in future Bluetooth specification as a Bluetooth ultra-low-power technology, known as Bluetooth Low Energy technology.
Integration of Bluetooth Low Energy with version 4.0 of the Core Specification was completed in early 2010. The first smartphone to implement the 4.0 specification was the iPhone 4S, released in October 2011. A number of other manufacturers released Bluetooth Low Energy Ready devices in 2012.
The Bluetooth SIG officially unveiled Bluetooth 5 on June 16, 2016 during a media event in London. One change on the marketing side is that they dropped the point number, so it now just called Bluetooth 5 (and not Bluetooth 5.0 or 5.0 LE like for Bluetooth 4.0). This decision was made allegedly to “simplifying marketing, and communicating user benefits more effectively”. On the technical side, Bluetooth 5 will quadruple the range by using increased transmit power or coded physical layer, double the speed by using optional half of the symbol time compared to Bluetooth 4.x, and provide an eight-fold increase in data broadcasting capacity by increasing the advertising data length of low energy Bluetooth transmissions compared to Bluetooth 4.x, which could be important for IoT applications where nodes are connected throughout a whole house.
The Bluetooth SIG released Mesh Profile and Mesh Model specifications officially on July 18, 2017. Mesh specification enables using Bluetooth Low Energy for many-to-many device communications for home automation, sensor networks and other applications.
Borrowing from the original Bluetooth specification, the Bluetooth SIG defines several profiles — specifications for how a device works in a particular application — for low energy devices. Manufacturers are expected to implement the appropriate specifications for their device in order to ensure compatibility. A device may contain implementations of multiple profiles.
All current low energy application profiles are based on the generic attribute profile (GATT), a general specification for sending and receiving short pieces of data known as attributes over a low energy link. Bluetooth 4.0 provides low power consumption with higher bit rates.
In 2014, Cambridge Silicon Radio (now part of Qualcomm) launched CSR Mesh. CSR Mesh protocol uses Bluetooth Low Energy to communicate with other Bluetooth Low Energy devices in the network. Each device can pass the information forward to other Bluetooth Low Energy devices creating a “mesh” effect. For example, switching off an entire building of lights from a single smartphone. The Bluetooth SIG recently formed the Smart Mesh study group to research and define its use cases in an effort to define a standard specification.
Health care profiles
There are many profiles for Bluetooth Low Energy devices in healthcare applications. The Continua Health Alliance consortium promotes these in cooperation with the Bluetooth SIG.
- BLP (Blood Pressure Profile) — for blood pressure measurement.
- HTP (Health Thermometer Profile) — for medical temperature measurement devices.
- GLP (Glucose Profile) — for blood glucose monitors.
- CGMP (Continuous Glucose Monitor Profile)
Sports and fitness profiles
Profiles for sporting and fitness accessories include:
- BCS (Body Composition Service)
- CSCP (Cycling Speed and Cadence Profile) — for sensors attached to a bicycle or exercise bike to measure cadence and wheel speed.
- CPP (Cycling Power Profile)
- HRP (Heart Rate Profile) — for devices which measure heart rate
- LNP (Location and Navigation Profile)
- RSCP (Running Speed and Cadence Profile)
- WSP (Weight Scale Profile)
- IPSP (Internet Protocol Support Profile)
- ESP (Environmental Sensing Profile)
- UDS (User Data Service)
- HOGP (HID over GATT Profile) allowing Bluetooth LE-enabled Wireless mice, keyboards and other devices offering long-lasting battery life.
"Electronic leash" applications are well suited to the long battery life possible for 'always-on' devices. Manufacturers of iBeacon devices implement the appropriate specifications for their device to make use of proximity sensing capabilities supported by Apple Inc. compatible iDevices.
Relevant application profiles include:
- FMP — the "find me" profile — allows one device to issue an alert on a second misplaced device.
- PXP — the proximity profile — allows a proximity monitor to detect whether a proximity reporter is within a close range. Physical proximity can be estimated using the radio receiver's RSSI value, although this does not have absolute calibration of distances. Typically, an alarm may be sounded when the distance between the devices exceeds a set threshold.
Alerts and time profiles
- The phone alert status profile and alert notification profile allow a client device to receive notifications such as incoming call alerts from another device.
- The time profile allows current time and time zone information on a client device to be set from a server device, such as between a wristwatch and a mobile phone's network time.
- The Battery Service exposes the Battery State and Battery Level of a single battery or set of batteries in a device.
Starting in late 2009, Bluetooth Low Energy integrated circuit implementations were announced by a number of manufacturers. Implementations commonly use software radio so updates to the specification can be accommodated through a firmware upgrade.
Current mobile devices are commonly released with hardware and software support for both Classic Bluetooth and the Bluetooth Low Energy.
Bluetooth Low Energy technology operates in the same spectrum range (the 2.400–2.4835 GHz ISM band) as Classic Bluetooth technology, but uses a different set of channels. Instead of the Classic Bluetooth 79 1-MHz channels, Bluetooth Low Energy has 40 2-MHz channels. Within a channel, data is transmitted using Gaussian frequency shift modulation, similar to Classic Bluetooth's Basic Rate scheme. The bit rate is 1 Mbit/s, and the maximum transmit power is 10 mW. Further details are given in Volume 6 Part A (Physical Layer Specification) of the Bluetooth Core Specification V4.0.
Bluetooth Low Energy uses frequency hopping to counteract narrowband interference problems. Classic Bluetooth also uses frequency hopping but the details are different; as a result, while both FCC and ETSI classify Bluetooth technology as an FHSS scheme, Bluetooth Low Energy is classified as a system using digital modulation techniques or a direct-sequence spread spectrum.
|Distance/range (theoretical max.)||100 m (330 ft)||>100 m (>330 ft)|
|Over the air data rate||1–3 Mbit/s||125 kbit/s – 1 Mbit/s – 2 Mbit/s|
|Application throughput||0.7–2.1 Mbit/s||0.27 Mbit/s|
|Active slaves||7||Not defined; implementation dependent|
|Security||56/128-bit and application layer user defined||128-bit AES with Counter Mode CBC-MAC and application layer user defined|
|Robustness||Adaptive fast frequency hopping, FEC, fast ACK||Adaptive frequency hopping, Lazy Acknowledgement, 24-bit CRC, 32-bit Message Integrity Check|
|Latency (from a non-connected state)||Typically 100 ms||6 ms|
|Minimum total time to send data (det. battery life)||100 ms||3 ms|
|Power consumption||1 W as the reference||0.01–0.50 W (depending on use case)|
|Peak current consumption||<30 mA||<15 mA|
|Primary use cases||Mobile phones, gaming, headsets, stereo audio streaming, smart homes, wearables, automotive, PCs, security, proximity, healthcare, sports & fitness, etc.||Mobile phones, gaming, smart homes, wearables, automotive, PCs, security, proximity, healthcare, sports & fitness, Industrial, etc.|
More technical details may be obtained from official specification as published by the Bluetooth SIG. Note that power consumption is not part of the Bluetooth specification.
Advertising and discovery
BLE devices are detected through a procedure based on broadcasting advertising packets. This is done using 3 separate channels (frequencies), in order to reduce interference. The advertising device sends a packet on at least one of these three channels, with a repetition period called the advertising interval. For reducing the chance of multiple consecutive collisions, a random delay of up to 10 milliseconds is added to each advertising interval. The scanner listens to the channel for a duration called the scan window, which is periodically repeated every scan interval.
The discovery latency is therefore determined by a probabilistic process and depends on the three parameters (viz., the advertising interval, the scan interval and the scan window). The discovery scheme of BLE adopts a periodic-interval based technique, for which upper bounds on the discovery latency can be inferred for most parametrizations. While the discovery latencies of BLE can be approximated by models for purely periodic interval-based protocols, the random delay added to each advertising interval and the three-channel discovery can cause deviations from these predictions, or potentially lead to unbounded latencies for certain parametrizations.
All Bluetooth Low Energy devices use the Generic Attribute Profile (GATT). The application programming interface offered by a Bluetooth Low Energy aware operating system will typically be based around GATT concepts. GATT has the following terminology:
Some service and characteristic values are used for administrative purposes – for instance, the model name and serial number can be read as standard characteristics within the Generic Access service. Services may also include other services as sub-functions; the main functions of the device are so-called primary services, and the auxiliary functions they refer to are secondary services.
Services, characteristics, and descriptors are collectively referred to as attributes, and identified by UUIDs. Any implementer may pick a random or pseudorandom UUID for proprietary uses, but the Bluetooth SIG have reserved a range of UUIDs (of the form xxxxxxxx-0000-1000-8000-00805F9B34FB ) for standard attributes. For efficiency, these identifiers are represented as 16-bit or 32-bit values in the protocol, rather than the 128 bits required for a full UUID. For example, the Device Information service has the short code 0x180A, rather than 0000180A-0000-1000-... . The full list is kept in the Bluetooth Assigned Numbers document online.
The GATT protocol provides a number of commands for the client to discover information about the server. These include:
- Discover UUIDs for all primary services
- Find a service with a given UUID
- Find secondary services for a given primary service
- Discover all characteristics for a given service
- Find characteristics matching a given UUID
- Read all descriptors for a particular characteristic
Commands are also provided to read (data transfer from server to client) and write (from client to server) the values of characteristics:
- A value may be read either by specifying the characteristic's UUID, or by a handle value (which is returned by the information discovery commands above).
- Write operations always identify the characteristic by handle, but have a choice of whether or not a response from the server is required.
- 'Long read' and 'Long write' operations can be used when the length of the characteristic's data exceeds the MTU of the radio link.
Finally, GATT offers notifications and indications. The client may request a notification for a particular characteristic from the server. The server can then send the value to the client whenever it becomes available. For instance, a temperature sensor server may notify its client every time it takes a measurement. This avoids the need for the client to poll the server, which would require the server's radio circuitry to be constantly operational.
An indication is similar to a notification, except that it requires a response from the client, as confirmation that it has received the message.
Bluetooth Low Energy is designed to enable devices with low power consumption. Several chipmakers including Cambridge Silicon Radio, Dialog Semiconductor, Nordic Semiconductor, STMicroelectronics, Cypress Semiconductor, and Texas Instruments have introduced their Bluetooth Low Energy optimized chipsets over the last few years. Devices with peripheral and central roles have different power requirements. A study by beacon software company, Aislelabs, reported that peripherals, such as proximity beacons, usually function for 1–2 years with a 1,000mAh coin cell battery. This is possible because of power efficiency of Bluetooth Low Energy protocol which only transmits small packets as compared to Bluetooth Classic which is also suitable for audio and high bandwidth data.
In contrast, a continuous scan for the same beacons in central role can consume 1,000 mAh in a few hours. Android and iOS devices also have very different battery impact depending on type of scans and number of Bluetooth Low Energy devices in the vicinity. With the newer chipsets and advances in software, both Android and iOS phones now have negligible power consumption in real-life Bluetooth Low Energy use scenarios.
GATT is described in full in Volume 3, Part G of the Bluetooth 4.0 Core Specification.