bluetooth-le-overview.qdoc

Go to the documentation of this file.

// Copyright (C) 2017 The Qt Company Ltd.
// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
 
/*!
\ingroup technology-apis
\title Bluetooth Low Energy Overview
\page qtbluetooth-le-overview.html
\brief The Qt Bluetooth Low Energy API enables communication between Bluetooth
Low Energy devices.
 
\tableofcontents
 
    The Qt Bluetooth Low Energy API supports the peripheral/server and central/client roles.
    It is supported on all major Qt platforms. The only exception is the missing peripheral role
    support on Windows.
 
    \section1 What Is Bluetooth Low Energy
 
    Bluetooth Low Energy, also known as Bluetooth Smart, is a wireless computer
    network technology, which was officially introduced in 2011. It works on the same
    2.4 GHz frequency as ”classic” Bluetooth. The main difference is, as stated by its technology name,
    low energy consumption. It provides an opportunity for Bluetooth Low Energy devices to
    operate for months, even years, on coin-cell batteries. The technology was introduced by
    \l {https://www.bluetooth.org/en-us/specification/adopted-specifications}{Bluetooth v4.0}.
    Devices which support this technology are called Bluetooth Smart Ready Devices.
    The key features of the technology are:
 
    \list
    \li Ultra-low peak, average and idle mode power consumption
    \li Ability to run for years on standard, coin-cell batteries
    \li Low cost
    \li Multi-vendor interoperability
    \li Enhanced range
    \endlist
 
    Bluetooth Low Energy uses a client-server architecture. The server (also
    known as peripheral) offers services such as temperature or heart rate
    and advertises them. The client (known as central
    device) connects to the server and reads the values advertised by the server.
    An example might be an apartment with Bluetooth Smart Ready sensors such
    as a thermostat, humidity or pressure sensor. Those sensors are peripheral
    devices advertising the environment values of the apartment. At the same time
    a mobile phone or computer might connect to those sensors, retrieve their
    values and present them as part of a larger environment control application
    to the user.
 
    \section1 Basic Service Structure
 
    Bluetooth Low Energy is based on two protocols: ATT (Attribute Protocol)
    and GATT (Generic Attribute Profile). They specify the communication layers
    used by every Bluetooth Smart Ready device.
 
    \section2 ATT Protocol
 
    The basic building block of ATT is an \e attribute. Each attribute consists of
    three elements:
 
    \list
        \li a value - the payload or desirable piece of information
        \li a UUID - the type of attribute (used by GATT)
        \li a 16-bit handle - a unique identifier for the attribute
    \endlist
 
    The server stores the attributes and the client uses the ATT protocol to
    read and write values on the server.
 
    \section2 GATT Profile
 
    GATT defines grouping for a set of attributes by applying a meaning to predefined
    UUIDs. The table below shows an example service exposing a heart rate
    on a particular day. The actual values are stored inside the two characteristics:
 
    \table
    \header
        \li Handle
        \li UUID
        \li Value
        \li Description
    \row
        \li 0x0001
        \li 0x2800
        \li UUID 0x180D
        \li Begin Heart Rate service
    \row
        \li 0x0002
        \li 0x2803
        \li UUID 0x2A37, Value handle: 0x0003
        \li Characteristic of type \e {Heart Rate Measurement (HRM)}
    \row
        \li 0x0003
        \li 0x2A37
        \li 65 bpm
        \li Heart rate value
    \row
        \li 0x0004
        \li 0x2803
        \li UUID 0x2A08, Value handle: 0x0005
        \li Characteristic of type Date Time
    \row
        \li 0x0005
        \li 0x2A08
        \li 18/08/2014 11:00
        \li Date and Time of the measurement
    \row
        \li 0x0006
        \li 0x2800
        \li UUID xxxxxx
        \li Begin next service
    \row
        \li ...
        \li ...
        \li ...
        \li ...
    \endtable
 
    GATT specifies that the above used UUID \c 0x2800 marks the begin of a service definition.
    Every attribute following \c 0x2800 is part of the service until the next \c 0x2800 or the
    end is encountered. In similar ways the well known UUID \c 0x2803 states that a characteristic
    is to be found and each of the characteristics has a type defining the nature of the value.
    The example above uses the UUIDs \c 0x2A08 (Date Time) and \c 0x2A37 (Heart Rate Measurement).
    Each of the above UUIDs is defined by the \l {https://bluetooth.org}{Bluetooth Special Interest Group}.
    and can be found in the
    \l{https://www.bluetooth.com/specifications/assigned-numbers}{GATT specifications}. While it
    is advisable to use pre-defined UUIDs where available it is entirely possible to use new and not
    yet used UUIDs for characteristic and service types.
 
    In general, each service may consist of one or more characteristics. A characteristic
    contains data and can be further described by descriptors, which provide additional
    information or means of manipulating the characteristic. All services, characteristics and
    descriptors are recognized by their 128-bit UUID. Finally, it is possible to include
    services inside of services (see picture below).
 
    \image peripheral-structure.png
 
    \section1 Using Qt Bluetooth Low Energy API
 
    This section describes how to use the Bluetooth Low Energy API provided by Qt.
    On the client side, the API permits creating connections to peripheral devices, discovering
    their services, as well as reading and writing data stored on the device.
    On the server side, it allows to set up services, advertise them, and get notified when the
    client writes characteristics.
    The example code below is taken from the \l {heartrate-game}{Heart Rate Game} and
    \l {heartrate-server}{Heart Rate Server} examples.
 
    \section2 Establishing a Connection
 
    To be able to read and write the characteristics of the Bluetooth Low Energy peripheral device,
    it is necessary to find and connect the device. This requires the peripheral device to advertise
    its presence and services. We start the device discovery with the help of the
    \l QBluetoothDeviceDiscoveryAgent class. We connect to its \l {QBluetoothDeviceDiscoveryAgent::deviceDiscovered()}
    signal and start the search with \l {QBluetoothDeviceDiscoveryAgent::start()}{start()}:
 
    \snippet heartrate-game/devicefinder.cpp devicediscovery-1
    \snippet heartrate-game/devicefinder.cpp devicediscovery-2
 
    Since we are only interested in Low Energy devices we filter the device type within the
    receiving slot. The device type can be ascertained using the \l QBluetoothDeviceInfo::coreConfigurations()
    flag. The \l {QBluetoothDeviceDiscoveryAgent::}{deviceDiscovered()} signal
    may be emitted multiple times for the same device as more details are
    discovered. Here we match these device discoveries so that the user only
    sees the individual devices:
 
    \snippet heartrate-game/devicefinder.cpp devicediscovery-3
    \snippet heartrate-game/devicefinder.cpp devicediscovery-4
 
    Once the address of the peripheral device is known we use the \l QLowEnergyController class.
    This class is the entry point for all Bluetooth Low Energy development. The  constructor of the class
    accepts the remote device's \l QBluetoothAddress. Finally we set up the customary slots and
    directly connect to the device using
    \l {QLowEnergyController::connectToDevice()}{connectToDevice()}:
 
    \snippet heartrate-game/devicehandler.cpp Connect-Signals-1
    \snippet heartrate-game/devicehandler.cpp Connect-Signals-2
 
    \section2 Service Search
 
    The above code snippet shows how the application initiates the service discovery once the
    connection has been established.
 
    The \c serviceDiscovered() slot below is triggered as a result of the
    \l {QLowEnergyController::serviceDiscovered()} signal and provides an intermittent progress report.
    Since we are talking about the heart listener app which monitors HeartRate devices in the vicinity
    we ignore any service that is not of type \l QBluetoothUuid::ServiceClassUuid::HeartRate.
 
    \snippet heartrate-game/devicehandler.cpp Filter HeartRate service 1
 
    Eventually the \l {QLowEnergyController::discoveryFinished()} signal is emitted to indicate
    the successful completion of the service discovery. Provided a HeartRate service was found,
    a \l QLowEnergyService instance is created to represent the service. The returned service object
    provides the required signals for update notifications and the discovery of service details
    is triggered using \l QLowEnergyService::discoverDetails():
 
    \snippet heartrate-game/devicehandler.cpp Filter HeartRate service 2
 
    During the detail search the service's \l {QLowEnergyService::state()}{state()} transitions
    from \l {QLowEnergyService::RemoteService}{RemoteService} to
    \l {QLowEnergyService::RemoteServiceDiscovering}{RemoteServiceDiscovering} and eventually ends with
    \l {QLowEnergyService::RemoteServiceDiscovered}{RemoteServiceDiscovered}:
 
    \snippet heartrate-game/devicehandler.cpp Find HRM characteristic
 
    \section2 Interaction with the Peripheral Device
 
    In the code example above, the desired characteristic is of type
    \l {QBluetoothUuid::CharacteristicType::HeartRateMeasurement}{HeartRateMeasurement}. Since the application measures
    the heart rate changes, it must enable change notifications for the characteristic.
    Note that not all characteristics provide change notifications. Since the HeartRate characteristic
    has been standardized it is possible to assume that notifications can be received. Ultimately
    \l QLowEnergyCharacteristic::properties() must have the \l {QLowEnergyCharacteristic::Notify} flag
    set and a descriptor of type \l {QBluetoothUuid::DescriptorType::ClientCharacteristicConfiguration} must exist to confirm
    the availability of an appropriate notification.
 
    Finally, we process the value of the HeartRate characteristic, as per Bluetooth Low Energy standard:
 
    \snippet heartrate-game/devicehandler.cpp Reading value
 
    In general a characteristic value is a series of bytes. The precise interpretation of
    those bytes depends on the characteristic type and value structure.
    A significant number has been standardized by the
    \l {https://developer.bluetooth.org/gatt/services/Pages/ServicesHome.aspx}{Bluetooth SIG} whereas others
    may follow a custom protocol. The above code snippet demonstrates how to the read the standardized
    HeartRate value.
 
    \section2 Advertising Services
 
    If we are implementing a GATT server application on a peripheral device, we need to define the
    services we want to offer to central devices and advertise them:
 
    \snippet heartrate-server/main.cpp Advertising Data
    \snippet heartrate-server/main.cpp Start Advertising
 
    Now potential clients can connect to our device, discover the provided service and
    register themselves to get notified of changes to the characteristic value.
    This part of the API was already covered by the above sections.
 
    \section2 Implementing a Service on the Peripheral Device
 
    The first step is to define the service, its characteristics and descriptors. This is achieved
    using the \l QLowEnergyServiceData, \l QLowEnergyCharacteristicData and
    \l QLowEnergyDescriptorData classes. These classes act as containers or building blocks for the
    essential information that comprises the to-be-defined Bluetooth Low Energy service.
    The code snippet below defines a simple HeartRate service which publishes
    the measured beats per minute. An example where such a service could be used is a wrist watch.
 
    \snippet heartrate-server/main.cpp Service Data
 
    The resulting \c serviceData object can be published as described in the
    \l {Advertising Services} section above. Despite the partial information overlap between the
    information wrapped by \l QLowEnergyServiceData and \l QLowEnergyAdvertisingData the two classes
    serve two very different tasks. The advertising data is published to nearby devices and often
    limited in scope due to its size restriction of 29 bytes. Therefore they are not always 100%
    complete. By comparison the service data contained inside of \l QLowEnergyServiceData provides
    the complete set of service data and only becomes visible to the connecting client when a
    connection with an active service discovery has been performed.
 
    The next section demonstrates how the service can update the heart rate value. Depending on the
    nature of the service it may have to comply with the official service definition
    as defined on \l {https://www.bluetooth.org}. Other services may be completely custom. The
    heart rate service was adopted and its specification can be found under
    \l {https://www.bluetooth.com/specifications/adopted-specifications}.
 
    \snippet heartrate-server/main.cpp Provide Heartbeat
 
    In general characteristic and descriptor value updates on the peripheral device use the same
    methods as connecting Bluetooth Low Energy devices.
 
    \note To use \l{Qt Bluetooth} (in both central and peripheral roles) on iOS, you have to provide
    an Info.plist file containing the usage description. According to the CoreBluetooth's
    documentation: \e {Your app will crash if its Info.plist doesn’t include usage description
    keys for the types of data it needs to access. To access Core Bluetooth APIs on apps linked
    on or after iOS 13, include the NSBluetoothAlwaysUsageDescription key. In iOS 12 and earlier,
    include NSBluetoothPeripheralUsageDescription to access Bluetooth peripheral data.}
*/