Bluetooth is one of the most popular wireless protocols, and it has been available in smartphones, computers, and other devices for over a decade. Most of us are familiar with Bluetooth and how Bluetooth can allow us to connect a headset and make calls using our cellphones. The explosive growth in Bluetooth devices led the Bluetooth SIG and other companies to the realization that Bluetooth consumed too much power and took too long to connect in some applications. For example, a key chain finder using Bluetooth would not run very long and that it could take significant time for it to connect, frustrating users.
Bluetooth v4.0 introduced Bluetooth Low Energy in 2014, officially known as Bluetooth Smart. This specification introduced a completely different wireless radio that was smaller, cheaper and lower power to meet the needs of these new applications. The wide support for it in Smartphones, Tablets that makes BLE applications easy to deploy and easy for consumers to use.
Apple’s support for BLE in their devices, starting with iPhone 4s, opened the way to a massive number of small, battery operated devices.
Before this, using Classic Bluetooth both caused a large battery drain and also usually required an authentication chip which was costly and made these products too expensive. BLE is supported without this requirement in Apple devices.
One of the most powerful aspects of BLE is how extensible it is and how it allows any developer with an idea and a need to exchange information to do so, as opposed to the rigid structure of Classic Bluetooth.
Wireless Protocol Comparison
Bluetooth and BLE are great protocols that can simplify connectivity with products. But, it’s important to understand where they stand alongside other wireless technologies. WiFi, Zigbee, and other protocols are better at certain applications, and BLE can’t always be used. Below, we look at some of the characteristics that differentiate BLE from them:
|Frequency Band||2.4GHz||2.4GHz / 5GHz||2.4GHz|
|Range||<100m||<300m||< 100m Point to Point, More with Meshing|
|Data Rate||1Mbps||11Mbps, 54Mbps, 150Mbps+||250kbps|
|Peak Current Consumption||<15mA||60mA RX, 200mA TX||19mA RX, 35mA TX|
|Standby Current||< 2uA||< 100uA||5uA|
Zigbee, WiFi and BLE all use the 2.4GHz ISM band, but are all significantly different in their capabilities. The BLE radio is clearly a shorter range device that consumes less power than Zigbee and especially WiFi. The lower peak current consumption is critical when choosing a battery. WiFi, with up to 200mA or more of peak TX current could never operate out of a coin cell battery. BLE’s peak, however, is much lower.
As opposed to Zigbee, BLE cannot currently form a mesh network if the spec is followed. CSR, however, has created CSR Mesh which extends BLE to allow mesh communications. Typically, Point to Point connectivity means that the devices has to be in range for the smartphone to control it. There are solutions using a BLE gateway that connects BLE devices to a router, but these add costs that may not be acceptable in many applications.
BLE Physical Layer
Let’s look at some more details about BLE’s Physical layer specifications. The Physical layer relates directly to the way in which a BLE devices wirelessly transmits and receives data.
BLE uses the same 2.4GHz ISM band that is common since using it is “unlicensed”. This band starts at 2400MHz and continues until 2483.5MHz. The Bluetooth v4.0 and subsequent specifications divide the band into 40 channels. 3 Of these channels are called “advertising” and are used by devices to send advertising packets with information about them so other BLE devices can connect. These channels were selected at the lower upper and middle of the band to avoid interference which may interfere with a number of channels. For example, if Channel 38 and its surrounding channels are being interfered by WiFi, then there are still 2 other advertising channels 37 and 39 that will not be affected.
Understanding more about the physical layer isn’t needed, but a few words about the RF are in order. Like all wireless transmitters, BLE devices transmit and receive Radio Frequency signals which require an Antenna and careful design. Antenna design is a complicated topic, but practically any 2.4GHz antenna can be used. PCB and Chip Antennas are among the most common ones. We’ll discuss some of this later.
BLE Link Layer
The real operation of BLE happens on what is called the Link layer. This is the layer that manages connections, controls packet sending and receiving, etc. A BLE device has several states that it can be in (only one state is technically
allowed at a time):
- Standby – Basically the device isn’t transmitting or receiving. Usually associated with the system being asleep to conserve power
- Advertising – A device that has a Peripheral Role will enter the Advertising state where it will send packets on the advertising channels. In this state it will also listen to any responses to the packets from a Central device. This
mode is one of the most critical to understand from a power perspective because a Peripheral device will spend much of its time in Advertising mode (depending on application). The Advertising interval then directly affects power
consumption and life.
- Scanning – Scanning refers to listening to advertising packets that are sent over those channels. This mode is used to scan for devices
- Initiating – This state is the state a Central device usually enters before a connection is established. The Central device will listen for advertisements on peripherals, but once the advertisement from the desired peripheral
device is received, the Central may connect by sending the right data
For the slave device, the Advertising state is also the initial state before the Connection state. The connection state is the final state in which the Slave (Peripheral) and Master (Central) can exchange data.
In BLE, data is actually exchanged periodically on Connection Events.
Data is transmitted on the data channels which are the 37 channels not used for advertising. Both devices agree on the channels to use and alternate sending of data, with some rules as set by the specification.
A typical BLE application involves a BLE chip connected to various sensors. A user walks into the BLE range. For the User’s smartphone to connect and read the sensor data, the BLE device needs to be advertising.
Once the Smartphone receives the advertisement packet, it will begin the process to connect and obtain the sensor data. If the advertising interval is set too high, the devices will take time connecting and the user will experience this delay. If the advertising delay is too short, the frequent TX and RX will drain the battery more quickly.
Now that we know a bit about BLE, we can see that there are two ways to get data from a device:
- Advertisement and Scan Responses – Advertisement include payload fields that can provide information, and the central device can request more information. There is no security inherent in this method because broadcasts are public, but multiple devices can receive the information at the same time.
- Connections – During a connection, two devices connect and exchange information, which allows much more information to be transmitted than using other methods.