Understanding BLE: The Bluetooth Low Energy Protocol

Bluetooth LE is a Bluetooth protocol designed specifically for ultra-low power applications. In this article, we will explore how BLE has become an energy-efficient short-range wireless connection technology. Before diving into the details of BLE, let’s first understand what a communication protocol is.

What Is a Communication Protocol?

Over time, computing has evolved from standalone systems to computers connected in networks (Figure 1).

Understanding BLE: The Bluetooth Low Energy Protocol

Figure 1. A simplified diagram of connected computers in a Local Area Network (LAN).

Communication networks support shared computing, allowing many users or terminals to access the same computer system. In specific scenarios, shared computing enables a central system to handle primary tasks and distribute results to multiple users lacking that capability.

Communication networks are also used for distributed computing, where many computers can work together as peers. These networks help transmit data from one point to another. For example, any device you use to read this webpage can wirelessly connect to web servers around the world. From there, information is transferred from the server to your device, allowing you to read this article.

One important point to note is that for different devices in a network to communicate effectively or share data, they must use a common language. This is where communication protocols come into play.

Figure 2. A conceptual view of protocols as a common language that allows different devices to communicate.

A communication protocol is a rulebook that strictly defines how the hardware and software of devices should perform each communication function when exchanging information.

In a communication protocol (also known as a rulebook), you will find rules regarding:

How devices should identify themselves
How they should establish, maintain, and terminate connections
What types of data they can send and how they should format it
How they ensure data is correctly received and understood at both ends
What modulation and channels devices should use
What error coding techniques devices should employ
How the exchanged information should be protected
Communication protocols are based on various factors, including the characteristics of the information to be exchanged, the communication medium, and the capabilities of the devices that will use it.

From Protocol to Standard

Communication protocols can become standards through a standardization process. Standards are agreed-upon or widely adopted protocols that must be adhered to, regardless of the manufacturer. They allow different companies producing related products to follow the same protocol, enabling their devices to communicate.

This is why you can connect a Bluetooth headset from one company to a smartphone from another company. Although the manufacturers of these products are different, they both adhere to a standardized communication protocol, in this case, Bluetooth.

In addition to Bluetooth, there are several other popular communication standards:

Ethernet
Transmission Control Protocol/Internet Protocol (TCP/IP)
Wireless Networking
Lora
Bluetooth Low Energy

So far, you have only received some hints about BLE. As you can see, Bluetooth Low Energy is listed under standardized communication protocols. Now, let’s talk about this.

What Is Bluetooth Low Energy?

At its core, Bluetooth is a short-range connection technology that uses radio waves as the communication medium. The first standard or specification of this technology is called Bluetooth Classic. Its primary design goal was to replace wires and provide wireless connectivity between mobile phones and other portable devices.

BLE was launched in 2010 as part of the Bluetooth 4.0 specification, optimized for ultra-low power applications. Due to its use in ultra-low power applications, BLE serves the market for battery-powered devices that require wireless networking capabilities. Before BLE emerged, it was challenging for these types of devices to support interconnectivity because they consumed too much battery power.

Next, we will introduce some optimized features of Bluetooth LE to achieve ultra-low power goals.

Bluetooth Low Energy Power-Saving Features

1. LE Radio Speaks Less

Devices enabled with Bluetooth LE wake their radio only when necessary to save power. When a device wants to send or listen for data, the LE radio turns on to quickly perform the necessary tasks and then disconnects. This operation of the LE radio is different from that of the Bluetooth Classic radio, which remains on most of the time and maintains connections for hours or days.

Since the LE radio is not active frequently, it is suitable for applications where devices send small packets of data occasionally, ranging from once per second to once every few days. For example, a heart rate monitor in a fitness tracker can collect all heart rate data and send it to your smartphone once per hour. Or a temperature sensor might only be triggered to send temperature readings if the temperature is very high or very low.

2. LE Radio Has Shorter Connection Times

Bluetooth LE consumes less energy due to a reduced number of RF channels available for connections, resulting in faster connections and less scanning time. Bluetooth Classic has 32 RF channels available for establishing connections between devices, while Bluetooth LE has only three.

Additionally, BLE devices that want to be discovered send signals on the RF37, RF38, and RF39 channels (known as the primary advertising channels).

When a device wants to find other devices, it listens for advertising data packets on the primary advertising channels. Because there are only three primary advertising channels, the LE radio does not have to scan as many channels, reducing the time it is awake and therefore the energy used.

3. LE Radio Uses Smaller Data Packets

Bluetooth LE packets are much smaller than Bluetooth Classic packets. The smaller packet size requires less computational overhead during encoding and decoding, which reduces power consumption.

4. Battery Discharge Is Pulsed

One characteristic of batteries is that the way they discharge affects their capacity. Battery capacity is the energy that can be extracted from a battery under specific conditions. Constant current discharge shortens battery life. On the other hand, pulsed discharge (where there are idle times between discharges) helps keep the battery as close to its rated capacity as possible. This is known as the battery recovery effect.

Bluetooth LE takes advantage of this recovery effect. Data transmission using BLE is accomplished in periodic short bursts, followed by idle periods. During these idle periods, the battery can recover, helping to extend battery life.

5. Bluetooth Low Energy Protocol Is Asymmetrical in Design

The BLE protocol uses an asymmetrical design to assign tasks to devices that want to connect. The device with the most limited resources does the least work. For devices that want to connect, one must act as the central device, and the other as the peripheral device. Central devices usually have greater processing power and battery life, such as smartphones. In contrast, peripheral devices are typically less powerful and have limited energy resources, like fitness trackers and heart rate monitors.

Bluetooth Low Energy Protocol Stack

A protocol stack (or protocol suite or protocol architecture) is a set or group of sub-protocols that work together to achieve complete communication between two or more computers on a network. Each sub-protocol in the protocol stack is referred to as a layer. The Bluetooth LE protocol stack uses a divide-and-conquer approach. This means that the overall communication task is divided among layers, with each protocol layer responsible for specific communication functions.

Figure 3 shows an example of an LE protocol stack and its layers.

Figure 3. Bluetooth Low Energy Stack

Table 1 summarizes the main functions of each layer of the LE protocol stack.

Devices supporting BLE can communicate in one of two ways:

Unconnected communication: It broadcasts its data to any listening devices
Connected communication: It forms a dedicated connection with another device and communicates with it using a client-server mechanism

Figure 4 provides an overview of these two communication methods.

Figure 4. Different ways BLE devices communicate

1. Unconnected Communication

BLE devices that want to participate in unconnected communication are assigned two roles defined by the GAP layer. One device must be the advertiser, while the other must be the observer.

The GAP roles of devices control the link layer, which in turn controls the LE radio of the devices.

Step 1: The advertising device indicates its link layer to become an advertiser. As the advertiser, the link layer controls the LE radio to switch from standby or idle state to advertising state, and vice versa.

Step 2: When the LE radio is in advertising state, the advertiser (link layer) can send advertising data packets on the three dedicated advertising channels RF37, RF38, and RF39. The advertising data packets can include data such as the name and address of the broadcasting device.

Step 3: On the other hand, the observer indicates its link layer to become a scanner. As a scanner, the link layer controls the LE radio to switch from standby (idle) to scanning state, and vice versa.

Step 4: When the LE radio is in scanning state, the scanner tunes in and listens for data on the primary advertising channels (RF37, RF38, and RF39).

For example, Figure 5 shows an image of my phone as an observer receiving advertising data packets from two broadcasting beacons:

Figure 5. Scanner picking up devices

Common applications of unconnected communication in Bluetooth LE include beacons and IoT sensors that broadcast their readings.

2. Connected Communication

In connected communication, you must understand two main concepts: device discovery and the client-server relationship between connected devices.

(1) Device Discovery and Connection Setup

One important thing to note is that anything referred to as a profile in BLE can control and coordinate other layers of the stack for specific use cases. In this regard, the GAP manages how the link layer and PHY layer (LE radio) should function for BLE devices to discover and establish connections.

LE devices that want to participate in connected communication are assigned two roles defined by the GAP layer. One device must be the central device, while the other must be the peripheral device.

Step 1: The peripheral device’s advertiser indicates the LE radio to broadcast advertising data packets on the advertising channel and listen for connection request packets from the central device.

Step 2: When the central device wants to connect to the peripheral device, it first uses its link layer as a scanner. The scanner will use the LE radio to listen for advertising data packets.

Step 3: When the central device identifies the device it wants to connect to, the link layer switches the LE radio from advertising state to initiating state.

Step 4: In the initiating state, the LE radio sends a connection request data packet to the peripheral device. If the peripheral device accepts the connection request, the connection is established.

(3) Client-Server Relationship

Once the connection setup is complete, the Generic Attribute Profile performs three main functions: it uses the ATT protocol to construct the data to be exchanged. It defines a hierarchy of services and characteristics arranged like folders in a file system. This arrangement of data makes it easy to store and access.

It also defines how connected devices interact in a client-server relationship. GATT defines two roles for connected devices: one device will be the client, and the other will be the server.

Server: Acts as a database that holds data variables. The server stores data using the attribute data types of the ATT protocol. Data on the server is organized hierarchically according to GATT definitions. The data on the server can be accessed based on permissions. Some data may be both readable and writable. Some may be read-only.

Client: After connecting to the server, the client typically requests access to the data (attributes) stored on the server. The client can read and/or write the server’s attributes based on access permissions.

Examples of Bluetooth LE Devices in Connected Applications:

A sleep tracker with sensors can detect physiological changes that occur while we sleep. The tracker connects to your smartphone to share readings. The tracker acts as a server, while the smartphone acts as a client, reading the data stored in the tracker.

Bluetooth SIG (Special Interest Group)

What is Bluetooth SIG? Bluetooth SIG is an independent non-profit organization responsible for defining Bluetooth standards. Bluetooth SIG is not a company or consortium. It does not manufacture or sell Bluetooth products.

Instead, it has four main responsibilities, which are:

Publishing Bluetooth specifications
Protecting the Bluetooth trademark
Providing qualification certification programs before products enter the market
Promoting Bluetooth technology

Bluetooth SIG was established in 1998 with only five founding members. They were Ericsson, Intel, IBM, Nokia, and Toshiba. Over the years, the Bluetooth SIG organization has grown to include over 16,000 member companies.

Bluetooth SIG offers two membership levels for companies wishing to join: adopter membership and associate membership (Table 2).

Finally, Bluetooth SIG has seven member companies with founding member status. These include all founding companies, as well as Apple and Microsoft.

In our increasingly interconnected world, Bluetooth Low Energy has become an important communication technology for ultra-low power applications. As we have seen, it requires efficient protocol design and collaboration between companies to truly make Bluetooth LE or any communication standard have a significant market impact.

Source | Semiconductor Industry Observer

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