Bluetooth Classic is the version of Bluetooth you have most likely used before due to its prevalence in common items such as wireless headsets. It is quite evident that Bluetooth is a perfect fit for applications such as streaming music. The data throughput is high enough to support it without connection problems or packet loss, plus it’s very easy to use. You probably need to recharge your smartphone and your wireless speakers after some time, but that is not a problem in this kind of application.

However, for low-power wearables or massive IoT applications, frequent battery charging is not feasible, especially when such high data transfer speeds are not necessary. Therefore, starting from Bluetooth Core Specification version 4.0, the Bluetooth SIG (Special Interest Group) introduced Bluetooth Low Energy (LE), with the intention of making it a key enabler for low-power IoT applications.

Bluetooth LE features

Bluetooth LE, as its name implies, focuses on achieving low energy consumption by sacrificing data rate. Sacrificing data rate refers to two mechanisms here. Firstly, data packets are made smaller, ranging from 27 to 251 bytes. Secondly, data is being sent as seldom as possible to avoid long radio-on times, which is a significant factor in power consumption. This makes Bluetooth LE more suitable for battery-operated devices that need to operate on minimal power and only send small bursts of data.

Bluetooth LE differs from Bluetooth Classic in other aspects as well, such as supported topologies and node types. This is because Bluetooth LE was intended for completely different use cases than Bluetooth classic, therefore different network topologies were necessary.

The table below summarizes some of the key aspects of Bluetooth LE.

A key advantage of Bluetooth Low Energy is its low cost when compared to other low-power personal area networks, making it attractive for applications requiring mass deployments.

The technology is also prevalent in smartphones (most smartphones support both Bluetooth Classic and Bluetooth LE), making it easy to test and prototype applications everywhere. In addition to the smartphone, one more Bluetooth LE device is needed to be able to test two-way communication. Since practically everyone has a smartphone, this reduces the cost and complexity required to conduct tests compared to other technologies where specific hardware is needed.

With Nordic Semiconductor’s wide variety of Bluetooth LE offerings, you will also get implementation flexibility, open-source documentation and continuous customer support. You can read more about Nordic Semiconductor’s Bluetooth LE offerings on our website.

Bluetooth LE protocol stack

A deep understanding of all the individual layers of the protocol stack is not an absolute necessity for enabling and using Bluetooth LE in your application. Nonetheless, understanding the basics of the different layers and some of their main functions does help with having an overview of what’s happening under the hood with the Bluetooth LE stack. This is what we will cover in next section.

Certain layers, due to their importance, will be further explained in subsequent lessons.

The image below depicts the protocol stack architecture, showing which layers make up the Bluetooth LE host and which make up the Bluetooth LE controller.

At the top, you have the application. This is the layer that the user interacts with, through API’s, to make use of the Bluetooth LE protocol. Important parts of the application layer include profiles, services, and characteristics, which will be explained more thoroughly in the upcoming lessons. The next layers make up the host, which essentially determines how Bluetooth LE devices store and exchange data between each other. Lastly, the controller makes up the lower layers, with the most notable part being the physical radio which generates the radio waves and encodes the signals with the data you want to send.

Host

The Bluetooth LE host consists of the following layers:

• Logical Link Control & Adaptation Protocol (L2CAP): provides data encapsulation services to the upper layers.
• Security Manager Protocol (SMP): defines and provides methods for secure communication.
• Attribute Protocol (ATT): allows a device to expose certain pieces of data to another device.
• Generic Attribute Profile (GATT): defines the necessary sub-procedures for using the ATT layer.
• Generic Access Profile (GAP): interfaces directly with the application to handle device discovery and connection-related services.

The Zephyr Bluetooth Host implements all these layers and provides an API for applications.

Controller

The Bluetooth LE controller is comprised of the following layers:

• Physical Layer (PHY): determines how the actual data is modulated onto the radio waves, and how it is transmitted and received.
• Link Layer (LL): manages the state of the radio, defined as one of the following – standby, advertising, scanning, initiating, connection.

The Bluetooth LE controller implementation we will be using in this course is the SoftDevice Controller, found in the nRF Connect SDK. Both the SoftDevice Controller and the Zephyr Bluetooth Host form the full Bluetooth LE protocol stack that is available in the nRF Connect SDK.