BLE Technology
Bluetooth low energy is a brand-new technology that has been designed as both a complementary technology to classic Bluetooth as well as the lowest possible power wireless technology that can be designed and built.
Although it uses the Bluetooth brand and borrows a lot of technology from its parent, Bluetooth low energy should be considered a different technology, addressing different design goals and different market segments.
Instead of just increasing the data rates available, BLE has been optimized for ultra-low power consumption. This means that you probably won’t get high data rates or even want to keep a connection up for many hours or days. This is an interesting move, as most wired and wireless communications technologies constantly increase speeds.
This different direction has been achieved through the understanding that classic Bluetooth technology cannot achieve the low power requirements required for devices powered by button-cell batteries.
However, to fully understand the requirements around low power, another consideration must be taken. Bluetooth low energy is also designed to be deployed in extremely high volumes in devices that today do not have any wireless technology. One method to achieve very high volumes is to have extremely low costs.
Therefore, the fundamental design for low energy is to work with button-cell batteries. This means that you cannot achieve high data rates or make low energy work for use cases that require large data transfers or the streaming of data. This single point is probably the most important difference between classic and low-energy variants of Bluetooth.
Device Types
Bluetooth low energy makes it possible to build two types of devices:
Dual-Mode: Supports BLE and Bluetooth classic.
Single-Mode: Supports BLE only.
There is a third type of device, which is a Bluetooth classic-only device.
Design Goals
When reviewing any technology, the first question to be asked is how the designers optimized it. Most technologies have one or two things that they are very good at and many things that they are not. By determining what these one or two things are, a greater understanding of that technology can be achieved.
With Bluetooth low energy, this is very simple. It was designed for ultralow power consumption.
When the low-energy work started, the goal was to create the lowest-power short-range wireless technology possible. To do this, each layer of the architecture has been optimized to reduce the power consumption required to perform a given task. For example, the Physical Layer’s relaxation of the radio parameters, when compared with a Bluetooth classic radio, means that the radio can use less power when transmitting or receiving data. The link layer is optimized for very rapid reconnections and the efficient broadcast of data so that connections may not even be needed. The protocols in the host are optimized to reduce the time required once a link layer connection has been made until the application data can be sent. All of this is possible only when all parts of the system are designed at the same time by the same group of people.
For global operation, a wireless band that is available worldwide is required. Today, only one available band can be implemented using low-cost and high-volume manufacturing technology: the 2.45GHz band.
The 2.45GHz band that Bluetooth low energy uses is already very crowded. Just taking into account standards-based technologies, it includes Bluetooth classic, Bluetooth low energy, IEEE 802.11, IEEE 802.11b, IEEE 802.11g, IEEE 802.11n, and IEEE 802.15.4. In addition, a number of proprietary radios are also using the band, including X10 video repeaters, wireless alarms, keyboards, and mice. A number of devices also emit noise in the band, such as streetlights and microwave ovens.
It is, therefore, almost impossible to design a radio that will work at all times with all possible interferers unless it uses adaptive frequency hopping, as pioneered by Bluetooth Classic. Adaptive frequency hopping helps by not only detecting sources of interference quickly but also by adaptively avoiding them in the future. It also quickly recovers from the inevitable dropped packets caused by interference from other radios. It is this robustness that is absolutely key to the success of any wireless technology in the most congested radio spectrum available.
Robustness also covers the ability to detect and recover from bit errors caused by background noise. Most short-range wireless standards compromise by using a short cyclic redundancy check (CRC), although there are some that use very long checks. A good design will see a compromise between the strength of the checks and the time taken to send this information.
Short range is actually a slight problem. If you want a low-power system, you must keep the transmitted power as low as possible to reduce the energy used to transmit the signal. Similarly, you must keep the receiver sensitivity fairly high to reduce the power required to pick up the radio signals of other devices from amongst the noise. What short range means in this context is really that it is not centered around a cellular base station system. Short range means that Bluetooth low energy should be a personal area network.
The original Bluetooth design goal of low power hasn’t changed that much, except that the design goals for power consumption have been reduced by one or two orders of magnitude. Bluetooth classic had a design goal of a few days of standby and a few hours of talk time for a headset, whereas Bluetooth low energy has a design goal of a few years for a sensor measuring the temperature or how far you’ve walked.
Terminology
Just like in many high-tech areas, the people working in BLE use their own language to describe the features and technology with the specifications. This section enumerates each of the words that have special meaning and what they mean.
Adaptive Frequency Hopping (AFH): a technology whereby only a subset of frequencies is used. This allows devices to avoid the frequencies that other non-adaptive technologies are using (e.g., a Wi-Fi access point).
Architecture: The design of Bluetooth low energy is sometimes known as the Architecture.
Frequency Hopping: The use of multiple frequencies to communicate between two devices. One frequency is used at a time, and each frequency is used in a defined sequence.
Layer: a part of the system that fulfills a specific function. For example, the Physical Layer covers the operation of the radio. Each layer in a system is abstracted away from the layers above and below it. The Link Layer doesn’t need to know all the details of how the radio functions; the Logical Link Control Layer and Adaptation Layer don’t need to know all the details of how the Link Layer works. This abstraction is important to keep the complexity of the system at manageable levels.
Master: a complex device that coordinates the activity of other devices within a piconet.
Piconet: This is a contraction of the words pico and network. Therefore, a piconet is a very small network. A piconet has a single master device that coordinates the activity of all the other devices (slaves) in the piconet and one or more slaves.
Radio Band: Radio waves are defined by their frequency or wavelength. Different radio waves are then allocated different rules and uses. When a range of radio frequencies are grouped together using the same rules, this group of frequencies is called a Radio Band.
Slave: a simple device that works with a master. These devices are typically single-purpose devices.
Wi-Fi: a complementary wireless technology that is designed for high data rates to connect computers and other very complex devices with the Internet.
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