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What Is Bluetooth Low Energy?

Robin Heydon explains what BlueTooth low energy is, as well as relevant device types, design goals, and terminology.
This chapter is from the book
  • If I have seen a little further, it is by standing on the shoulders of Giants.
  • —Isaac Newton

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.

Classic Bluetooth was designed to unite the separate worlds of computing and communications, linking cell phones to laptops. However its killer application has proved to be as an audio link from the cell phone to a headset placed on or around the ear. As the technology matured, more and more use cases were added, including stereo music streaming, phone book downloads from the phone to your car, wireless printing, and file transfer. Each of these new use cases required more bandwidth, and therefore, faster and faster radios have been constantly added to the Bluetooth ecosystem over time. Bluetooth started with Basic Rate (BR) with a maximum Physical Layer data rate of 1 megabit per second (Mbps). Enhanced Data Rate (EDR) was added in version 2.0 of Bluetooth to increase the Physical Layer data rates to 3Mbps; an Alternate MAC1 PHY2 (AMP) was added in version 3.0 of Bluetooth that used IEEE3 802.11 to deliver Physical Layer data rates of up to hundreds of megabits per second.

Bluetooth low energy takes a completely different direction. Instead of just increasing the data rates available, it 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, as illustrated in Table 1–1.

Table 1–1. Speeds Almost Always Increase





















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 be extremely low cost. For example, Radio frequency identification (RFID) tags can be deployed in very high volumes because they are very low cost, ultimately because they work by scavenging power delivered by a more expensive scanner.

Therefore, it is crucial to also look at the Bluetooth low energy system design from the requirements of low cost. Three key elements within this design point to very low cost:

  1. ISM Band

    The 2.4GHz ISM band is a terrible place to design and use a wireless technology. It has poor propagation characteristics, with the radio energy readily being absorbed by everything, but especially by water; consider that the human body is made up primarily of water. These rather significant downsides are made up by the fact that the radio spectrum is available worldwide and there are no license requirements. Of course, this Free Rent sign means that other technologies are also going to use this space, including most Wi-Fi radios. But the lack of licensing doesn’t mean that anything goes. There are still plenty of rules, mainly related to limiting the power output of devices that use the spectrum, limiting the range. However, these limitations are still more attractive than paying heavily for licensed spectrum. Therefore, choosing to use the ISM band lowers the cost.

  2. IP License

    When the Wibree technology was mature enough to be merged into an established wireless standards group, Nokia could have taken the technology to any such group. For example, it could have taken it to the Wi-Fi Alliance, which also standardizes technology in the same 2.4GHz ISM band. But they chose the Bluetooth Special Interest Group (SIG) because of the excellent reputation and licensing policy that this organization has. These policies basically mean that the patent licensing costs are significantly reduced for a Bluetooth device when compared with a technology developed in another SIG or association that has a FRAND4 policy. Because Bluetooth has a very low license costs, the cost per device is also significantly reduced.

  3. Low Power

    The best way to design a low-cost device is to reduce the materials required to make such a device—materials such as batteries. The larger the battery, the larger the battery casing needs to be, again increasing the costs. Replacing a battery costs money, not just for a consumer who needs to purchase another battery, but replacement also includes the opportunity costs of not having that device available. If this device is maintained by a third party, perhaps because it is part of a managed home alarm system, there are additional labor costs to change this battery. Therefore, designing the technology around low power consumption also reduces the costs. As a thought experiment, how would things be different if a megawatt battery were available for a single penny?

    Many devices could accommodate a larger battery. A keyboard or mouse can easily take AA batteries, yet the manufacturers want to use AAA batteries not because they are smaller, but because their use reduces the bill of materials and therefore the cost of the device.

Therefore, the fundamental design for low energy is to work with button-cell batteries—the smallest, cheapest, and most readily available type of battery available. 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. This is discussed further in the next section.

1.1. Device Types

Bluetooth low energy makes it possible to build two types of devices: dual-mode and single-mode devices. A dual-mode device is a Bluetooth device that has support for both Bluetooth classic as well as Bluetooth low energy. A single-mode device is a Bluetooth device that only supports Bluetooth low energy. There is a third type of device, which is a Bluetooth classic-only device.

Because it supports Bluetooth classic, a dual-mode device can talk with the billions of existing Bluetooth devices. Dual-mode devices are new. They require new hardware and firmware in the controller and software in the host. It is therefore not possible to take an existing Bluetooth classic controller or host and upgrade it to support low energy. However, most dual-mode controllers are simple replacement parts for existing Bluetooth classic controllers. This allows designers of cell phones, computers, and other device to replace their existing Bluetooth classic controllers with dual-mode controllers very quickly.

Because it does not support Bluetooth classic, a Bluetooth low energy single-mode device cannot talk with the existing Bluetooth devices, but it can still talk with other single-mode devices as well as dual-mode devices. These new single-mode devices are highly optimized for ultra-low power consumption, being designed to go into components that are powered by button-cell batteries. Single-mode devices will also not be able to be used in most of the use cases for which Bluetooth classic is used today because single-mode Bluetooth low energy does not support audio for headsets and stereo music or high data rates for file transfers.

Table 1–2 shows what device types can talk with other devices types and what Bluetooth radio technology would be used when they connect. Single-mode devices will talk with other single-mode devices using low energy. Single-mode devices will also talk with dual-mode devices using low energy. Dual-mode devices will talk with other dual-mode devices or classic devices using BR/EDR. A single-mode device cannot talk with a classic device.

Table 1–2. Single-Mode, Dual-Mode, and Classic Compatibility
















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