Bluetooth Low Energy (LE) Fundamentals
Bluetooth Low Energy (LE), used in Bluetooth 4.2 and 5.0, is the greatest leap so far in the evolution of Bluetooth. Previous versions of Bluetooth focused on expanding features and increasing the potential transmission size of packets. Bluetooth LE instead centered on radically reducing power consumption. Since Bluetooth LE was released in December 2014, additional advancements have been made which increase the range and transmission size of packets. Bluetooth 5.0 builds on the capabilities of 4.2 in ways that promise to make Bluetooth the ubiquitous Internet of Things protocol of choice.
- Low Energy Secure Connection with Data Packet Length Extension
- Link Layer Privacy with Extended Scanner Filter Policies
- Internet Protocol Support Profile (IPSP) version 6 ready for Bluetooth Smart things to support connected home
- Slot Availability Mask (SAM)
- 2 Mbit/s PHY for LE
- LE Long Range
- High Duty Cycle Non-Connectable Advertising
- LE Advertising Extensions
- LE Channel Selection Algorithm #2
LE was designed from scratch to ensure that all possibilities to achieve ultra low power consumption have been incorporated. It is designed to achieve simplicity, low cost and ultra-low power consumption without compromising robustness, security, global usage, or ease of use.
Are all Bluetooth devices mutually intelligible?
There are billions of BR/EDR devices on the market today which have a nearly perfect attach rate with laptops, tablets, and mobile phones. LE devices are designed to be backward compatible with previous Bluetooth versions. Laptops, tablets, and mobile phones will be upgraded to run Bluetooth 4.0+ devices, while remaining compatible with prior versions. With new protocols being released every couple of years, it is reassuring to learn that compatibility is major concern of the Bluetooth SIG (Special Interest Group). There is only one case when trademarks are not mutually compatible: In interactions between BR/EDR and Single Mode LE. Here is a breakdown of the modes and trademarks to explain BR/EDR and Single Mode LE:
Bluetooth devices are classified by three modes:
1. BR/EDR Devices
Basic Rate/Enhanced Data rate devices which do not support the LE functionality.
2. LE Only Devices aka Single Mode LE Devices
These devices support only LE functionality, not BR/EDR functionality. Examples of these devices include heart rate monitors, thermostats, health and fitness equipment, watches, and key fobs. These devices are expected to have ultra low power consumption and last for several months or years on coin cell batteries.
3. BR/EDR/LE aka Dual Mode Devices
These devices support both BR/EDR and LE functionality. Examples of these devices are smartphones, tablets, PCs, etc. These devices are expected to communicate with both the BR/EDR devices and single mode LE devices even at the same time. These devices don’t have as stringent requirements on power consumption as the single mode LE devices since these have bigger batteries or are generally recharged frequently.
Bluetooth LE devices are also classified by two trademarks:
1. Bluetooth Smart
These are sensor devices which are employed to collect and relay specific pieces of data. Examples of these devices include heart rate monitors, sports equipment, temperature monitors, etc. These devices transmit data to Bluetooth Smart Ready devices. Bluetooth Smart devices must meet three requirements:
- Conform to Bluetooth 4.0 or higher with GATT based architecture.
- Contain Single mode LE radio.
- Use GATT-based architecture to enable a particular functionality
2. Bluetooth Smart Ready
These devices are able to receive data from both traditional Bluetooth and Bluetooth Smart devices. Smart Ready devices implement dual mode which allows them to listen for simultaneous data from both traditional and Smart devices. Examples of Bluetooth Smart Ready Devices are laptops, tablets, and mobile phones. There are also three requirements for manufacturers to qualify for Bluetooth Smart Ready branding:
- They must conform to Bluetooth 4.0 or higher with GATT based architecture.
- They must contain Dual mode radio.
- They must provide a means by which the end user can choose to update the functionality for a Bluetooth Smart device on a Bluetooth Smart Ready device. For example if the user buys a new Bluetooth Smart device then new software can be installed on the smart phone to communicate to that device.
Features of Bluetooth LE
LE devices are only expected to need to transmit data occasionally and are in a switched off state for the remainder of their time. Some uses of Bluetooth LE are locating and alerting devices, proximity detection, sensors, fitness and healthcare wearables, mobile payments, targeted advertising and marketing, etc. None of these use cases require constant data transfer or high data thoroughput – they focus on transferring small packets of information as needed.
LE takes much less time to establish connections than BR/EDR. This is a result of LE’s reduced number of dedicated advertising channels. LE uses just 3 channels to create connections, while BR/EDR uses 32. Scanning just 3 channels to establish connections reduce connection times to 3-4 milliseconds. Connection time between devices is typically so low that it is simpler to re-establish device connections each time data transfer is required. As a result, battery usage is drastically reduced.
LE has trimmed away some of the less necessary functionality of Bluetooth to appeal to the Internet of Things market which values low power consumption and speed. Some of the major reductions are:
1. Dual transmitting and receiving is an option: not a requirement – It is no longer necessary for the device to operate as a transmitter and receiver of data at the same time. To save battery life, LE devices have the option of only sending packets of data instead of simultaneously listening to receive packets. For example, a fitness watch takes readings of BPM and transmits that data to user’s Bluetooth Smart Ready mobile phone. The fitness watch does not need to listen to the phone for instructions.
2. No support for voice channels—LE is not intended for devices that must be constantly connected such as headphones. SCO/eSCO functionality has been removed.
3. No more Scatternets —LE provides support for only a piconet which has simplified the state machine of the link layer since a device can only be in one piconet at a particular time. The benefit of a scatternet topology is not lost: BR/EDR may support only up to 7 devices in a piconet, whereas LE piconets can have any number of connected devices.
4. No Master/Slave role switches —Role switches are no longer supported to simplify the link layer state machine.
5. No more constantly checking connections – BR/EDR devices constantly transmit POLL or NULL packets to check if remote devices are still there. LE devices terminate and re-establish links as needed, consuming far less energy.
6. No support for sniff and park modes. These modes for BR/EDR devices are intended as power saving modes. LE does not require these modes as it terminates and re-establishes links as needed, and conserves power for the rest of its time.
LE packets may be a maximum of 27 bytes long, whereas BR/EDR packets may be up to 1021 bytes. Shorter packets require less time and energy to transmit and keeps the radio from needing constant re-calibration due to high power states.
Reduced Memory Footprint
LE optimizes the use of dynamic memory so as to reduce the silicon area required in the hardware. Dynamic memories must be refreshed constantly so as not to lose content, while ROM and FLASH memories do not require this. LE uses shorter packets, shorter headers, simpler protocols, and uniform packet formats to reduce the need for dynamic memory.
LE is helping cities, institutions, businesses, and individuals realize the potential of IoT. By connecting everyday objects to the internet, we are arming ourselves with the data to make smart, automated, and unbiased decisions. At 4x the range and 2x the speed of Bluetooth 4.2, Bluetooth 5.0 is poised to usher in an affordable and ultra low-power era of connection.