Bluetooth Low Energy

The recent weeks I have been investigating the Bluetooth Low Energy (BLE) technology. Honestly I can say there has probably been no other wireless technology designed so well and complete as BLE is. On the other hand BLE is the most confusing and most miscommunicated standard I can remember. The reason for this post is to clear the fog a little.

So first, what is the purpose of BLE and why we need yet another standard for wireless communications? Bluetooth has always been about interconnectivity in PAN or Personal Area Network. Interconnectivity means various devices from different vendors can connect and talk to each other. Most typical use case is the wireless headset connecting to a phone. Or wireless mouse connecting to a computer (sans dongle). PAN means the target scenarios are about devices in the close proximity. Like all the gadgets you wear or carry. Your personal cloud, usually with a radius of  one meter (can go up to around ten meters). Bluetooth (the classic Bluetooth, up to version 3) as we know it, has always ruled this space.

There are two problems with the classic Bluetooth however. The first is it consumes too much energy. A Bluetooth mouse won't last a year on a battery. And a headset has to be charged almost daily. The power consumption problem has one root cause. Bluetooth classic is a connection-oriented protocol, meaning there has to be a "link" established between the devices and that link has to be maintained. Meaning the devices exchange empty packets of data even if they are not really active. Setting up classic Bluetooth connections takes relatively long time. A mouse cannot start establishing a connection to a computer as soon as it starts moving, because a user will experience annoying delays. A connection has to be established beforehand and has to stay up eating batteries. Even when nothing happens. The Bluetooth classic is also designed for data streaming. Like music or bidirectional voice conversation. It is not optimized for just very short data packets, like hear rate monitor belt sending a few bits of data every second.

The second problem with the classic Bluetooth is there are just a few profiles defined. A profile is a use case. Or how devices use the Bluetooth radio. We have the headset profile (most commonly used), we have the stereo audio profile, the serial port emulation and a few more. The problem with profiles is they have to be very strictly defined and it takes a long time to push them through the Bluetooth standardization process. This is the reason why many wireless gadgets, especially various types of sensors, do not use Bluetooth. It has been easier for the manufacturers to design their own proprietary radio protocol and not bother about the lengthy profile definition and product specification.

So here comes the Bluetooth Low Energy to the rescue. First I have to stress BLE and Bluetooth classic have two things in common: the name Bluetooth and the frequency band of 2.4GHz. Apart from that they are completely different protocols. This is where the confusion starts. BLE is not Bluetooth as we know it. BLE devices do not work and will never be able to work with Bluetooth classic (up to version 3.0) devices. To bridge the compatibility gap the Bluetooth 4.0 standard has been introduced. Bluetooth 4.0 is DUAL-MODE. Meaning it can communicate with BOTH classic Bluetooth devices (v1, v2, v3, as we know them) AND Bluetooth LE as well. Typically a laptop or a smartphone will have Bluetooth 4.0, so you can connect both a Bluetooth classic headset and a BLE heart rate monitor to it. But the heart rate monitor will typically be BLE-only and the headset will be Bluetooth classic only. I hope you are still with me.

Now to add more confusion. The Bluetooth SIG (the Special Interest Group) renamed the Bluetooth Low Energy (BLE) to Bluetooth Smart. So Smart in Bluetooth equals LE. And consequently they renamed the Bluetooth 4.0 (the dual mode BLE + Classic Bluetooth) to Bluetooth Smart Ready. Yes it makes sense, but nobody understands it. Even the Blueooth SIG members. Only last week I attended a presentation by the SIG evangelist / engineer, who had a headset pictured as an example of the Bluetooth Smart, which was completely WRONG!. I will continue here on the blog with the BLE acronym, as it is commonly best understood and does not add to the Smart vs Smart-Ready confusion.

BLE is designed for telemetry and control. It can exchange only very tiny pieces of information (like sensor values or control commands). It uses the least amount of power possible to transport things like "19°C" or "LAMP POWER ON". It is also optimized to set up connections on demand, as they are needed and tear them down often. This is what I call the "telemetric" behavior. Very low duty cycle. Very low bandwidth. Do nothing (meaning do not consume energy nor occupy bandwidth) most of the time, do what is needed quickly and then fall back to sleep. Telemetric is the essence of BLE. The opposite of streaming. By the way it turns out low bandwidth can be as sexy as high bandwidth. Especially as it can run on almost no power at all. The common goal of BLE devices is to power them by coin batteries, which should last about two years. And I am sure we will find many BLE devices using various energy harvesting techniques, bringing what I call the truly wireless freedom.

Trying to keep the essay short enough for the content snickers to reach to the end, this is just about it when it comes to the very brief introduction to Bluetooth Low Energy. It is a fascinating technology with bright long term future and many near term roadblocks. I will be covering both here. Stay in touch!