Bluetooth Smart: How to avoid making dumb devices CONNECTED DEVICES
Bluetooth Smart: How to avoid making dumb devices white paper Development activity for new Bluetooth Smart devices rocketed after Apple included support for the standard in its iphone 4S and Google subsequently supported it in Android v4.3. The standard offers a new level of low-power, low-cost, licence-free communications between smartphone apps and accessory devices. However, Bluetooth Smart poses some unexpected challenges to accessory device makers if they are to realise the potential offered by the standard and achieve the long battery life and small size they have been promised. Whilst the smartphone appears to be an open and transparent platform, the way that the Bluetooth Smart interface is configured can have a direct impact on the performance of the accessory device. Managing this challenge is made more acute by frequent operating system updates and new handset releases. Leading accessory device makers will have to take a detailed and flexible design approach to create and maintain a great user experience for their products. Bluetooth Smart 1 was formally adopted into the Bluetooth v4.0 core specification in June 10, but it was only with the release of the iphone 4S in October 11 that the technology hit the mainstream. Apple s decision to support the standard and make it easy and flexible to use has created the environment in which we now see the emergence of new and exciting devices. Google added support for Bluetooth Smart into Android v4.3 in July 13. With the top two operating systems now supporting the standard, it looks to have cemented its position as the winning solution for low-power devices to connect to smartphones. Low power consumption is one of the key benefits of Bluetooth Smart over other wireless connectivity standards. It achieves its low-power characteristics primarily by ensuring that the radio is switched off as much as possible. This means that, for applications requiring only occasional transfer of small amounts of data, a Bluetooth Smart device can run for a year or more on a coin-cell battery. This long battery life, coupled with the ability to share data with a smartphone, is stimulating the creation of new products in applications from home automation and asset tracking through to sports and fitness or personal medical devices. Many of these application areas have challenging power consumption or size requirements (consider a wrist-worn pedometer or hearing aid). To get the best performance from a device, developers must consider how the device and smartphone interact at a very detailed level (a device will be wasting precious energy if it is transmitting when the smartphone is not receiving). Unfortunately, the key smartphone parameters needed to optimise the interaction and hence the device performance are not usually revealed by the smartphone manufacturer. In addition, these key parameters can change depending on the mode of the smartphone and from one version of operating system to another. These uncertainties pose a significant challenge to device makers who, to get the best performance from their products, must understand in detail how every smartphone behaves in each mode and for each version of operating system. However, not grappling with this challenge can result in significantly higher power consumption and a poorer overall customer experience. Taking a low-energy approach Bluetooth Smart is designed to transfer relatively small amounts of data from devices that may need to run for many weeks from a small battery. These use cases mean that a different approach to the communications scheme is needed over higher-power technologies. Whereas a smartphone might maintain an active link to a WiFi access point or Bluetooth headset to provide a user with instant access to these resources when they need them, a Bluetooth Smart device often cannot afford to spend energy maintaining a link which may not be used. Rather, many of the new products we see being developed will establish a new connection with the smartphone only when the device has data to exchange. A simple example is a set of weighing scales that transfers data to a smartphone each time the user weighs themselves. There is no need to maintain a link with the smartphone for the vast majority of the time. This approach enables the device to conserve energy in exchange for some reduction in responsiveness. However, it also means that optimising the efficiency of the process by which the device connects to the smartphone becomes increasingly important for the overall device performance. Figure 1 shows the radio power consumption for an example Bluetooth Smart use case. In this case, we assume a device is trying to connect to a smartphone in standby mode to exchange a single packet of data. For this example use case, 1 Initially, the Bluetooth SIG called the standard Bluetooth Ultra-Low Power before renaming it Bluetooth Low Energy and now finally Bluetooth Smart. 1
white paper Bluetooth Smart: How to avoid making dumb devices 82% of the radio power budget can be used in establishing a connection. From our research, this is the best-case situation the power needed to set up a connection in this use case can be much higher. smartphones when connecting in the background. (For more details on our analysis see About the Study) The study had three startling findings: Handshaking 17% Transferring data 1% 1. Varying the connection attempt interval used by the accessory device created more than a threefold variation in the mean number of connection attempts required (and hence power consumed) to connect to the smartphone 2. The number of connection attempts required to connect to different handsets and different operating systems (and hence the power consumed) can vary almost 24 times Connecting 82% 3. When the smartphone s operating system was upgraded, the mean number of connection attempts required (and hence power consumed) to connect increased by up to 35% Finding the sweet spot Figure 1: An example Bluetooth Smart device can expend 82% of its radio power budget connecting to the smartphone. Source: Cambridge Consultants Getting connected The connection process involves a device transmitting connection adverts at regular intervals and then listening for a reply from any other device that is able to connect. The speed with which a connection can be established between an accessory device and smartphone depends upon how actively the accessory device transmits adverts, and how actively the smartphone listens for these incoming requests. When a smartphone is active, and the app associated with the accessory device is running in the foreground 2, the smartphone will actively listen for connection attempts. However, when the smartphone is in standby mode or the accessory device s app is running in the background, the operating system will often reduce the app s access to the Bluetooth Smart radio this reduces the overall radio activity to conserve the smartphone battery. For many device makers, the ability to connect to a smartphone whilst it is in standby is a key requirement, but is one over which they have little control. Rather the performance of their accessory device is to a large degree controlled by the operating system on the smartphone. To understand this issue better, we analysed the behaviour of the Bluetooth Smart connection process for representative We measured the relationship between the connection attempt interval, and the number of attempts needed to make a successful connection, for an LG Nexus 4 running Android 4.3. The summary results are shown in Figure 2. The figure shows that an accessory device transmitting connection attempts at ms intervals required on average 3 12 transmissions before successfully connecting to the smartphone. Conversely, an accessory device transmitting at 80ms intervals only required an average of 4 transmissions to connect. This variation translates into a more than threefold uncertainty in the power a device needs to use to connect to the Average number of attempts to connect 14 12 10 8 6 4 2 0 12 9 8 Nexus 4 (Android 4.3) 40 50 60 70 80 Connection attempt interval (ms) Figure 2: Connection attempt interval can increase energy used to connect by more than 3 times. Source: Cambridge Consultants 9 7 5 3x 4 2 Running in the foreground refers to an app being granted control of most of the smartphone s resources by the operating system. The app will usually be shown on screen and will be actively engaged with the user. 3 Based on a sample of 0 connections 2
Bluetooth Smart: How to avoid making dumb devices white paper smartphone. If connecting represents 82% of the total radio budget as shown in Figure 1, this would cause a 2.7x variation in the accessory s total power consumption 4. A new day, a new handset As we all know, smartphone handsets are not all equal, and this is no less true for how devices are able to connect with them. Accessory device makers need to consider the range of performance they are likely to achieve across a number of smartphone platforms which their customers will use. This means that one user gets a battery life of 2 days, and another gets over a month We studied how the average number of connection attempts needed to successfully connect varied across handsets and operating systems. Specifically we compared the performance of an LG Nexus 4 running Android 4.3, Samsung Galaxy S4 running Android 4.2 and Apple iphone 4S running ios v6. The results are shown in Figure 3. The figure shows that an accessory device optimised for the lowest average number of connection attempts for the Nexus (4 attempts) would, on average, require an incredible 95 connection attempts to connect to the Galaxy S4. Assuming again that connecting consumes 82% of the total radio power budget, this means that one user gets a battery life of 2 days, and another gets over a month. This is an extreme assumption since the radio is unlikely to consume the entire power budget but, nevertheless, this indicates that, for some applications, the handset can have a significant impact on the accessory device performance. Everything changes With many handsets receiving an operating system update every few months and a continual stream of new handsets reaching the market, change is a fact of life. Coping with these changes poses a significant challenge for device makers seeking to support the largest number of smartphones and operating systems possible. We studied how the average number of connection attempts needed to successfully connect changed when the operating system on an iphone 4S was upgraded to a newly released version. The summary of these results is shown in Figure 4. The figure shows that, in most cases, the average number of connection attempts required increased between ios v5 and upgrading the smartphone to ios v6. This will have a direct impact on the battery life of the accessory device. Most alarmingly, the connection attempt interval that had previously provided the lowest average power consumption (80ms) experienced the greatest increase in the number of attempts needed to establish a connection. The average number of connection attempts when transmitting at 80ms intervals increased from when connecting to an iphone running ios v5 to 27 when connecting to the same phone running ios v6. This variation would cause a 35% increase in the total power the accessory device used to connect to the smartphone purely due to the user upgrading the operating system. Average number of attempts to connect 0 150 100 50 0 12 41 165 9 28 135 121 33 22 8 9 1 26 Nexus 4 (Android 4.3) iphone 4S (ios v6) Galaxy S4 (Android 4.2) 107 7 5 24 100 24x 18 4 40 50 60 70 80 Connection attempt interval (ms) 95 Average number of attempts to connect 50 40 10 0 40 37 24 29 26 22 35 35 25 22 iphone 4S (ios v5) iphone 4S (ios v6) +35% 26 27 24 40 50 60 70 80 Connection attempt interval (ms) Figure 3: The number of connections needed to connect to a smartphone can vary by 24 times from one handset to another. Source: Cambridge Consultants Figure 4: A device using the optimum connection parameters for ios v5 would suffer a 35% increase in power needed to connect when the user upgrades the smartphone to ios v6. Source: Cambridge Consultants 4 For cases where the radio dominates the device s total power budget 3
white paper Bluetooth Smart: How to avoid making dumb devices Thriving on uncertainty Managing these challenges requires accessory device makers to take a rigorous yet flexible approach to their communications system. To maximise the user experience of a product, devices need to be designed to optimise the connection parameters for the specific use case. In addition, devices should be architected such that they can change the way they connect with a smartphone as operating systems and handsets evolve. This approach will enable the leading device makers to establish market-leading products that can maintain their performance over time despite their dependence on the smartphone manufacturers. Key to enabling the leading players to stay ahead will be an ability to quickly characterise new operating systems and handsets and to analyse the impact of any changes on their products. This detailed knowledge of how the smartphone is behaving is not publically released by the smartphone manufacturers and so understanding this offers a competitive advantage to accessory developers. About the study The data presented here is based on our automated characterisation system which allows us to assess a smartphone s Bluetooth Smart radio implementation over a range of modes and operating conditions. We use this characterisation system, along with a suite of other development tools, to enable us to rapidly create robust Bluetooth Smart devices for our clients. Our approach is scalable to easily allow coverage of new handsets and operating systems. We collected tens of thousands of characterisation data points and combined these with a mathematical model of the Bluetooth Smart protocol. The model, populated with this large dataset, allows us to quickly identify average connection times for different connection attempt intervals across many different scenarios. The data set is large enough to give greater than 95% confidence that the mean number of connection attempts differs across connection attempt interval and operating system version. This approach can be used during the initial design phase to create a detailed model of the performance of a device. By modelling the device performance for multiple different scenarios, the optimum design parameters can be identified quickly. Figure 5 shows the distribution of connection times at a ms connection attempt interval. From this set of data the mean number of adverts required to establish a connection can be calculated. In addition to calculating the mean number of adverts, the distribution is important for answering questions such as: When should a device stop attempting to connect if the phone isn t there? With more detailed requirements for a specific device, this approach can be readily extended to estimate the trade-offs between power consumption, transmission range and physical size. This is achieved by incorporating the effects of other electronics within the device and antenna performance into the power model. From this, the battery size needed to achieve any target battery life can be estimated. Taken together, this approach can rapidly provide a view of some of the key design parameters for any accessory device which needs to connect to a smartphone. 25 Probability Density (s -1 ) 15 10 5 0 0 1 2 3 4 Connection time (s) Figure 5: Distribution of connection times for multiple connection attempts at ms advertising interval Source: Cambridge Consultants 4
About Cambridge Consultants Cambridge Consultants is a world-class supplier of innovative product development engineering and technology consulting. We work with companies globally to help them manage the business impact of the changing technology landscape. With a team of more than 400 staff in Cambridge (UK), Boston (USA) and Singapore, we have all the in-house skills needed to help you from creating innovative concepts right the way through to taking your product into manufacturing. We re not content just to create me-too products that make incremental change we specialise in helping companies achieve the seemingly impossible. We work with some of the world s largest blue-chip companies as well as with some of the smallest, innovative start-ups who want to change the status quo fast. We have one of the largest independent radio design teams in the world and our wireless communications division has created a number of world firsts, ranging from the miniature to the global from radios that are implanted into the human body through to ones that allow air traffic control to talk to aircraft across the globe. We re experts in a bewildering array of wireless technologies but agnostic to all of them. What we care about most is creating the right solutions for a client s problem in order to give them a truly world-class product. Cambridge Consultants is part of the Altran Group, a global leader in innovation. www.altran.com For further information or to discuss your comments, please contact: Tim Ensor, Head of Connected Devices Tim.Ensor@CambridgeConsultants.com Rob Milner, Senior Consultant Robert.Milner@CambridgeConsultants.com Bluetooth, Bluetooth Smart and Bluetooth Smart Ready are registered trademarks of the Bluetooth SIG Inc. iphone is a registered trademark of Apple Inc. The contents of this report are the proprietary information of Cambridge Consultants 13 Cambridge Consultants Ltd. All rights reserved.
Cambridge UK Cambridge USA Singapore www.cambridgeconsultants.com Cambridge Consultants is part of the Altran group, a global leader in innovation. www.altran.com