This article is part of a series exploring the role of networking in the Internet of Things.
Previously, we set out to choose the wireless technology standard that best fits the needs of our hypothetical building monitoring and energy application. Going forward, we will look at candidate technologies within all three networking topologies discussed earlier: point-to-point, star, and mesh. We’ll start with Bluetooth, the focus of this post.
Bluetooth is the most common wireless point-to-point networking standard, designed for exchanging data over short distances. It was developed to replace the cables connecting portable and/or fixed devices.
Today, Bluetooth is well suited for relatively simple applications where two devices need to connect with minimal configuration setup, like a button press, as in a cell phone headset. The technology is used to transfer information between two devices that are near each other in low-bandwidth situations such as with tablets, media players, robotics systems, handheld and console gaming equipment, and some high-definition headsets, modems, and watches.
When considering Bluetooth for use in our building application, we must consider the capabilities of the technology and compare these capabilities to the nine application attributes outlined in my previous post. Let’s take a closer look at Bluetooth across these eight key attributes.
The range of a class II Bluetooth radio, which is used in a majority of Bluetooth deployments, is about 10 meters (about 30 feet) depending on the environmental conditions.
- Power Consumption
Bluetooth technology is deigned to consume a relatively low level of power. Class II devices use 2.5 mW of power. Battery life is typically measured in days. It is therefore typically necessary to periodically recharge devices that utilize Bluetooth networking technology.
In general, Bluetooth provides a relatively high degree of interference immunity. An often cited scenario is a WiFi-enabled cell phone being used with a Bluetooth ear piece. In this case, WiFi and Bluetooth are sharing the same ISM 2.4 GHz band. The cell phone call is utilizing adjacent frequency bands, typically 850MHz or 1.9 GHz. All communication is being done without interference or interruption.
Specifically, Bluetooth utilizes adaptive frequency hopping (AFH), a technique designed to reduce interference between wireless technologies sharing the 2.4 GHz spectrum. AFH works within the spectrum to take advantage of the available frequencies. This is done by detecting other devices in the spectrum and avoiding the specific frequencies they are using. Each device uses 1 MHz of the spectrum at a time. Adaptive hopping among 79 frequencies at 1 MHz intervals gives a high degree of interference immunity and also allows for more efficient transmission within the spectrum.
- Scalability and Flexibility
Bluetooth technology establishes a point-to-point network with one device acting as a master and the other a slave. The technology scales in a limited way, because one master can establish a connection with up to 7 active slaves, creating a Bluetooth “Piconet”. Note that this may seem like a limited star topology, but it differs in a key way. WA Bluetooth Piconet lets the master control the communication with each slave, but it doesn’t provide the ability for slave-to-slave communication. In a true star topology, communication can occur between any two nodes in the network. The total range of the Piconet is therefore limited to the transmission range of one radio, about 30 feet as stated earlier.
Bluetooth v4.0 provides a maximum data transfer speed of 1–3 Mbits/sec. This is sufficient for streaming data and audio transmission, as cellphone headsets and wireless speakers do.
Bluetooth is managed by the Bluetooth Special Interest Group (SIG), which has more than 19,000 member companies in the areas of telecommunication, computing, networking, and consumer electronics. Bluetooth has been in use for over 15 years, undergoing constant enhancement and refinement. Today it enjoys wide interoperability across numerous application areas such automotive, consumer electronics, health and fitness, mobile telephony, and more.
- Component Availability
Bluetooth is a mature technology and is widely available from multiple vendors, including Texas Instruments and Broadcom.
The cost of a Texas Instruments CC2540F128/CC2540F256 2.4 GHz Bluetooth System-on-Chip (SOC) is about $5.50 and drops to a price slightly greater than $2.50 in quantities greater than 2,500.
Bluetooth also offers a new, low-power-consumption version called Bluetooth Low Energy (BLE). The protocol employs a “low energy stack” designed for short burst message traffic. Although the radio uses the same amount of power to transmit, the protocol has been optimized for very low utilization rates (around 0.25%). Depending on the duty cycle, coin cell battery lives can be extended to more than a year.
Bluetooth Low Energy is aimed at new, principally low-power and low-latency applications for wireless devices within a short range (up to 50 meters/160 feet). The protocol has a lower application throughput and is not capable of streaming voice. The data rate is 1 Mb/s and the packet length ranges from 8 to 27 Bytes.
So Bluetooth has important strengths that make it well-suited for the consumer devices where it is commonly found. It has a very easy and automatic network setup process to establish a connection with another device. It also has relatively high bandwidth (data rate) sufficient for streaming the data and audio needed for these applications.
One new and growing segment of the IoT industry ideally suited to Bluetooth is wearable devices, or “wearables”. An exploding portfolio of new products are coming to market, ranging from fitness monitors, smart watches, bracelets, and necklaces to baby clothes, running shoes, etc. These products typically employ embedded sensors to collect data and use Bluetooth technology to communicate sensor information to the user’s mobile device, which usually includes its own Bluetooth interface. Data can then be presented to the user over the smart phone’s screen or transmitted via the cellular network to the cloud and to other laptop or mobile devices.
Wearables illustrate the kinds of IoT environments suitable for Bluetooth: they involve only a few devices (usually just one tracker and a cell phone, switching to other protocols on the cell phone side). As personal devices, they need only a very small transmission range. And finally, Bluetooth provides security through a form of public key cryptography called Secure Simple Pairing (SSP). As the name implies, it just works. No user interaction is required, although a device may prompt the user to confirm the pairing process.
Regarding our building monitoring and energy management application, there are two critical limitations that eliminate Bluetooth as a candidate technology. First, the master-to-slave, point-to-point network has a total range of only about 30 feet. Our application requires network ranges up to 1,000 feet. And second, the network scales to only 7 nodes (plus the master), far fewer than our 100-node solution.
Next up, we’ll explore the attributes of star networks, including the most common–WiFi–and its applicability for our building monitoring and energy management application.