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A primer on Bluetooth Low Energy (Part 1)

Posted: 08 Aug 2011     Print Version  Bookmark and Share

Keywords:Bluetooth  4.0  BLE  ZigBee 

Bluetooth is well known to the public as a technology that provides a wireless link for local connectivity between a phone and a headset, or as a cable replacement. Its new extension, Bluetooth Low Energy (BLE) is a hallmark in the Bluetooth 4.0 specification adapted last year. As its name implies, BLE is intended for such energy-constrained applications as a sensor or a disposable device. BLE was designed to enable wireless connectivity with small devices running on a coin cell battery.

Generation "W" for wireless
In the modern world, wireless connectivity is considered a given in any more-or-less user-friendly device. In particular, local wireless connectivity has developed and successfully penetrated into many consumer products. The biggest leap to date is Wi-Fi: it is a standard feature in laptops, play stations, smart phones—you name it. Wi-Fi provides substantial bandwidth for data transfer, however it's bulky based on the size of the protocol stack and it is definitely not lean in power consumption. Recent efforts to reduce Wi-Fi power requirements yielded Low Energy Wi-Fi, which is better suited to embedded M2M designs [1].

However, battery powered applications often do not require many data transfers; rather the opposite, wireless connectivity in such applications is used to send just a few bytes at very low duty cycles. IEEE 802.15.4 is a commonly used standard to create these types of networks. Yet, it only defines PHY/MAC layers, while actual network layers are determined independently. The ZigBee Alliance maintains the ZigBee standard, which is the best-known network layer running on top of 802.15.4. There are a number of other network protocols such as RF4CE, WirelessHART, and 6LoWPAN, optimised for different applications but that use common low layers defined by the 802.15.4 specification. In addition, a number of proprietary wireless protocols are successfully competing in the same space, such as Z-Wave in applications for home automation and ANT protocol in sports-related products.

The market for wireless solutions is still very segmented, not due to standardisation difficulties, but because there are no "one-size-fits-all: solution.

A place in the crowd
Bluetooth was considered to be in the Wi-Fi camp due to such factors as standardised data transfer with fairly high-power consumption. In its original release, Bluetooth supported 1Mbit/sec data transfer (Bluetooth 1.2). Since then, this number was increased to 3Mbit/sec with an Enhanced Data Rate version (Bluetooth 2.0 + EDR), and even further with a High-Speed version, Bluetooth 3.0 + HS. Now Bluetooth 4.0 moves to the other end of the spectrum targeting lower power, small size of data transfer with low duty cycle types of applications.

You may find yourself questioning if BLE even has a place in this game since applicable markets already have established technologies in both camps, proprietary and standard wireless protocols.

A place for BLE does exist, but it may not be easy to identify. BLE is intended for light duty cycle devices that support small data throughput and operate a long time on a coin-sized battery. Such wireless connectivity comes with a minimal price (inexpensive silicon, light MCU processing requirements, reduced memory footprint), and it can be used in applications related to the Body Area Network (BAN) which represents a connectivity bubble that moves along with the individual. After putting all requirements together it is easy to see that none of existing wireless solutions is a good fit in this case.

Let's first eliminate candidates that require a rechargeable battery or are capable of supporting streaming data—no Wi-Fi. The primary candidate for this job is an 802.15.4 based network and it has such multiple variations as ZigBee, 6LoWPAN, etc. In general, however, the average power consumption for a ZigBee node is in the range of 30ma, which is above the capacity of a coin cell battery.

Given that a ZigBee network with a 30ma power consumption budget allows nodes to be placed within a range of 30.48 Meters (100 feet), BLE is designed to work within a 30-foot range. BLE use cases circle around the user and their immediate surroundings. Since all devices are within a reachable distance, no retransmission using an intermediate node is required. It not only provides a cut in power consumption but also reduces network organisation to a simple star type of connectivity.

It is fair to say that BLE has a respectable contestant from the camp of proprietary wireless technologies, the ANT protocol. Used with a Nordic ultra low power SoC, it operates within a required power budget and it has a compelling use case: ANT is used in Garmin devices that communicate with pedometers and other sport related products [2].

However, here is a rocket booster for BLE—under the Bluetooth umbrella, BLE is not only standardised, but will also inhabit a huge number cell phones, which will have regular Bluetooth and it smaller brother, Bluetooth LE.

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