Bluetooth Low-Energy: An IntroductionBy
This week the Bluetooth SIG formally adopted Bluetooth Core Specification Version 4.0, which adds Bluetooth Low-Energy (LE) to what’s now being called Classic Bluetooth or Basic Rate (BR), namely what you have in your Bluetooth headset. Designed to work from a coin cell for up to a year, Bluetooth LE is targeting healthcare, sports and fitness, security and home entertainment applications.
Not Your Dad’s Bluetooth
The Basic Rate system includes optional Enhanced Data Rate (EDR) Alternate Media Access Control (MAC) and Physical (PHY) layer extensions. Classic Bluetooth offers synchronous and asynchronous connections with data rates of 721.2 kbps for Basic Rate, 2.1 Mbps for Enhanced Data Rate and high speed operation up to 24 Mbps with 802.11 alternate MAC/PHY (AMP) controllers.
The LE system includes features designed to enable products that require lower current consumption, lower complexity and lower cost than BR/EDR. The LE system is designed for use cases and applications with lower data rates and shorter duty cycles.
Bluetooth LE chips will feature a lightweight Link Layer providing ultra-low power idle mode operation; simple device discovery; and reliable point-to-multipoint data transfer with advanced power saving and secure encrypted connections. Bluetooth LE technology supports very short data packets (8 octet minimum up to 27 octets maximum) that are transferred at 1 Mbps. LE packets are far shorter than Classic Bluetooth ones, which include a 68-72 bit access code, a 54-bit header and a payload of up to 2745 bits—for a maximum packet size of 2971 bits. In addition all connections use advanced sniff-subrating to achieve ultra low duty cycles.
It Pays to Advertise
Bluetooth’s handshaking scheme, while secure, isn’t exactly dynamic. Setting up a trusted link is a time-consuming process, which doesn’t lend itself to ad hoc mobile networks, where nodes can join or drop out as they move into or out of range.
Like the BR/EDR radio, the LE radio operates in the unlicensed 2.4 GHz ISM band. According to the 4.0 spec, LE employs two multiple access schemes: Frequency division multiple access (FDMA) and time division multiple access (TDMA). Forty physical channels, separated by 2 MHz, are used in the FDMA scheme. Three are used as advertising channels and 37 are used as data channels. The physical channel is sub-divided into time units known as events. Data is transmitted between LE devices in packets that are positioned in these events. There are two types of events: Advertising and Connection events.
Devices that need to form a connection to another device listen for connectable advertising packets. The advertising event is ended and connection events begin if the advertiser receives and accepts the request for a connection be initiated. Once a connection is established, the initiator becomes the master (host) device in the piconet and the advertising device becomes the slave device.
The slave device then synchronizes with the host’s clock, mirrors its frequency hopping algorithm and goes to sleep, waking up only periodically as dictated by the application to pass data to the host. Nodes in low-power sensor networks might only go into active mode for a few microseconds every several seconds to pass along a short data string, leading to long battery life. This is a far simpler scheme with less overhead than that used in classic Bluetooth.
Bluetooth LE ≠ ZigBee
At first glance it would seem that Bluetooth LE would be going after low-power ZigBee applications. There would be some irony in that, since the ZigBee camp broke off from Bluetooth several years ago in order to develop a smaller, low-power stack for energy-efficient applications.
But Bluetooth is a very different animal than ZigBee and suited to a different range of applications. ZigBee was designed for mesh networking, using asynchronous communication requiring dedicated routers that must always be powered up since nodes can wake up at any time. It’s basically a low-power wireless LAN protocol with flexible routing to deal with nodes that may not respond. ZigBee is suited to fixed-location networks, with always-on routers to track the network status.
Bluetooth LE was designed to accommodate portable devices in a star network. Nodes can dynamically connect with and drop off the network as they move into or out of range. Bluetooth LE uses a synchronous protocol, allowing both master and slave to wake up simultaneously. Since both can be powered down most of the time, this keeps energy consumption low.
Bluetooth LE is suited to dynamic mobile networks where energy efficiency is paramount. It’s also a leading candidate for ultra-low power wireless sensor networks, such as those used to monitor vibrations on aircraft wings and industrial motors as well as cracks in bridges and buildings. According to analysts at Research and Markets, “Bluetooth Low Energy will be a significant contributor to the overall Wireless Sensor Network market, representing nearly half of all shipments in 2015.”
Despite its early designs on medical and other ultra-low power applications, ZigBee seems to have homed in on the one application where it’s found commercial success, namely smart metering. Expect it to expand later out the power line to other ‘smart grid’ applications. Meanwhile Bluetooth LE could have a great run in low-power medical applications, where energy efficiency is paramount and cost is apparently no object.
Despite the lack of apparent overlap with ZigBee, the Bluetooth SIG is also promoting LE applications in home automation and smart energy, so in these areas the horse race may just be getting started.
Are We Really Compatible?
With any technology advance there’s always the issue of backward compatibility. Here the answer is “yes, but.” According to the Bluetooth SIG:
“This enhancement to the Bluetooth Core Specification allows two types of implementation, dual-mode and single-mode. In a dual-mode implementation, Bluetooth low energy functionality is integrated into an existing Classic Bluetooth controller. The resulting architecture shares much of Classic Bluetooth technology’s existing radio and functionality resulting in a minimal cost increase compared to Classic Bluetooth technology. Additionally, manufacturers can use current Classic Bluetooth technology (Bluetooth V2.1 + EDR or Bluetooth V3.0 + HS) chips with the new low energy stack, enhancing the development of Classic Bluetooth enabled devices with new capabilities.”
In short, your current Bluetooth phone can’t communicate with LE devices but future ones will be able to since they’ll include Bluetooth BR and LE (dual mode) in the same chip. When a Bluetooth LE peripheral wants to come onto a BR network, the host will downshift to LE mode and be able to communicate with it. This compatibility will have to wait for the next generation of consumer devices that incorporate dual mode Bluetooth chips.
First Silicon is Already Here
On the same day that the Bluetooth SIG released the new spec, Texas Instruments announced it had “achieved complete Bluetooth v4.0 controller qualification on the CC2540 low-power, single-mode system-on-chip (SoC) running both protocol stack and application software. TI has now certified the full set of components, including ATT, GATT, GAP and SMP that encompass Bluetooth protocol implementation.”
In addition to TI, Broadcom, CSR, Atheros, Wicentric and Nordic Semiconductor have all qualified 4.0 products (silicon) since qualification opened on Tuesday. With more than 13,000 members of the Bluetooth SIG, you can be sure that plenty of Bluetooth LE chips are in the pipeline, with products starting to appear early next year.
“The finalization of Bluetooth low energy wireless technology within the Core Specification is a monumental achievement,” said Michael Foley, Ph.D., executive director, Bluetooth SIG. Amen to that. The SIG and its members deserve a lot of credit for a disciplined development process that didn’t give rise to a lot of “pre-LE” products as happened with 802.11n, where “pre-n” silicon appeared months before the spec was finalized. With a stable spec and the SIG’s rigorous certification process, Bluetooth LE should see a fast ramp, leveraging the huge base of Bluetooth applications.
In low-power sensor networks, medical, security and home automation applications, Bluetooth LE looks set to be a major player in the low-power wireless world for years to come.