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Getting Familiar with Bluetooth 4.0 Low Energy

Bluetooth 4.0, with the hallmark feature of Bluetooth low energy wireless technology, is ready to roll. The Bluetooth SIG formally adopted the radio and host layers of the specification, and opened qualification for new products in July of last year.

MICHAEL FOLEY, BLUETOOTH SIG

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Bluetooth low energy is designed to enable new markets requiring low cost, low latency, low duty cycle, low data rate and low power consumption devices. These markets—healthcare, fitness, proximity, automotive, smart grid—may contain device categories that are completely new to Bluetooth technology, or which will use multiple features—low energy and high speed, for instance—of Bluetooth technology within the same device. This is an exploding area of opportunity. The industry analyst firm Gartner listed Bluetooth 4.0 as one of its top 10 technologies to watch through 2011. 

New Classes of Bluetooth Devices

With the introduction of Bluetooth 4.0 with low energy, several new classes of devices in the Bluetooth ecosystem will exist: Bluetooth low energy only devices, Bluetooth devices plus low energy support, and Bluetooth devices without low energy support. Some combination of devices will not be able to communicate with each other. For instance, Bluetooth low energy only devices will not be able to communicate with traditional Bluetooth devices without low energy support. Although this may initially be confusing to users who are accustomed to all Bluetooth devices having the ability to interact with other Bluetooth devices at a basic level, it does allow for much lower cost and single purposed devices to be manufactured for very specific markets containing only the Bluetooth low energy feature. For markets that are not as cost sensitive and/or devices that are multifunctional, the incremental cost of adding Bluetooth low energy technology to an existing Bluetooth device is extremely low. This is due to the amount of reuse of the existing Bluetooth wireless technology implementations. Due to the low incremental cost, the attach rate of Bluetooth devices plus low energy is expected to be significant. 

Bluetooth low energy was optimized for occasional connections that allow for longer sleep times between connections, small data transfers, very low duty cycles and simpler topology while Classic Bluetooth technology devices are optimized for connection-oriented applications where the connection is always on or present, highly secure and with more complex topologies. There are numerous basic characteristics of the Bluetooth low energy feature that should be considered when determining if a new application can best leverage the strengths of Bluetooth low energy technology.

In terms of latency, Bluetooth low energy can support connection setup as low as 3ms, which allows an application to quickly form a connection, transfer its data, and then quickly tear down the connection. In order to support such low latencies the remote device needs to be continuously broadcasting at a high duty cycle for a very limited period of time. This scenario is workable if the scanning device is plugged in since it needs to be continuously scanning for devices that may be using this broadcasting method. For this scenario, connection setup is highly optimized, and the need to stay connected to service low latency applications is not necessary.  

A typical application for this scenario is a remote control for an audio/video device. However, applications between battery-operated devices will typically have higher connection latencies in the range of 20ms to 10.24s (nominally around 1.28s). For special situations a fast connection is possible, but the frequency of use for battery-operated devices should be low.

Bluetooth low energy supports very short data packets (8 octets minimum up to 27 octets maximum) and is optimized to transmit commands and data in single packets. Packets of this size are not optimal for transmitting large amounts of data (hundreds of octets) frequently or continuously for extended periods of time. It is likely to be more efficient to use Classic Bluetooth wireless technology when data packets averaging hundreds of octets are necessary. The use of the term “octet” here may seem synonymous with “byte,” but it is preferred for expressing 8-bit quantities in network protocols because it avoids some legacy ambiguity that has been associated with “byte.”

The greatest benefit in power savings with Bluetooth low energy is from having the controller and host sleep for longer periods of time. It is extremely important for applications that wish to take full advantage of these power savings not to require that the connections exist between devices for long periods of time. Virtual connections may be used to handle situations where maintaining state information is desired in place of a full-time connection. Duty cycles in the 1% or less range are recommended.

There are also differences in terms of security. While Bluetooth low energy shares many of the security traits of Classic Bluetooth wireless technology, it currently lacks the ability to perform the numeric comparison used in secure simple pairing (SPP) defined in Bluetooth wireless technology v2.1 or later.  SPP is a method where the user compares two numbers displayed on each device’s screen. SPP may be added in the next version of the Bluetooth low energy specification. Instead, pass key entry is necessary if man-in-the-middle protection is desired.  Bluetooth low energy also permits the use of authentication of a remote device without requiring an encrypted link between the devices. The authentication key generation needed for pass key entry in Bluetooth low energy uses a different algorithm than Classic Bluetooth wireless technology.

Bluetooth low energy places a significant amount of intelligence in the controller, which allows the host to sleep for longer periods of time and only be awakened by the controller when the host needs to perform some action. This allows for the greatest savings in current since the host is assumed to consume more power than the controller. Applications for which the host needs to have greater control may not benefit from the current savings Bluetooth low energy enables. For example, in Bluetooth low energy, the controller will ignore or accept connection requests without involvement from the host.

There are also somewhat more limited options for connection topology. Bluetooth low energy is currently optimized for one-to-one connections while allowing one-to-many connections using a star topology. Complex networks like mesh and scatternets are not supported in the initial release of Bluetooth low energy. With the use of quick connections and disconnections, data can move in a mesh-like topology without the complexities necessary to maintain a mesh network.  Bluetooth low energy devices can be asymmetric in that they are not required to support both the master and slave roles. In some cases, a Bluetooth low energy device will only support a single topology role. The topology roles are also fixed at the time of connection creation. Switching topology roles requires the connection to be terminated and recreated with the new topology role. An example of a typical mix of devices is shown in Figure 1.

Figure 1
In a personal network, Classical Bluetooth devices, such as the phone and the earpiece, could play their traditional roles, but with the phone also acting as the central device for Bluetooth low power sensors such as thermometer, pedometer and heart rate monitor, which play the role of broadcasters.

Bluetooth 4.0 with Low Energy: The Technology Basics

Devices utilizing the low energy feature of Bluetooth 4.0 differ from those built upon Bluetooth 3.0 or earlier versions in a number of ways. First and foremost is the lack of symmetry between peer devices. Strictly optimized for low cost and low power, Bluetooth low energy devices are permitted to support only unidirectional communication (transmit only or receive only). Bi-directional communication may still be supported but, unlike existing Bluetooth devices, Bluetooth low energy devices do not support master-slave role changes while in a connection at the present time. This means that application profiles need to specify which device in a profile role will be master and which will be slave to avoid a two-master or two-slave situation.

The other most significant difference is in device discovery and connection setup. Bluetooth low energy uses a push model instead of the pull model used in traditional Bluetooth wireless technology. Devices that wish to be discovered by other devices use broadcast advertising (or beacons) in an area to devices listening for such broadcasts, whereas traditional Bluetooth wireless technology places devices wishing to be discovered by other devices in a listening mode for broadcasts from inquiring devices before sending device information. In Bluetooth low energy, advertising broadcasts are also used as a method for a device to indicate to surrounding devices that it is connectable with other devices or a specific device. Devices listening for advertising broadcasts may also be in an exclusive connectable mode where they respond only when receiving connectable advertising broadcasts from devices wishing to connect to another device.

Due to the lack of symmetry and fixed discovery and connection modes, it was necessary to establish specific profile roles that represent these various combinations of capabilities. It is extremely important that profile roles are clearly established to enable Bluetooth low energy devices to communicate with one another when both devices support the same profile. The profile roles are described in Table 1.

Table 1
The four profiles of devices for Bluetooth low power are broadcaster, such as a temperature sensor; an observer, such as a temperature display; a peripheral, such as a printer; and a central, such as a laptop or a mobile phone.

A device may support multiple profile roles as long as the required functionality is supported while operating in that profile role. Each profile role is optimized for certain types of applications. If the application is only transmitting small amounts of information on a periodic basis and does not require bi-directional communication (e.g. data reception acknowledgement) then the broadcaster and observer profile roles should be considered. Even in these cases, the application may change into the peripheral and central profile roles under particular circumstances, such as device configuration, but operate most of the time in just the broadcaster and observer profile roles. Some applications will require the additional capabilities of the peripheral and central profile roles. The important consideration will be which higher layer profile role is best suited for the peripheral profile role or the central profile role.

The Bluetooth SIG publishes all specification documents on the main website, www.bluetooth.com. The Bluetooth 4.0 specification can be found in the section Get Technical/Building with Bluetooth Technology. Membership in the organization is recommended to all and required for the qualification of products and the licensing of the brand. To become a Bluetooth SIG member, visit www.bluetooth.org.  

 

Bluetooth SIG

Kirkland, WA.

(425) 691-3535.

[www.bluetooth.org].