Joining the Dot(dot)s with Thread and Zigbee


RF protocols Thread and Zigbee join forces to create a single language that handles the connectivity attributes of everyday objects

Smart home. Connected home. IoT in the home. The concepts have been around for some time, and have developed into healthy—if, thus far, sometimes niche—markets for a list of end-equipment makers. The pace of adoption looks set to accelerate with the increasing use of voice command devices, such as Amazon’s Echo and the Apple HomePod. “Alexa, make the lights in here brighter,” may be the spur to boost installation of the systems that give “Alexa” the sensor data to know which spaces are occupied, and what the ambient conditions are, together with the control channels to switch ‘things’ on and off. At the same time, new developments in the technology for wireless networking continue to offer improved performance, increased flexibility, and greater interoperability.

The appeal of reliable, robust, and low power wireless networks using mesh principles has long been obvious, in both the industrial and domestic environments. To succeed in the latter, it is imperative that operation is simple, out-of-the-box, and faultless. For the homeowner, anything other than simple set-up and consistent operation means a product returned to the store. Similarly, while early adopters might settle for proprietary families of devices, the full potential of a market sector can only be realized when products from multiple sources can be used together. Standards are required.

The sequence of events triggering standards is familiar. The bright idea of the moment takes hold in the industry; there is more than one concept of how the idea might be set on course to become a successful product line; groupings and collaborations take root; and in a very short time, competing candidates vie to become, ‘the standard’.

When compared to some of the epic standards ‘wars’ that have marked the development of other categories of technology-based products, low power, short-range radio has spawned relatively few such efforts, to the sector’s benefit. Bluetooth, ANT, Zigbee, and Thread—accompanied by a host of proprietary and non-interoperable offerings using many of the same basic elements—basically covers the technology space.

Standards mature
The basic ideas of low-power, modest-data-rate RF protocols for wireless monitoring and control applications date back more than 20 years. The Zigbee Alliance was constituted in 2002 and has developed a broad-ranging standard. It currently has specifications that span a range of application spaces including home automation lighting control, and smart energy. Member companies now have many millions of products deployed in markets worldwide.

The Thread protocol was a later arrival on the low-power mesh networking scene, with the Thread Group being constituted in 2014; membership at its debut included names such as Arm, Samsung, and Qualcomm. The Thread protocol is structured around use of Internet Protocol (IPv6 addressing) and uses the 6LoWPAN standard; it therefore employs IEEE 802.15.4 RF hardware, as does Zigbee, with silicon available from multiple suppliers. The Thread Group presents the protocol as being specifically designed for the home automation space, with built-in security using established IP-based techniques. It also claims “battery-friendly” operation and enables the use of “... Internet standards with constrained [limited-resource] embedded devices.”

Also present in the home-automation networking space is Bluetooth, coming from a different starting point, as it was conceived to enable wireless point-to-point, device-to-device data links. Through its evolution through Bluetooth Low Energy (Bluetooth LE), Bluetooth recently gained range and throughput enhancements (albeit not concurrently) in the latest iteration (Bluetooth 5) and mesh networking support with the Bluetooth mesh 1.0 release. The Bluetooth Special Interest Group (SIG) reports market success for its mesh network option in, in particular, the smart-lighting sector. Bluetooth LE is supported natively in all currently available smartphones and tablets, which the Bluetooth SIG sees as an advantage because it offers consumers a familiar interface for controlling wireless home-automation devices.

Creating a single language
Thread and Zigbee have not stood idle while the Bluetooth SIG has enhanced its product for home-automation applications. Rather the groupings have joined forces to champion a new standardization effort; Dotdot. Dotdot represents a move to extend standardization in the higher, application layers of the stacks of existing standards. Dotdot is an effort to create a single language that handles the connectivity attributes of the everyday objects that the IoT aims to animate. It is an “open, common protocol” developed by Zigbee and using the IEEE 802.15.4-compliant physical layer (PHY). At the same time, it runs on Thread’s IP-based platform.

Dotdot is not, therefore, a new standard in its own right – the Zigbee positioning is that it is a “common layer for IoT [devices] with Thread as a common platform”. Dotdot also isn’t a ground-up exercise but is based on application-level features already present in the Zigbee specification – the Zigbee Cluster Library (ZCL). ZCL, now “re-branded” as Dotdot, is a set of commands and attributes, spanning multiple Zigbee profiles, that a developer uses to construct products that will interoperate with products from other makers, or that will use Zigbee public profiles (for example, Zigbee Smart Energy). In an analogy with the App Store model of smartphone usage, Dotdot will enable applications to run on any suitable, and compliant, device.

The person who most needs visibility of Dotdot is the developer, so that he or she can readily write the interoperable, device-level code. While consumers will start to see the Dotdot brand name and logo they will be less interested in hearing about the technical detail. For them, the essential attribute is that operation of their networked devices will be simple, intuitive and that everything “just works”.

Standards wars are resolved in a number of ways; there may be a process of attrition, with some contenders disappearing; or one of collaboration and convergence. Help may also come from the hardware part of the equation. Successive generations of integrated circuits in a particular technology space build on experience gained; and integration levels increase. In early generations of a product line, there may be different silicon for different firmware/software standards. When later iterations are taped out, the optimum choice for configuring the feature set may be to produce a multiprotocol part that will accommodate all of the relevant protocols and standards freeing the developer from the risk of narrowing his or her options too soon.

An example is Nordic’s nRF52840 multiprotocol System-on-Chip (SoC) built around an ARM 32-bit Cortex-M4F microprocessor with 1MB Flash and 256kB RAM. The chip supports Bluetooth 5 and ANT, in addition to configurable, proprietary 2.4GHz operation. Additionally, the nRF52840 includes an IEEE 802.15.4 PHY to support any protocol based on the standard.

The chip is Thread 1.1-certified (running the OpenThread stack) and is accompanied by the nRF5 Software Development Kit (SDK) for Thread and Zigbee. Concurrent operation between the Zigbee/Thread and Bluetooth LE environments enables development of most flexible products to populate the next generation of home, commercial, and industrial networks.

A familiar interface

The Bluetooth SIG champions its technology’s mobile-device interoperability because it offers consumers a familiar interface to control Bluetooth LE home-automation networks