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April 6, 2020
Protocols like 6LoWPAN and Zigbee add a range of capabilities to IEEE 802.15.4-enabled devices while retaining the latter’s distinct advantages.
Any developer who works with short-range wireless networking standards is almost certainly familiar with IEEE 802.15.4, one of the most widespread technical standards in modern IT. This low-power connectivity standard encompasses a physical base layer and a data handling control layer, leaving the top layers open for additional development.
Because of its ubiquity and upper-layer compatibility, IEEE 802.15.4 serves as the foundation upon which many other well-known standards are layered, including 6LoWPAN, Thread, and Zigbee.
IEEE 802.15.4 derives its name from the Institute of Electrical and Electronics Engineers, an organization that supports working groups as they develop and maintain various communication standards. First defined in 2003, the IEEE 802.15.4 standard was developed specifically for applications that can perform well with a low data rate and need only minimal power. In the decade-and-a-half since, the standard has become a popular alternative to WiFi (which requires both more power and more bandwidth) among developers looking for a low-cost, low-infrastructure communication protocol.
The IEEE 802.15.4 standard defines both a physical layer (PHY) and a media access control layer (MAC). The former defines features of a wireless link like frequency and modulation, whereas the latter defines how the data transmitted over the link is handled. The standard also incorporates a logical link control layer and service specific convergence sublayer that together enable communication with any additional upper layers other standards define.
IEEE 802.15.4-enabled devices may use one of several frequency bands to send packets: sub-GHz (868 MHz and 915 MHz) or, most importantly, 2.4 GHz (one of the most widely-used free radio frequencies in the world). These devices’ maximum data transfer rate is typically 250 kbit/s, though lower rates can be used when minimizing power consumption is of the essence.
While IEEE 802.15.4-enabled devices’ transmission range can vary significantly depending on conditions, it typically falls between 10 and 75 meters. Longer-range communication is possible, but such connections may not be very reliable. In most cases, IEEE 802.15.4 is best-suited to short-range communication between two nodes, although networking can increase overall communication distance.
When multiple IEEE 802.15.4-enabled devices are combined into a low-power network, they can function in one of two network topologies. Within a star topology, communication between nodes must first pass through a central PAN coordinator node. Conversely, within a peer-to-peer topology, nodes can communicate with each other directly. This latter topology makes it possible to use IEEE 802.15.4 as a base layer for mesh networking.
Although the IEEE 802.15.4 standard only provides the PHY and MAC layers, this doesn’t mean developers have to build additional layers from scratch when they want to create a mesh network. The standard’s PHY and MAC layers are designed to integrate with communication standards like Zigbee, WirelessHART, 6LoWPAN, IS100.11a, Thread, and SNAP.
These standards define the upper layers that IEEE 802.15.4 leaves undefined, giving developers a range of options they can use to achieve the requisite capabilities for various use cases.
IPv6 over Low-Power Wireless Personal Area Network (6LoWPAN) is a specification that allows networks built on IEEE 802.15.4 to transmit IPv6 and IPv4 packets. This standard was developed specifically to enable small devices with limited processors to connect over IP. You might be familiar with this protocol thanks to Thread, which is essentially a standardized version of 6LoWPAN combined with a defined transport layer that is meant to support home automation use cases.
Building a mesh network using 6LoWPAN can be a solid option if one needs a low data rate network that offers wireless internet connectivity. Built-in IP connectivity can be useful for a number of purposes, including allowing devices to connect to the cloud to receive manufacturer updates. Note that, unlike the Zigbee protocol, 6LoWPAN does not include a defined application layer, meaning this choice lacks the ready-made functions offered by the Zigbee framework.
In many circumstances, utilizing Zigbee on top of IEEE 802.15.4 is a highly attractive option. By introducing powerful network and application layers that enable reliable short-range connectivity, Zigbee enhances the communication between IEEE 802.15.4-enabled devices, regardless of the network topology being used. Among other promising use cases, combining Zigbee and IEEE 802.15.4 has the potential to be a game-changer for smart home applications.
In short, the Zigbee network layer builds on the IEEE 802.15.4 MAC sublayer to allow the creation of a network, and handles routing management by acting as a message broker. It also streamlines the connecting, disconnecting, and configuring of smart devices.
The Zigbee application layer includes an application framework that allows end users to interface with devices, often through a basic request/confirmation structure. This layer also includes an application support sublayer that bridges a device’s entire stack, as well as the Zigbee Device Object layer, which plays a critical role in initializing the network layer and ensuring device security.
Notably, Zigbee is designed to provide more flexibility when it comes to choosing a network topology, and it can support star, tree, and mesh networks. The value of being able to arrange IEEE 802.15.4-enabled devices in a mesh network should not be underestimated. A mesh network allows any node to communicate with any other node both directly and through intermediary nodes, which can extend a network’s range far beyond even the top end of the standard IEEE 802.15.4 range. What’s more, if any one node within a mesh network goes down, a transmission can easily be rerouted, a failsafe mechanism that enhances the overall reliability of the network.
Clearly, protocols like 6LoWPAN and Zigbee add a range of capabilities to IEEE 802.15.4-enabled devices while retaining the latter’s distinct advantages, particularly its minimal power consumption. What’s more, newer solutions like Dotdot and Connected Home over IP (CHIP) are giving developers even more options when working with IEEE 802.15.4 PHY and MAC layers.
Dotdot is an application layer based on the Zigbee Cluster Library that enables devices that are built on an 802.15.4 radio hardware layer to interface with each other even if their network layers are different. Only recently announced by a working group comprised of the likes of Amazon, Apple, Google, and the Zigbee Alliance, CHIP will “[build] upon Internet Protocol (IP)…to enable communication across smart home devices, mobile apps, and cloud services and to define a specific set of IP-based networking technologies for device certification.”
Ultimately, while developers of simple devices may be tempted to rely exclusively on the IEEE 802.15.4 standard to save money, the added connectivity, reliability, and security delivered by protocols like 6LoWPAN and Zigbee (and Dotdot and, eventually, CHIP) make investing in a hybrid solution well worth the modest premium.