Protocols for Sensor Networks
Unlike Wi-Fi, which has become universal for wireless data networks, and Bluetooth, which is included in every cell phone shipped today, there is no ubiquitous protocol for wireless sensor networks. ZigBee is the best known, has the largest alliance, and will likely dominate in home and building automation applications. But there are many other protocols competing for sensor networking applications including: WirelessHART; DASH7; 6LoWPAN; and others. There are also dozens, if not hundreds of proprietary protocols like JenNet, DigiMesh, SNAP and Z-Wave.
Generally speaking, all of the suppliers of wireless ICs for sensor networking applications provide ZigBee and other popular stacks at no charge to customers who use their devices. But they also encourage the use of their own proprietary protocols, which are typically much simpler and easier to use, and don't require membership in an industry alliance.
Whether proprietary or backed by a large industry alliance, every protocol has its strengths. DASH7 is one of the few non-profit alliance-supported protocols that use the 433-MHz ISM band, giving it a range advantage, and its open source protocol stack is small, needing less than 32 kB. The 6LoWPAN protocol adapts IPv6 packets to run on the small frame size of the IEEE Std 802.15.4 radio, giving IP addressing to individual sensors and enabling their visibility on a global scale.
While there is no ubiquitous sensor networking protocol, many of them rely on the IEEE Std 802.15.4 radio. It defines sophisticated MAC+PHY layers that support self-forming networks using mesh, star and cluster-tree topologies, with spread spectrum signaling in both the 2.4 GHz band and several sub-GHz bands. The 2.4-GHz band supports 16 non-overlapping channels, each with up to 250 kbps data rates. The various protocols that use this standardized radio define the upper protocol layers – the network layer up through the application layer, to suit their own specific needs.
ZigBee was the earliest protocol defined to run on the IEEE Std 802.15.4 radio, and has been widely deployed in sense and control applications like home/building automation, advanced metering, and health/fitness monitoring. Today there are two versions – ZigBee and ZigBee PRO, with most new development using ZigBee PRO. New variants of ZigBee include RF4CE – the emerging standard for RF-based remote controls for consumer electronics, ZigBee Green Power – a lightweight stack for energy harvesting nodes, and ZigBee IP – which replaces the network layer with 6LoWPAN for IP connectivity.
The ZigBee Alliance has also developed a variety of ZigBee application profiles (application layer protocols) for specific applications like home automation, lighting and smart energy. These profiles define the device types that participate in the application, their characteristics, and control interfaces simplifying the development of ZigBee-based products and ensuring interoperability at the application level. The ZigBee Smart Energy 2.0 profile, released in draft in July, 2011, is a coordinated effort with the Wi-Fi Alliance, the HomePlug Powerline Alliance, and others to define an IP-based application layer protocol for the smart grid's Advanced Metering Infrastructure (AMI) and Home Area Networks (HAN).
One of the emerging trends in wireless hardware design is the growing abundance of pre-certified, integrated wireless modules. Modules are offered as radio-only modules which interface to existing processors, pre-integrated with onboard processors, or as host-less network processors in which case the protocol stack itself is also embedded in the module.
A serial-to-Wi-Fi module for example, presents the simplest way to integrate Wi-Fi connectivity for machine-to-machine(M2M) applications by using a common UART interface and simplified API that lightens the software effort of integrating the TCP/IP stack. Pre-certified modules can also alleviate the burden of RF design and test, FCC/ETSI/IC certification testing, and provide a significant time-to-market advantage.
About the author
Joe Tillison is a technology director at Avnet Electronics Marketing Americas. For the first 13 years of Joe's career, he designed electronics for spacecraft and launch vehicles at Lockheed Martin Space Systems in Denver. For the last 13 years, Joe has worked in the semiconductor distribution channel, developing tools and technical training that helps design engineers shorten design cycles by using increasingly more complex semiconductor products. Joe has a BSEE from the University of Oklahoma and a MSE from the University of Colorado at Boulder.
When it's time to design for the wireless model, the good news is that it's easier than ever before to add wireless connectivity. Embedded systems developers have a diversity of networking protocols to choose from that are uniquely suited for M2M communications, automation, smart energy, and many other applications. And the growing abundance of pre-certified, integrated wireless modules give the hardware designer a straightforward path to wireless, even with little or no RF design experience.