PrintByline: Cees Links
Coming in the wake of their voice and data counterparts, wireless networks for sensing and control represent the exciting third wave ofwireless networking. The combination of wireless communications withemerging energy-harvesting and low-power sensor networks will deliver entirely new classes of electronic devices that could revolutionizehow we play, work and conduct the business of day-to-day life.
While connected "smart" sensors present a unique market opportunity, they also pose unique engineering challenges, particularly with respect to cost; global regulatory compliance; interoperability across brands; second sourcing; and, of course, powering and maintaining reliable, always-on connectivity. That connectivity must be maintained over many years in home or harsh environments, as well as in "green" buildings with environmental controls, health and safety devices, and automotive and security apps.
By using a combination of standards for the radio and software stacks, developers can avail themselves of a strong, clear road map to the future of low-power wireless sensor nets.
The need for standards
It is not enough to remove cabling for connectivity; a truly wireless device also eliminates the power wires. Installing and routing power cables can greatly complicate deployment and increase installation costs. Further, requiring "wireless" devices to use power cabling tethers them to a location and inhibits network efficiency and flexibility.
Wireless power exists today in the form of batteries. But battery-operated devices typically are not maintenance free. Periodic batteryreplacement is inconvenient and costly, and disposal of used batteries is not a green proposition.
To reap the benefits of low-cost, wireless, green devices, we musteither eliminate the batteries (by converting to energy harvesters) or at least get extremely long life (more than eight years) out of inexpensive batteries. Only power-efficient devices, communicating via specialized, low-power protocols, can make that possible.
Several proprietary protocols are available today. But to accelerate the acceptance of the third wave of wireless, we need industry standards, for several important reasons:
* Market acceptance. Consumers have grown accustomed to buying devices that conform to industry standards.
* Interoperability. Devices from Manufacturer X should work with those from Manufacturers Y and Z.
* Lower cost. Multiple suppliers must compete with each other on price. Economies of scale lower the prices of common components.
* Silicon business model. High volumes, enabled by global markets,are required to justify the high investment costs for silicon chips.
The wireless networking industry is moving toward adoption of standards governing extremely low-power wireless networks for industrial,home, enterprise, and sense and control applications. Powered by batteries and energy-harvesting devices, truly wireless products and technologies are becoming a reality.
The development and acceptance of technology standards are important factors for the adoption of new technologies. Product integrators require technology standards because standards provide product interoperability, a large body of knowledge and development sources, and second-sourcing flexibility.
Which wireless standard?
The requirements for sensor applications are completely different from those for wireless voice and data networks. The availability of power is the most conspicuous difference; sensors often have to work for years on a nonrechargeable coin cell battery or on energy harvested from the environment through a solar panel or a vibration harvester. Other sensor-specific application requirements are related to automatic network organization, reliability, communications range and thelarge number of nodes to be supported in a single network.
For wireless sensor transceivers, the dominant standard--indeed, probably the only real standard--is IEEE 802.15.4, for which several vendors offer compliant transceiver chips. Some are minimal implementations of the standard; others offer add-ons that are useful in some application segments, such as power-reducing features targeted toward coin-cell and batteryless applications.
One key aspect when choosing any wireless standard is the network stack. The stack has two responsibilities, the first of which is to form and maintain the network.
An important consideration in wireless network stack design is theability to cope with the constantly varying quality of the wireless links between nodes. For example, in a building automation application the movement of people about the building can compromise the link quality; a link might "disappear" at any moment, possibly isolating a node or even a whole branch of the network. To provide uninterrupted connectivity to all parts of the network, the stack must be able to reorganize the communication routes by establishing new links.
The other responsibility of the network stack is to ensure that messages can travel from a source node to a destination node in a reliable and efficient way. Efficiency here means that latency requirements must be met and message routing bottlenecks avoided.
The underlying technology driving the third generation of low-power networks is IEEE 802.15.4, part of the IEEE 802.x family of networking standards.
In addition to 802.15, there are four other wireless subfamilies that are based on 802: IEEE 802.11, also known commercially as Wi-Fi; IEEE 802.16, for wireless metropolitan area networks such as WiMax; IEEE 802.20, for mobile WMANs such as WiMax Mobile; and 802.22, governing wireless radio area network apps such as TV band reuse and cognitive radio.
For our purposes, 802.15, the standard for wireless personal area nets, further breaks down into a subset of standards. The subset includes 802.15.1, for wireless PANs such as Bluetooth and WiBree; IEEE 802.15.3, for high-rate WPANs such as ultrawideband, Wireless USB and WiMedia; and IEEE 802.15.6, for body area networks. Also in this family is IEEE 802.15.4, the standard for low-rate WPANs, including the various flavors of ZigBee as well as WirelessHART (the wireless Highway Addressable Remote Transducer protocol) and various proprietary stacks.
The 802.15.4 standard is targeted for low-power, midrange applications of up to 100 meters at a low data rate (100 kbits/second). IEEE 805.15.4 can support data rates of up to 250 kbits/s in the 2.4-GHz frequency band. Within the 2.4-GHz ISM band, it supports 16 channels; within the U.S. 900-MHz ISM band, it supports 10 channels. When used in the European 868-MHz band, 802.15.4 supports a single channel.
An 802.15.4 network can be set up as a star or a peer-to-peer topology. It supports low-latency devices, CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) channel access and dynamic device addressing, and uses a full-handshake protocol for transfer reliability.
An 802.15.4 net can provide an effective data rate of 100 kbits/s while only requiring 100 mW when operating at full speed. Bluetooth (802.15.1) can provide a much higher effective data rate (about 0.5 Mbit/s) but requires 10 to 100 times more power. Wi-Fi (IEEE 802.11) can provide even higher data rates but requires 100 to 1,000 times morepower. By using new processor architectures and ways of operating, ultralow-power 802.15.4 networks can provide reliable communications while only requiring 150 microjoules per packet.
ZigBee is backed by a strong and dynamic standards body with an active marketing wing and is focused on a variety of commercial, home automation and industrial markets. ZigBee Pro is similar but offers more interference robustness for radio-unfriendly environments, as wellas an increased range and a higher number of nodes.
ZigBee Specialized Profiles consist of ZigBee subsets for specificfunctions. These smaller, more task-oriented stacks require fewer Mips and less flash memory, and they consume less power.
Specialized profiles for smart energy and home automation have already been developed and released. A ZigBee green power profile is still in development. Aimed at energy-harvesting solutions, it will provide a guideline for the seamless connectivity of ultralow-power energy supplies with ZigBee networks and sensors. It is expected to be completed by the end of this year.
According to the ZigBee organization, "Green Power will enable newcapabilities for ZigBee and ZigBee Pro networks and will offer an established, competitive marketplace for deploying switches, sensors and controllers using harvested energy in residential, commercial and industrial environments."
Earlier this year, ZigBee announced a partnership with four of thelargest global consumer electronics manufacturers. The result is theZigBee RF4CE standard, which provides a common platform for the development of ZigBee-powered RF remote controls and devices for home automation and entertainment. ZigBee RF4CE will provide consumers with highly featured remote controls that can transmit through walls and cabinet doors; provide interactivity; and, by using ZigBee low-power features, run for a decade or more on a single coin cell battery hardwired inside.
Other standards are largely focused on industrial applications andthus tend to add complexity that does not fit into consumer and commercial cost budgets. For factory automation, ISA100 provides IPv6 Internet Protocol capability at the expense of longer headers. Currentlythe standard is the target of some political wrangling, with open conflict among participants. WirelessHART is expected to merge with ISA100, but for now politics are holding back the merger.
In addition to the approved and in-development IEEE and industry standards, several companies have developed proprietary solutions and are pushing to have them accepted as standards. "Proprietary" does not mean that a specification is not open, because sometimes it is; butif a single company controls the direction of the technology, it effectively has a monopoly.
Proprietary standards have often been designed around a single or a limited set of applications. In practice, a proprietary technology can develop much faster than an industry technology standard because there is no need to reach consensus. Sometimes the proprietary standard may have advantages over other standards when used within its limited set of target applications. Conversely, however, a proprietary technology rarely is able to address the broad space of applications that an industry standard addresses.
ZigBee advantages
ZigBee is a well-known, well-defined standard with a great deal ofindustry acceptance and resources behind it. It also offers the advantages of reliability and interoperability.
When choosing to go with a proprietary solution, a developer needsto remember that not all software stacks are the same. Some are dumbed-down ZigBee; others are akin to supercharged ZigBee. Some proprietary protocol stacks have severe limitations regarding performance andend-use applications. At the same time, some currently proprietary stacks have generated considerable industry interest and may wind up being adopted as ZigBee profiles down the road.
One advantage of using an 801.15.4 wireless network is the abilityto create mesh networks. A "pure" ZigBee network consists of a network coordinator (NC) device and numerous full-function devices (FFDs) and reduced-function devices (RFDs). In a standard ZigBee network, only the FFDs are meshed and can talk to one another. The FFDs require power (hooked up to a power main); the RFDs don't require as much power and may be powered by batteries or even energy harvesting.
Low-power routing (LPR) networks add synchronization features and "mesh to the edge." All the nodes can talk to each other. The nodes are lower-power routers but still require battery power; energy harvesting is not sufficient to power the nodes.
By building a mesh network that combines the essences of both ZigBee and LPR networks, it is possible to design and develop a "batteryless" network; the mesh node routers still require batteries to operate, but the batteryless end devices can be powered by energy-harvesting solutions. The entire network is then controlled by a powered NC unit.
Cees Links ( cees.links@greenpeak.com ) is founder and chief executive officer of GreenPeak Technologies.
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