Understand the ZigBee Spec
Here is a little info that describes the Zigbee spec and its uses quite nicely. Keep in mind that this does not cover all potential applications processes that are/will be possible..EOM
By Jon Adams, Freescale Semiconductor Courtesy of Wireless Net DesignLine
Building wireless into an application isn't a simple thing—it takes an understanding of radio performance, propagation, modulation, coding, protocol, and more to make a good wireless link. It's no wonder that so many designers throw up their hands in despair when attempting to get rid of a simple serial connection. Until recently, the wireless choices were either expensive, proprietary, and ill-suited to the developer's need, or less expensive, proprietary, with no hope of broader interoperability, etc. However, ZigBee and IEEE 802.15.4 may put an end to those problems.
The ZigBee Alliance is a rapidly growing, open alliance of over 160 companies that have a common goal, namely cost-effective, simple, ubiquitous wireless connectivity between nearly any devices. Cost-effective means that uncomplicated products at retail could be $20 or less. Simple means that the time it takes from idea to engineering sample is measured in work-weeks, not work-years. And ubiquitous means that if the solution meets the first two requirements, then nearly anything that has sensing ability or control needs can be made wireless effectively and quickly.
IEEE 802.15.4: the wheels and chassis for ZigBee wireless
Many of the individuals that work Alliance members are also voters in the IEEE 802 standards group, specifically the 802.15 Wireless Personal Area Networking group. Through their efforts over the past five or so years, these engineers created the 802.15.4 wireless standard which serves as the basis of ZigBee, and made sure that the world had an open, wireless standard that made good on broad applicability, low-cost, and low complexity. Because of their efforts, there are already many silicon radio platform vendors building and selling 802.15.4 radios with competition heating up and driving down prices. While at first generation silicon right now, the prices are already on par with current fourth-generation Bluetooth silicon, primarily because of ZigBee technology's lower complexity (Fig. 1).
1. ZigBee technology sits on top of the IEEE radio.
IEEE 802.15.4 is the undercarriage of the ZigBee vehicle, so ZigBee networking counts on the IEEE standard to deliver the robustness and reliability that one expects from a wireless network. Developed to be a low-duty cycle, acknowledgement-based protocol, the spec brings together multiple RF channels in the 868-, 915-, and 2400-MHz bands. While the lower bands have some attractive performance characteristics, it's the 2400-MHz band that attracted the most interest, not only because of the worldwide availability of that band for unlicensed operation, but also because of the performance of the products developed for that band (Fig. 2).
2. Shown are the available channels and the unlicensed nature of each band..
ZigBee wireless technology takes advantage of the robustness inherent in the IEEE radio and layers upon it a strong mesh network that can self-form and self-heal, route messages quickly and accurately, provide interoperability mechanisms and testing capabilities to ensure that like devices can talk to like devices, and that all devices have can take advantage of the ZigBee network. The network functions allow for three device classes, the coordinator, the router, and end device. There are some important differences in the home space for these devices. The coordinator, of which only one is needed per network, sets up the initial address space and network configuration.
Profiles and device descriptions
ZigBee specifies how a device, like a light switch, must behave in a given environment. These behaviors are in documents published by the Alliance and available on the Web. The Alliance divides up functionality according to markets. For example, the home space differs from the industrial controls space, as installing a control/sensor network in a home presumes a different competence than would be expected in an industrial setting. The home space must be consumer-friendly, and that means OEMs can't expect consumers to have knowledge of (or even care about) networking, radios, device functionality, etc. It must be simple to be successful, yet with the right features presented in a straightforward manner so that consumers won't return it after the out-of-the-box experience.
The ZigBee Web site provides the necessary documentation to develop an application. For example, the Home-Control-Lighting (HCL) profile document specifies the overall home control lighting environment, types of devices, and cluster IDs used to transmit information. The HCL Switch-Remote-Control (SRC) specifies the required ZigBee-specific physical device I/Os. These two documents, available to the general public, provide the core of what the device needs to do to be considered a ZigBee product.
Designing a ZigBee product
As an example design, let's choose a battery-operated, remote-control light pad with one push button for the home controls space. Note that the device uses the Simple Binding process. In the SRC device document (Figure 6b) we discover that the OnOffSRC cluster has an attribute called OnOff, with three possible data values. 0x00 specifies OFF, 0xFF specifies ON, and 0xF0 means TOGGLE (if it was on, turn it off, and vice-versa). For the simple light switch, we'll use only the TOGGLE value, because we've only got a momentary switch on a faceplate. The code will issue a ZigBee packet that specifies the Cluster ID of 0x13 and a data value of 0xF0 every time the button is pushed (Fig. 3).
3. Taken from the HCL Profile document, this figure diagrams the SRC device, with the mandatory and optional interfaces.
According to the specification (paragraph 1.4.2.2 End Device Bind Overview), simple binding means: Provides the ability for an application to support "simple binding" where user intervention is employed to identify command/control device pairs. Typical usage would be where a user is asked to push buttons on two devices for installation purposes.
This is an intuitive method of binding and one that's used in many applications. In our light-switch product, this method may be cast as an instruction to the user: "Hold down the button on the remote light switch for five seconds, then push the bind button on the master home controller within 60 seconds." There could also be other physical instantiations of the required approach, but it's always essentially the same.
The binding comes from user physical input to both the device that can issue the command (the switch) and the device that will respond to the command (the load control). This binding information will then be stored, at a minimum, in the ZigBee coordinator assigned to the network, and may also be stored in the end devices as a fallback.
Keeping the task as straightforward and simple as practical, you may choose to start with someone else's ZigBee Compliant Platform, where they've already demonstrated to the Alliance that the silicon platform with the appropriate software meets the Alliance's technical specification requirements. As of April 2005, four manufacturers offered a compliant platform.
Using the development tools provided by the silicon vendor or a third party, craft the ZigBee-specific parts of the application code while maintaining your overall look and feel that differentiates your product from others on the market. Spend the majority of your time on how the product interacts with the user. Using a compliant platform allows you to not worry as much about the other required "under the hood" parts.
Interoperability and Certification Testing
Before releasing the product, the designer should check to ensure that it interoperates with other ZigBee devices, first at a basic IEEE 802.15.4 level, then at a ZigBee network level, and finally at the device profile and description level. The Alliance schedules quarterly interoperability events called ZigFests, in which developers can perform interoperability and functionality testing in a secure environment.
Once the product reaches engineering sample stage in its final form factor, it's time to bring it to a ZigBee Alliance certified test house to verify its adherence to the chosen profile. This is an important step and necessary for two reasons: 1) You want your customers to know that the product is compatible with other ZigBee-certified products in their environment, and 2) The Alliance wants to ensure that a product that wearing the ZigBee logo has been checked for adherence to the same specs that other similar devices have undergone. Certification is a two-way street, one that benefits both the developer and the consumer.
For the simple battery-operated light switch, the certification testing is probably less than a day's work at a test house. But in those six to eight hours, many things are checked. It starts with quick spot checks, like RF frequency and basic functionality, like whether the product works on all the frequencies allocated for that band, whether the modulation is clean enough, whether the range is sufficient, and so on. Once the basic functional tests have been completed, the majority of the effort is spent making sure that the device under test meets the requirements of both the ZigBee profile and device description documents. The testing wraps up with a written report and the developer and Alliance notified that a light switch has successfully passed the certification testing.
After receiving the certification notification, the developer may request the use of the ZigBee logo for that particular product. The developer must supply the product part number and as long as the Alliance has proof from the test house that the product passed the certification testing and the developer has demonstrated interoperability at least at one ZigFest, the Alliance can issue authorization for logo usage.
Because it's a wireless device, the developer must also ensure that it meets the radio regulatory rules for the countries in which it'll be sold or used. For the U.S. and Canada, that means at a minimum getting transmitter testing done at an FCC/IC-certified test house. Fortunately for the developer, the ZigBee-authorized test houses are also regulatory type-certification test houses, as are many other companies. This testing is generally lengthier than the ZigBee certification testing, but for a simple device, not much more so. This testing is a requirement for all wireless devices sold commercially, whether ZigBee or not.
About the author
Jon Adams is the director of radio technology for Freescale Semiconductor's Wireless Mobile Systems Group. He's also a member of the ZigBee Alliance Board of Directors. Adams can be reached at jta@freescale.com.
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