Ushering in a new technology or standard to the market always seems to take longer than expected. In fact, the time between concept and actual adoption often can be measured with a stack of calendars. ZigBee is no exception. For years now, followers of wireless technology have heard about the enormous effort to develop the ZigBee standard and its applications.
Now that the silicon and software are available, we can expect real-world products this year. While the entire development cycle was shorter than that of Bluetooth and Wi-Fi, lots of work was poured into the effort to reach this point. Today, ZigBee is ready for those designers who have waited to add wireless to their designs.
Consider how long it took Bluetooth to become successful. Today, Bluetooth is everywhere—cell phones, headsets, laptops, PDAs, and peripheral devices. It's so common, it's taken for granted—a telltale sign of success.
According to Mike Foley, director of the Bluetooth Special Interest Group (SIG), over 500 million Bluetooth chips have been shipped to date, and the current ship rate is 9.5 million chips per week. Does ZigBee have that potential? Bob Heile, the ZigBee Alliance's chairman, thinks so. In fact, its overall potential may be greater.
IEEE 802.15.4 Standard 101
ZigBee is the nickname for a short-range wireless technology used for personal-area networks (PANs). It's based on the IEEE 802.15.4 standard and guidelines developed by the ZigBee Alliance.
The IEEE standard defines the physical layer (PHY) and mediaaccess-control (MAC) layer of the radio. The ZigBee standard effectively extends from that, covering the network and security layers of the protocol, as well as application frameworks and profiles (Fig. 1). The ZigBee Alliance also has established a testing and certification program to ensure interoperability between products from different ZigBee vendors.
The 802.15.4 standard defines three license-free (FCC Part 15) bands of operation: 868 MHz, 915 MHz, and 2.4 GHz. The 868-MHz band is for European use, the 915-MHz band is for the U.S., and the 2.4-GHz version is available worldwide. It's no surprise that most vendors have selected the 2.4-GHz band for maximum volume.
Data rates are lower than most other wireless standards—20 kbits/s for the 868-MHz band, 40 kbits/s for the 915-MHz band, and 250 kbits/s for the 2.4-GHz band. These low rates are fast enough for monitoring and control applications. The 868- and 915-MHz radios use direct-sequence spread-spectrum (DSSS) with binary phase-shift keying (BPSK) modulation. The 2.4-GHz radio employs DSSS with offset-quadrature phase-shift keying (O-QPSK).
Other features of the 802.15.4 standard include receiver energy detection, link quality indication, and clear channel assessment. It supports contention-based and contention-free access methods. Maximum packet size is 128 bytes, including a variable payload of up to 104 bytes. The 802.15.4 standard also uses 64- and 16-bit addresses that support over 65,000 nodes per network.
The MAC enables network association and dissociation. It has an optional superframe structure with beacon for time synchronization. Also, its guaranteed time-slot mechanism supports higher-priority communications. The access method is carrier-sense multiple access with collision avoidance (CSMA-CA).
Furthermore, the 802.15.4 standard radios are tops when it comes to bit error rate (BER) for a given signalto-noise ratio (SNR). ZigBee easily beats out Bluetooth, Wi-Fi, and even ISM-band (industrial, scientific, medical) frequency-shift keying (FSK). And for robustness and reliability, you'll be hard-pressed to find anything that's superior to ZigBee.
ZigBee Adds Networking
The ZigBee standard builds on the 802.15.4 stack, defining how devices are networked. It supports three major types of ad-hoc, self-forming wireless networks—star, mesh, and cluster-tree.
These topologies support three types of nodes. First is the ZigBee Coordinator (ZC), which initiates the network formation. There's only one ZC per network. Next is the ZigBee Router (ZR). It serves in a monitor or control function, but it's also a router or repeater for multi-hop messaging. Third is the ZigBee End Device (ZED). It simply serves in a monitor or control function, but it doesn't route or repeat.
In the IEEE standard, the ZED is called a reduced-function device (RFD), and the ZC and ZR nodes are called full-function devices (FFDs). Each node includes the radio transceiver plus an embedded controller with the IEEE and ZigBee stacks and minimum RAM and ROM.
The main function of these battery-powered, single-function devices is to send data from a sensor or receive a command from a master controller. The FFDs are more powerful and have additional memory. They can serve as repeaters or network coordinators. ZCs and ZRs can talk to one another or to any ZED, but ZEDs can only talk to ZCs or ZRs.
The star topology is the most popular method in simple systems (Fig. 2). It uses a central FFD and multiple RFDs. ZigBee's usefulness really comes alive in mesh topologies, though, where multiple nodes talk to each other over short distances (Fig. 3). But each node also can serve as a repeater for other nodes. If one node is too far from the destination node, the message can be sent through two other nodes acting as repeaters.
This feature greatly extends the range of any given node beyond its normal line-of-sight permitted by radio propagation physics. It also makes communications more reliable. If one node goes down, the message still gets through via other network paths in the mesh.
ZigBee networks are so appealing because they're self-forming and self-healing. Nodes seek one another out and automatically link up. This occurs after each node is " commissioned" by the software. It assigns addresses and provides routing tables that identify approved communications buddies. For security, ZigBee uses the AES-128 encryption method to provide authentication and encryption.
While the networking capabilities are inherent in the 802.15.4 and ZigBee standards, the application defines the overall function. Applications are implemented according to a specific need or to profiles like those used in Bluetooth. A profile defines node behavior in a particular application, such as sensor networks or industrial control. The ZigBee Alliance has already finished a home lighting and control profile. It's now hard at work developing application profiles for building monitoring and control, security, automatic meter reading (AMR), industrial machine and process monitoring, ZigBee gateways, and other areas.
ZigBee wireless networks are intended for low-duty-cycle applications, such as applications that are active less than 1% of the time. Sensor networks are the most common example. Others include control of lights, security systems, and AMR. With such low duty cycles, nodes can be battery-operated with a battery life of many years.
When they aren't transmitting, nodes go into a sleep mode and draw only a few microamps of power. They can wake up in about 15 ms. Sensor nodes can be interrogated periodically, or they can wake up on their own as programmed. With its attributes of low power and many years of battery life, ZigBee fits applications other wireless technologies simply can't handle.
And if you're looking to take advantage of the benefits of mesh wireless, ZigBee is a great choice. It substantially minimizes the required amount of mesh design and software development.
Let's put it this way. ZigBee addresses most of the applications that Bluetooth, Wi-Fi, Ultra-Wideband (UWB), cell phones, and other wireless technologies don't cover.
ZigBee's low cost, low power, small size, and ensured interoperability-will lure engineers who want to make some part of their product wireless. And with a fully defined standard as well as available hardware and software, adding ZigBee wireless will be relatively easy. Get ready for the "peel and stick" phase of wireless.
It's essential for the the industry to first address the most interesting and potentially voluminous applications. Once volume is established (as was the case with Bluetooth in cell phones), a ZigBee module that costs less than $5 will become reality. As a result, ZigBee will hop into many other low-volume applications.
The sector with the greatest potential is home automation. The industry has identified over 200 possible home wireless applications, creating the volume needed to make ZigBee the wireless king. But because consumers demand the cheapest and easiest-to-use products, companies are proceeding with caution.
So far, the industrial sector heads the list for ZigBee adoption, with products for building monitoring and control and AMR. Testing, validating, and improving industrial/commercial ZigBee will lead to better home products. As they say, applications are limitless (see "Top 12 ZigBee Apps,").
Designing With ZigBee
Assuming you have a wireless application, your first task is to decide which technology to use. Cost, availability, speed, range, and power consumption apply. But don't forget to factor in time-tomarket and the engineering talent at your disposal, mainly wireless expertise and software-development capabilities. There are six technology options:
- Infrared (IR): It's the cheapest of all, but it offers the shortest range and operates via line-of-sight only. Data rates reach 16 Mbits/s.
- ISM wireless: These inexpensive chips operate in the 315-, 433-, and 902-to 928-MHz range. There are lots of inexpensive modules too, but no standard protocol. It's best for really simple applications.
- Bluetooth: This fast (to 3 Mbits/s), low-cost technology features a range of typically 10 to 100 m. It's highly developed, with many profiles. It also offers PAN ad-hoc (non-mesh) networking capability.
- Wi-Fi: Also known as 802.11a/b/g, it's widely available. Though more expensive, Wi-Fi offers very high data rates to 54 Mbits/s and the longest range (100 m). Wi-Fi transceivers consume more power and are more difficult to integrate into a mesh network.
- ZigBee: It's the newest technology. It trades off a low data rate (up to 250 kbits/s) for very low power consumption and an inherent mesh capability. Few formal profiles are available now, but more are on the way. Prices will drop as volume builds. It has the greatest potential among the wireless options.
- UWB: Ultra-Wideband is now available in chip and module form. Its very high speed (100 to 480 Mbits/s) and short range (less than 10 m) tend to typecast it as a video and high-speed data (USB) solution in consumer products. No mesh capability exists at this time.
Mesh networking extends your wireless possibilities by offering longer range at lower cost via radio relay. Mesh also adds a major benefit—reliability. It solves many problems associated with wireless adoption. Oh, by the way, it just so happens that ZigBee offers mesh.
If you select ZigBee, your next choice is the frequency band. In Europe, there's only one choice—868 MHz. In the U.S. and the rest of the world, the 2.4-GHz band is the most useful, and that's where most chips and modules operate.
Only a few 915-MHz modules are available, but their advantage is longer range (up to 1000 m outside) and greater penetration of walls and other obstacles. ZigBee's 2.4-GHz maximum range is 400 m outside and about 30 m inside. It can penetrate one concrete wall, but not two.
If you feel secure in your wireless design abilities and programming acumen, you can buy your own chips and dig right in. Don't forget the need for a good antenna. Reference designs can really help here too. The ZigBee Alliance has "blessed" six ZigBee-compliant development platforms, and more are on the way.
Because chip design is tied so closely to the IEEE 802.15.4 and ZigBee Alliance standards, the features and specs are very similar. How do you distinguish one from another? Besides a good reference design and development platform, look for specs like high receiver sensitivity (for longer range and greater reliability) and sleep current drain (for longer battery life).
If you want to avoid the whole wireless part of the design, go for a complete module. A wide variety of companies makes complete packaged modules on pc boards or in a housing or dongle, including antenna. To use such a module, all you have to do is connect a battery and your sensor for monitoring applications or a battery and your actuator in a control application.
At this point, your design involves the writing or the code for your unique application. Keep in mind that most chip and module vendors have software development platforms. Also, third-party software is becoming more common for popular applications, especially sensor nets and mesh.
ZigBee Down the Line
Like most wireless technologies, there's a roadmap. While most of us aren't part of the inside group planning the future, we can guess what's on the roadmap based on tradition, innuendo, and common sense.
First, you can expect more profiles from the Alliance. You'll also see more products, mainly software, to speed your development. Hardware-wise, we might expect a higher-speed version some time in the distant future. Right now, though, speed isn't a major issue for most applications. But by adjusting the modulation, designers should be able to double today's 250-kbit/s speeds.
Finally, changes to the radio to permit mobile applications are in the works. ZigBee, as is, probably can handle modest speeds on moving nodes from 10 to 15 mph. But higher-speed handling needs a change in technology if speeds to 100 mph are desired, such as in RFID or other mobile applications.