Demystifying ZigBee and its Application
A look at the differences between IEEE 802.15.4 and ZigBee.
John Schwartz, Digi International -- Control Engineering, 8/1/2008
Wireless networks are becoming more ubiquitous and are implemented using a variety of protocols specifically designed for radio frequency systems. Some protocols that are in use are proprietary to individual vendors while others are industry standards.
| ONLINE Extra: link to more than 30 wireless resources, below. |
Recently, a lot of attention has been given to 802.15.4 and ZigBee, but there is still some ambiguity as to what is different about them and what kind of networks or systems would benefit from these particular protocols.
What is IEEE 802.15.4?
IEEE 802.15.4 is a standard for wireless communication established by the Institute for Electrical and Electronics Engineers (IEEE). A few examples of wireless IEEE standards include the 802.11 standard which defines communication for wireless local area networks (LANs) and 802.16 that is used for broadband wireless communication in Metropolitan Area Networks.
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| At 2.4 GHz, communication occurs in one of sixteen 5 MHz channels ranging from 2.405 to 2.480 GHz. |
This standard is designed specifically for communications in a point-to-point or a point-to-multipoint configuration with sleeping and security also being integral parts of the standard. A typical application involves a central coordinator that often acts as a data collector, with multiple remote nodes connecting back to this central host. While both 802.11 and 802.16 standards are concerned with higher bandwidth Internet access applications, 802.15.4 was developed with a lower data rate, simple connectivity, and battery-powered applications in mind. The 802.15.4 standard specifies that communication can occur in the 868.0-868.8 MHz, the 902-928 MHz or the 2.400-2.4835 GHz industrial scientific and medical (ISM) bands. The 868 MHz and 902 MHz bands offer the least amount of bandwidth under the specification, with 20 kHz and 40 kHz available, respectively, for each channel. While any of these bands can technically be used by 802.15.4 devices, the 2.4 GHz band seems to be most popular because it allows for data rates up to 250 kHz and this frequency is allowed by most countries worldwide for data communications. The 868 MHz band is specified primarily for European use, whereas the 902-928 MHz band can only be used in the United States, Canada, and a few other countries and territories that accept the FCC regulations.
The standard requires the use of direct sequence spread spectrum (DSSS) encoding, and offset quadrature phase shift keying (O-QPSK) modulation with half-sine pulse shaping in the 902-928 MHz band.
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| A typical IEEE 802.15.4 application involves a central coordinator that often acts as a data collector, with multiple remote nodes connecting back to this central host. |
At 2.4 GHz, communication occurs in one of sixteen 5 MHz channels ranging from 2.405 to 2.480 GHz, with a maximum over-the-air data rate of 250 kbps and an address space of 65,535 nodes per every 802.15.4 personal area network (PAN).
The possible selection of one of 16 channels helps avoid interference with other 2.4 GHz networks that may be in the same area. Protocol overhead limits theoretical maximum throughput rate to speeds closer to 125 kHz, and only approximately 2 MHz of the channel is consumed with the specified 5 MHz occupied bandwidth.
If communication in a particular network is infrequent enough, there is no reason that a single PAN cannot support all 65,535 nodes. In many cases, the amount of data being sent and the frequency of those data required by the system end up limiting the number of nodes that can be supported below the maximum.
In essence, 802.15.4 defines PHY and MAC layers that are ideal for low-data rate, low-power applications.
What is ZigBee?
ZigBee is a protocol that uses the IEEE 802.15.4 standard as a baseline and adds an additional network layer to give the system routing and networking functionality. Since the ZigBee protocol uses the 802.15.4 standard to define the PHY and MAC layers, the frequency, signal bandwidth, and modulation techniques are identical.
The ZigBee protocol was developed by the ZigBee Alliance to allow companies to cooperate in developing a mesh network protocol that can be used in a variety of commercial and industrial low-data-rate applications. Mesh networking is useful in applications where the range between two points may be beyond the range of the two radios located at those points, but intermediate radios are in place that could forward any messages between them.
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| To transmit data from poing A to point B when the distance is too great for a single jump, the message could be transmitted through point C to reach the destination. |
The ZigBee protocol is designed so that if a number of different radios were deployed in a given area, the radios would automatically form a network without user intervention. The ZigBee protocol within the radios will take care of retries, acknowledgements, and data message routing.
ZigBee also has the ability to self-heal the network. For example, if the radio at node C (intermediate between nodes A and B) was removed for some reason, a new path would be used to route messages from A to B.
The real advantages of mesh networks are that they improve data reliability by providing multiple redundant paths in areas where a lot of nodes are deployed. They are not designed, however, for every application. It takes time for paths to form and devices to associate. Additional system delays occur as messages must be forwarded on through the network. The 250 kHz of over-the-air bandwidth gets used up very quickly with overhead—making video, audio, and other high bandwidth applications are poor choices for mesh networks.
ZigBee devices can either be used as end devices, routers or coordinators. Routers can also be used as end devices, but the main difference is that end devices are allowed to “sleep” (modules and chipsets used to implement ZigBee have sleep currents down to a few microamps, so battery life can extend beyond several years).
Because ZigBee was designed for low-power applications, it fits well into systems that use small, low-power microcontrollers, and applications where reliability, battery life, and versatility are important but high bandwidth is not.
Some examples include networks for home automation, industrial monitoring, and control; pressure, temperature, moisture, and humidity sensors; commercial and industrial meter reading; home fitness machines, and water quality oxygen content monitoring.
ZigBee devices’ lower data rate allows better sensitivity and range, making IEEE 802.15.4 and ZigBee one of the longer range technologies that exist at 2.4 GHz. The comparison graphic illustrates the similarities between IEEE 802.15.4 and other popular wireless technologies.
Making the decision
Bandwidth, battery requirements, code space, frequency, and range are all factors that should be considered if designing a wireless version of a product is on your company’s roadmap.
If the application strictly needs to communicate in a point-to-point or a point-to-multipoint fashion, 802.15.4 can handle all the communications, and will be simpler to implement than a module or chipset with ZigBee firmware. ZigBee is necessary if you need to use repeating, mesh networking functionality in your system, or if you require the added robustness of a mesh network.
In any case, these protocols might be useful in any application where battery operation is required.
| ZigBee & 802.15.4 | GSM/GPRS CDMA | 802.11 | Bluetooth | |
| Focus Application | Monitoring and Control | Wide area voice and data | High-speed internet | Device connectivity |
| Battery Life | Years | 1 week | 1 week | weeks |
| Bandwidth | 250kbps | up to 128k | 11Mbps | 720kbps |
| Typical Range (meters) | 100+ | Several Km | 50-100 | 10-100 |
| Advantages | Low power, Cost | existing infrastructure | Speed, Ubiquity | Convenience |
ONLINE extra
Wireless Communications for Industry
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Industrial Wireless Implementation Guide (With links to more than 30 wireless resources)
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Survey Says: Users Demand Wireless Monitoring, Control Products
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