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ZigBee Solution for Wireless Apps

Mon, 01/31/2005 - 10:29am

Low-power, low-cost, and robust ZigBee hardware is beginning to show up in the marketplace.

By Atmel

Glossary of Acronyms

BPSK - Binary Phase Shift Keying
CSMA - Carrier Sense Multiple Access with Collision Avoidance
DSSS - Direct Sequence Spread Spectrum
ES - Advance Encryption Standard
FFD - Full-Function Device
GTS - Guaranteed Time Slot
HVAC - Heating , Ventilation and Air Conditioning
ISM - Industrial, Scientific, Medical
MAC - Media Access Controller
MCU - Microprocessor Control Unit
O-QPSK - Offset-Quadrature Phase Shift Keying
RFD - Reduced-Function Device
RISC - Reduced Instruction Set Computer
SRAM - Static Random Access Memory
WPAN - Wireless Personal Area Network

ZigBee is a low data rate, ultra-low power, networked wireless standard for control and monitoring applications based on IEEE Standard 802.15.4. ZigBee-enabled products do not need hard wiring between a powered device and its switch and offer the potential to save millions of dollars in wiring costs in new and existing construction.

There are a number of vendors offering ZigBee-based 802.15.4 radios. Some are designed to be used with general purpose MCUs and do not have security hardware or memory structures to cost effectively meet the program, data storage and network security requirements of the ZigBee standard. These units require encryption to be implemented in software. This approach adds to the processor load, power consumption, and required memory.

A complete solution is being offered by companies like Atmel. The AT86ZL3201 Z-Link Controller is the next generation of ZigBee hardware that integrates some of the functions left to peripheral hardware. Devices like the AT86ZL3201 can integrate encryption hardware with an 8-bit RISC processor and memory structures that are optimized specifically to implement the ZigBee standard.

The ZigBee Standard

The ZigBee standard targets industrial monitoring and control applications that have relatively small amounts of data to transfer and are turned off most of the time (duty cycles of <1% in some cases) but also respond immediately when needed (low latency). They must also be highly reliable, secure, and consume almost no power. They must support very large networks (e.g., lighting or HVAC for a large office building in a variety of topologies (mesh, star, and cluster tree).

Many ZigBee devices, such as wireless light switches, offer consumers the convenience of being able to instantly add a light switch and a light anywhere in their home or office. Since there is no hard wiring between the light and the light switch, these units will use standard alkaline batteries with a battery life of two or more years. When the battery dies, it is simply replaced. Other ZigBee devices will be used in large networked applications (such as emergency lighting for large office buildings) with thousands or even tens of thousands of devices. Unlike hard-wired lighting systems, ZigBee devices can be networked in such a way that the lighting system will continue to function even if there is a catastrophic failure (e.g. fire or earthquakes) somewhere in the hard-wired system. ZigBee devices must also have low latency. No one wants to wait three seconds for the light switch to work.

Achieving this vision requires that ZigBee devices be extremely low-cost and have exceptionally low power consumption. ZigBee networks must be highly reliable and secure. Attached devices must function consistently and only be accessible by authorized persons. Toward this end, the ZigBee standard specifies a battery life of two years or more, 128-bit symmetric key AES in CCM1 mode, network join time of 30 ms, sleeping slave to active time and active slave channel access times of 15 ms, and a peak information rate of 128 kb/s.

The ZigBee standard is less complex than other wireless standards (currently estimated to be 32 kB for the ZigBee protocol stack vs. 250 kB for Bluetooth and 1 MB for 802.11b). ZigBee applications require less processing and have much smaller data and program memory requirements than other wireless standards. In addition, the ZigBee Alliance has defined two device types: FFDs for network routing and link coordination, and RFDs that are used as simple send/receive devices in the network. ZigBee is based on the tri-band IEEE WPAN Standard 802.15.4 with 16 channels at 250 kb/s in the 2.4 GHz ISM band, 10 channels at 40 kb/s in the 915 MHz ISM band, and one channel at 20 kb/s in the European 868 MHz band. The physical layer includes receiver energy detection, link quality indication, and clear channel assessment, while the 802.15.4 MAC allows network association and disassociation, has an optional superframe structure with beacons for time synchronization and a GTS mechanism for high priority communications. IEEE 802.15.4 supports up to 65,000 nodes (there can be two nodes for a network) in star, cluster, or mesh networks, with a range of one to 100 meters. Carrier Sense Multiple Access with CSMA-CA is used for channel access, using contention-based and contention-free access modes.

What’s Next

Until now, ZigBee designs have been implemented using 802.15.4 radios with standard microcontrollers that have less than optimal memory configurations and lack certain vital peripherals. In some cases, these features are offered on the radio. In other cases, ZigBee-mandated tasks, such as encryption/decryption, must be done in software, increasing program size, load on the processor, power consumption, and cost. Another drawback of general purpose MCUs is that few have more than 4 kB of SRAM. Early estimates suggest that the 802.15.4 and ZigBee standards alone are likely to need 4 kB SRAM to execute, meaning there may not be enough on-chip SRAM to execute encryption algorithms or the application itself.

The 802.15.4 and ZigBee stacks are estimated to require 32 kB. Up to 8 kB of additional program storage will be required for the application profile and application. Thus, general purpose MCUs with flash densities of 64 kB or 128 kB MCUs will be required, even though much of the flash will not be used.

Finally, the 802.15.4 and ZigBee firmware must be modified to fit the architecture of any general-purposed MCU used in the application. Developing the firmware, integrating it with the hardware and getting it qualified could easily add a year to the design cycle.

Footnote

1. Encryption/decryption in enhanced counter with cipher block chaining message authentication code mode.

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