Abuzz About ZigBee

Low-Power Self-Organizing Data Networking
Contributed editorial appearing in
Scientific Computing 22:11, October 2005, pg. 14.

I recently purchased a 2006 Toyota Corolla with the goal of reducing the frequency of trips to my local filling station. In time, I may begin to miss my close friends behind the counter, but one thing I was startled to miss right away was the throttle cable to the Corolla's VVT-i engine. Similar to the "fly-by-wire" transformation experienced by the U.S. Air Force, "drive-by-wire" technology has trickled down to entry-level automobiles. Rather than adjusting the fuel flow mechanically, a sensor in the gas pedal communicates with the electronic throttle control (ETC) unit, which subsequently follows its own internal logic to meet the new set point. This can be disquieting to folks with control issues, but the pedal does not "tell" the ETC to accelerate the car, it simply "asks" for the new speed and the ETC responds. The concept of "control" is evolving into the concept of "re-goal. "

In order to control a device completely, it must be given its every move. If there is a lag between the device and the controller, it either becomes dormant or, worse, it fails to react to its dynamic environment and develops into a hazard to itself or others. My son and I have lost multiple remote-control cars to high-speed collisions with the curb when the cars traveled beyond the range of their transmitters. One solution is to use a wired communication system, but the tether severely limits the range and location of the device. Increasing the range of the transmitters and their communication rate so there is enough bandwidth to accommodate error correction can help as well. This is easily stated, but it requires a large amount of power. Wireless WiFi laptops, cell-phones and Bluetooth-enabled handhelds transmit and display voice and data at high rates, but their batteries must be recharged daily.

Enter ZigBee technology. Based on the IEEE 802.15.4 standard, ZigBee is a wireless communication specification designed with smart devices in mind that do not have to be controlled continuously, just re-goaled occasionally. This is tailor-made for industrial sensors and control units that already know how to perform their function and need to communicate small packets of measurement values when they are available, and then accept new set points. Since there is only occasional traffic on a ZigBee network, it is based on a Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) protocol such that the sensor waits for the network to be available before it transmits its packet, thereby reducing the amount of bandwidth and power required to coordinate handshaking. Because a ZigBee sensor can spend most of is life in deep sleep, as in the case of a proximity sensor, it can operate on one set of commercial AAA batteries for months or years depending on its task.

A ZigBee Network Coordinator (ZNC) broadcasts a beacon signal to all of the devices in the network, such that each device knows when the time is proper to transmit or receive. When a new ZigBee node enters the network, the ZNC assigns it one of more than 18 x 1018 unique addresses (IEEE 64-bit addressing). Even though each device has a typical range of 50 meters, the network can grow to any size provided each ZigBee device is within range of another, as the specification supports the relaying of messages along the nodes of the network back to the ZNC.

Patrick Kinney, Chair of the IEEE 802.15.4 Task Group, presents a compelling comparison of ZigBee with current 802.11 WiFi technology. Consider a near-future home containing 100 smart sensors and controllers providing security, energy optimization and convenience. As each WiFi transmitter requires 667 mW of power, a city of 50,000 homes would require 3.33 megawatts of power for the sensors. Equivalent ZigBee devices only require 30 mW to remain active, dropping the requirement to 150 kilowatts. Add in the ability of each ZigBee device to sleep until needed at a typical duty cycle of 0.1 percent, and all five million sensors in the entire city can be powered at a total cost of 150 watts.

ZigBee devices require one percent of the power required to operate Bluetooth (IEEE 802.15.1) devices and have an advantage in latency. It typically takes 20 seconds for a Bluetooth device to join the network, while typical join times are 30 ms for ZigBee. A sleeping ZigBee sensor can wake up and communicate its data within 15 ms; 200 times faster than the three seconds typically required by Bluetooth. As both specifications are subsets of the same working group, they are not direct competitors. Rather, they are addressing different demands for wireless personal area networks (WPANs). While Bluetooth continues to connect cell phones, handhelds, computer peripherals and other remote controlled devices, ZigBee is an excellent choice for sensors and controls that go about their duties autonomously and only occasionally need to re-goal.