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1 January 2006

ZigBee: Working with Wireless Control

By William C. Craig

Wireless monitoring and control applications for industrial and home markets require longer battery life, lower data rates, and less complexity than those available from existing wireless standards. An IEEE 802.15 task group is investigating a low data rate solution with multi-month to multi-year battery life and low complexity. A highly integrated single-chip approach is the preferred solution of semiconductor manufacturers developing IEEE 802.15.4 compliant transceivers. The IEEE standard at the PHY is the significant factor in determining the radio frequency (RF) architecture, and topology of ZigBee-enabled transceivers.

ZigBee defines the network, security, and application framework profile layers for an IEEE 802.15.4-based system. ZigBee's network layer supports three networking topologies: star, mesh, and cluster tree. Star networks are common and provide for very long battery life operation. Mesh, or peer-to-peer, networks enable high levels of reliability and scalability by providing more than one path through the network. Cluster-tree networks use a hybrid star/mesh topology that combines the benefits of both for high levels of reliability and support for battery-powered nodes.

Devices and networks

To provide for low-cost implementation options, the ZigBee physical device type distinguishes the type of hardware based on the IEEE 802.15.4 definition of reduced function device (RFD) and full function device (FFD). An IEEE 802.15.4 network requires at least one FFD to act as a network coordinator.

An RFD sees implementation with minimum RAM and ROM resources as a simple send or receive node in a larger network. With a reduced stack size, it needs less memory, and thus a less expensive IC. ZigBee RFDs are generally battery powered. RFDs can search for available networks, transfer data from its application as necessary, determine whether data is pending, request data from the network coordinator, and sleep for extended periods of time to reduce battery consumption. RFDs can only talk to an FFD, a device with sufficient system resources for network routing. The FFD can serve as a network coordinator, a link coordinator, or as just another communications device. Any FFD can talk to other FFD and RFDs. FFDs discover other FFDs and RFDs to establish communications, and they're typically line powered.

The ZigBee logical device type distinguishes the physical device types (RFD or FFD) deployed in a specific ZigBee network. The logical device types are ZigBee coordinators, ZigBee routers, and ZigBee end devices. The ZigBee coordinator initializes a network, manages network nodes, and stores network node information. The ZigBee router participates in the network by routing messages between paired nodes. The ZigBee end device acts as a leaf node in the network and can be an RFD or FFD. ZigBee application device types distinguish the type of device from an end-user perspective as specified by the application profiles.

Heal thyself

ZigBee's self-forming and self-healing mesh network architecture permits data and control messages to pass from one node to other node via multiple paths. This extends the range of the network and improves data reliability. You could use the peer-to-peer capability to build large, geographically dispersed networks where smaller networks link together to form a cluster-tree network.

ZigBee networks consist of multiple traffic types with their own unique characteristics, including periodic data, intermittent data, and repetitive low latency data. The characteristics of each are as follows:

• Periodic data is usually defined by the application such as a wireless sensor or meter. You would typically handle data using a beaconing system where the sensor wakes up and checks for the beacon, exchanges data, and goes to sleep.
• Intermittent data is either application- or external stimulus-defined, such as a wireless light switch. You can handle data in a beaconless system or a disconnected operation.
• Repetitive low latency data uses time slot allocations, such as a security system. These applications may use the guaranteed time slot (GTS) capability.

ZigBee networks are primarily for low duty cycle sensor networks (<1%). They may recognize new network nodes and associate them in about 30 ms. Waking up a sleeping node takes about 15 ms, as does accessing a channel and transmitting data. ZigBee applications benefit from the ability to quickly attach information, detach, and go to deep sleep, which results in low power consumption and extended battery life.

About the Author

William C. Craig is program manager, wireless communications at ZMD America Inc. in San Diego. Reach him at craig@zmda.com.