Kontron
InterphaseTECHNOLOGY IN CONTEXT
I/O and Sensor Technology
Using ZigBee Wireless Networking to Develop Commercial Products
ZigBee devices bring simple, effective wireless connectivity to low-rate sensors and control devices at an effective cost. The ZigBee Alliance has built up an interoperability, compliance and certification program to validate products performance in a ZigBee environment.
JON ADAMS, FREESCALE SEMICONDUCTOR
ZigBee wireless mesh technology has been developed to address sensor and control applications with its promise of robust and reliable, self-configuring and self-healing networks that provide a simple, cost-effective and battery-efficient approach to adding wireless to any application, mobile, fixed or portable. A typical IEEE 802.15.4-based, ZigBee-compliant device is shown in Figure 1.
The ZigBee Alliance released their specification to the public in June 2005, and since then the playing field has become much simpler for product designers who want to add wireless to their sensor or control application. An open and growing industry group of nearly 200 companies from product/system OEMs to applications developers to semiconductor companies, the Alliance has worked hard to provide a technology that takes best advantage of the robust IEEE STD 802.15.4 short-range wireless protocol, adding flexible mesh networking, strong security tools, well-defined application profiles, and a complete interoperability, compliance and certification program to ensure that end products destined for residential, commercial and industrial spaces work well and network information smoothly.
Zigbee Relies on the IEEE 802.15.4 Standard
The IEEE standard brings with it the ability to uniquely identify every radio in a network as well as the method and format of communications between these radios, but does not specify beyond a peer-to-peer communications link, a network topology, routing schemes or network growth and repair mechanisms.
The IEEE 802.15.4 standard, released in May 2003, was selected by the ZigBee Alliance as the wheels and chassis, upon which ZigBee networking and applications are to be constructed. This is not without its challenges, as the Alliance does not control the IEEE specification. However, many of the same people who sit in the IEEE 802.15 Working Group are deeply involved in the ZigBee standard. This relationship has meant that both the IEEE and the ZigBee specifications track one another fairly well. Figure 2 shows the relative organization of the IEEE radio with respect to the ZigBee functionality.
The IEEE standard specifies the RF link parameters, including modulation type, coding, spreading, symbol/bit rate and channelization. Currently, the standard identifies 27 channels spread across three different frequency bands (Table 1).
The 802.15.4 PHY physical layer contains specific primitives that manage the radio channel, and control packet data flow. This is a packet radio specification, and the PHY defines four different frames that have unique functions: Data, Acknowledgement, Beacon and MAC Command. The Data frame (Figure 3) can carry up to 104 bytes of payload. The Acknowledgement frame is used by a receiving station to acknowledge to the transmitting station that a data packet was received without error. The Beacon frame is used by stations that may be implementing significant power saving modes, or by Coordinator and Router devices that are attempting to establish networks. The MAC Command frame provides some unique abilities to send low-level commands from one node to another.
Figure 4 demonstrates the timing involved in a data transaction between two devices. The sensor wakes up either on an event or at the end of an interval, checks the channel, transmits its message, awaits the acknowledgement, then may go back to sleep or first receive data intended for the node before going back to sleep.
The 802.15.4 MAC medium access control layer contains over two dozen primitives that allow data transfer, both inbound and outbound, as well as management by higher-level entities of the RF and PHY. The IEEE 802.15.4 specification uses a 64-bit unique address for every radio node. The 64-bit address is used in peer-to-peer communications, where no established network is available. However, once in a network, the 64-bit address is traded for a 16-bit address to reduce packet overhead.
There are two physical types of devices specified in 802.15.4, and three logical types. The two physical types are the Full Function Device (FFD) and the Reduced Function Device (RFD). While either device may act as a sensor node, control node or composite device, only the FFD may perform routing tasks for a network. FFDs may, depending on their location in a network, have child devices for which the router performs routing functions. RFDs do not route, and therefore cannot have child devices. Figure 5 is a graphical representation of the connectivity practical in a ZigBee network using 802.15.4 devices.
