ZigBee is a published specification set of high level communication protocols designed to use small, low power digital radios based on the IEEE 802.15.4 standard for wireless personal area networking. The relationship between IEEE 802.15.4 and ZigBee is analogous to that existing between IEEE 802.11 and the Wi-Fi Alliance. The ZigBee 1.0 specifications were ratified on December 14, 2004, and are available to members of the ZigBee Alliance. An entry level membership in the ZigBee Alliance costs $3500 and provides access to the specifications.
The technology is designed to be simpler and cheaper than other WPANs such as Bluetooth. The most capable ZigBee node type is said to require only about 10% of the software of a typical Bluetooth or Wireless Internet node, while the simplest nodes are about 2%.
As of 2004, the estimated cost of the radio for a ZigBee node is about $6 to the manufacturer.
ZigBee is aimed at applications with low data rates and low power consumption. ZigBee's current focus is to define a general-purpose, inexpensive self-organizing mesh network that can be shared by industrial controls, medical devices, smoke and intruder alarms, building-automation and home automation. The network is designed to use very small amounts of power, so that individual devices might run for a year or two with a single alkaline battery. The killer app is probably meter-reading, although other applications, such as wireless light controls, should also be popular.
There are three different types of ZigBee devices: The most capable is a "ZigBee coordinator." It might bridge to other networks, and forms the root of the network tree. It is able to store information about the network. There is exactly one ZigBee coordinator in each network. A "full function device" (FFD) can act as an intermediate router, passing data from other devices. A "reduced function device" (RFD) is just smart enough to talk to the network; it cannot relay data from other devices. An RFD requires less memory, and therefore should be less expensive to manufacture, than an FFD. Similarly, an FFD requires less memory, and therefore should be less expensive to manufacture, than a ZigBee coordinator.
The protocols build on recent algorithmic research to automatically construct a low-speed ad-hoc network of nodes. In most large cases, the network is a cluster of clusters. It can also form a mesh or a single cluster.
The ZigBee protocols support both beaconing and non-beaconing networks. In beaconing networks, the network nodes transmit beacons to confirm their presence to other network nodes, and to allow nodes to sleep between beacons, thereby lowering their duty cycle and extending their battery life. Beacon intervals may range from 15.36 milliseconds to 15.36 ms * 2^14 = 251.65824 seconds; to obtain the benefits of low duty cycle operation with long beacon intervals, however, precise timing is needed, which can conflict with the need for low product cost. In non-beaconing networks, most devices typically have their receivers continuously active, requiring a more robust power supply; however, this enables heterogeneous networks, in which some devices receive continuously while some remain asleep, transmitting only when an external stimulus is detected. The typical example of a heterogeneous network is the wireless light switch: The ZigBee node at the lamp may receive constantly, since it is connected to the mains supply, while the battery-powered light switch remains asleep until the switch is thrown. The switch then wakes up, sends a command to the lamp, receives an acknowledgement, and returns to sleep. In such a network the lamp node is at least an FFD, if not the ZigBee coordinator; the switch node is typically an RFD.
In general, the ZigBee protocols minimize the time the radio is on in order to reduce the power used by the radio. In beaconing networks the network synchronizes nodes to talk and listen at particular times when they have anything to hear or say. In non-beaconing networks, power consumption is more asymmetrical; some devices are constantly active, while others (if present) are almost always asleep.
ZigBee uses the IEEE 802.15.4 Low-Rate Wireless Personal Area Network (WPAN) standard to describe its lower protocol layers--the physical layer (PHY), and the medium access control (MAC) portion of the data link layer (DLL). This standard specifies operation in the unlicensed 2.4 GHz, 915 MHz and 868 MHz ISM bands. The radio uses DSSS which is managed by the digital stream into the modulator. Conventional DSSS is employed in the 868 and 915 MHz bands, while an orthogonal signaling scheme that transmits four bits per symbol is employed in the 2.4 GHz band. The raw, over-the-air data rate is 250 kb/s per channel in the 2.4 GHz band, 40 kb/s per channel in the 915 MHz band, and 20 kb/s in the 868 MHz band. Transmission range is between 10 and 75 metres (33~246 feet).
The basic mode of channel access specified by IEEE 802.15.4 is "carrier sense, multiple access" (CSMA), that is, the nodes talk in the same way that people converse--they briefly check to see that no one is talking before they start. Beacons, however, are sent on a fixed timing schedule, and do not use CSMA. Message acknowledgements also do not use CSMA.
Software and Hardware
The software is designed to be easy to code for small, cheap microprocessors. The radio design utilized by ZigBee has been carefully optimized for low cost. It has few analog stages and uses digital circuits wherever possible. Most vendors plan to put the radio on a single chip.
ZigBee-style networks began to be conceived near 1998, when many engineers realized that both WiFi and Bluetooth were going to be unsuitable for many applications. In particular, many engineers wanted to design self-organizing ad-hoc networks of digital radios. The simple one-chip design of Bluetooth digital radios was also inspirational for many engineers. The IEEE 802.15.4 standard was completed in May 2003. In the summer of 2003, Philips Semiconductors, a major promoter, ceased its investment. Philips Lighting has, however, continued Philips' participation, and Philips remains a promoter member on the ZigBee Alliance Board of Directors. The ZigBee Alliance announced in October 2004 that its membership had more than doubled in the past year and had grown to more than 100 member companies, in 22 countries. The ZigBee specifications were ratified on 14 December 2004.