ZigBee

ZigBee is a proprietary 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. In order to gain access to the protocol specifications, one must tender quite a large fee to join the ZigBee Alliance.

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 (RFDs) are about 2%.

As of 2004, the estimated cost of the radio for a ZigBee node is about $6 to the manufacturer. This implies that a ZigBee radio will add about $20 to the retail cost of any device that uses it.

Uses

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.

Device types

There are three basically different types of ZigBee devices: The most capable is a routing function device. It might bridge to other networks, and forms the root of the network tree. It is able to store information about the network. A "full function device" can act as an intermediate router, passing data from other devices. A "reduced function device" is just smart enough to talk to the network.

Protocols

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 minimize the time the radio is on in order to reduce the power used by the radio. The network synchronizes nodes to talk and listen at particular times when they have anything to hear or say. At the longer sleep intervals (up to 33 seconds), nodes need more precise timing.

The basic mode within a cluster is "carrier sense, multiple access," (CSMA) that is, the nodes talk in the same way that people converse. The problem with CSMA is that the nodes have to listen all the time. To reduce power further, ZigBee permits an optional "beacon" mode, in which a selected node transmits a periodic beacon message. In beacon mode, each node listens and responds near one of 16 times evenly distributed between beacons. This is operationally similar to people meeting on a regular schedule to talk.

The IEEE network layers used by ZigBee operate in the unlicensed 2.4GHz, 915MHz and 868MHz ISM bands. The radio uses DSSS which is managed by the digital stream into the modulator. The IEEE specification describe only the lower protocol layers, the link-level protocols, (PHY and MAC). The data rate is 20kb/s per channel, and transmission range is between 10 and 75 metres (33~250 feet).

Software

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.

History

• 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. ZigBee can be interpreted as an attempt to use ad-hoc networking software with slightly redesigned BlueTooth radios.
• The IEEE protocol layers were completed in May 2003, roughly a year late.
• The ZigBee protocols have failed to release several times, and the most recently advertised release date, November 2004, is about two years after the original.
• Some observers have speculated that the closely-held proprietary protocols are advancing more slowly precisely because public input is not available, as it is in most protocol development.
• Some people advocate further IEEE (i.e. public) protocols to define the higher layers of such networks. If this activity proceeds, and succeeds, ZigBee as such may fail.
• Some observers say that for the above reasons, ZigBee by that name may be losing momentum. In the summer of 2003, Philips, formerly a major promoter, ceased significant investment. Motorola, also an early advocate, has also reduced it activities, and possibly its investment.