Zigbee

Zigbee is the popular name of the IEEE 802.15.4 standard. IEEE 802.15.4 Zigbee is a standard for a radiocommunication link between equipment over rather short distances to replace a cable. Zigbee is especially designed for a low power consumption and low cost of the infrastructure, both the device and the installation and maintenance. Therefore the data rate is not very high, it ranges from 20 kbps to a maximum of 250 kbps. The range is limited to typically 50 m (5-500 m based on the environment). This makes Zigbee very suitable for control functions and sensor networks, such as building automation, personal health care, industrial control and remote control of consumer electronics.

Radio characteristics
Zigbee can operate in three different frequency bands.

The radio interface makes use of Direct Sequence Spread Spectrum (DSSS) with 11 chips/symbol.

Zigbee is optimized for a low duty-cycle applications, with a transmission time of less than 0.1% of the total time.

Network characteristics
Zigbee is based on a master-slave configuration. Zigbee defines two different kind of devices:

1. ZigBee Full Function Devices (FFD)
   This device can act as a coördinator (master). The coordinator is responsible for setting up the network and the routing of messages. Therefore the coordinator has to listen to the network continously and is not battery powered;
2. ZigBee End Devices
   End devices can only act as a slave. They can find a network and transfer data from its application if necessary. These devices are designed for high energy savings and are typically battery powered.

A Full Function Device can also act as a router to make very large and extended networks. The maximum number of network nodes is 65.536. Zigbee can make use of different network topologies, including a full mesh network.

Multiple access
Zigbee defines two different access mechanisms:

1. Beacon enabled,
2. Non-beacon enabled.

In a non-beacon enabled network the end devices can transfer data whenever they want as long as the radio channelis free. Before they transmit their data they listen if the radio channel is free. This is called Carrier Sense Multiple Access - Collision Avoidance (CSMA/CA).

In a beacon enabled network the coordinator transmits a beacon to all slaves in the network for synchronisation purposes. Every node gets its own time slot.

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