ZigBee is an IEEE 802.15.4-based specification for a suite of high-level communication protocols used to create personal area networks with small, low-power digital radios. It operates on top of IEEE 802.15.4 to address issues in the upper protocol layers and application profiles by adding logical network, security and application software.
Sensors and control devices don’t need high bandwidth. They need low latency and very low power consumption. ZigBee offers product interoperability and vendor independence – win-win for customers. It defines modules for quality assessment, receiver energy detection and clear channel assessment.
The maximum distance between nodes is up to 70 m. ZigBee supports 3 network topologies: star, mesh and cluster tree (hybrid of the first two). It uses a master-slave configuration suitable for star networks comprising of many infrequently used devices that transmit small data packets. Master nodes can talk to each other. It supports up to 254 nodes and can be increased if necessary.
It incurs low latency – a node that is powered down can wake up and transmit a packet in 15 ms, which is an important feature for time-critical messages, e.g. in a nuclear plant. A node may act as a coordinator (FFD), router (FFD) , or end device (FFD or RFD).
The functions performed by a coordinator include setting up a network, transmits network beacons, management of network nodes, stores network node information and routes messages between paired networks.
A network node searches for available network, transfers data from its application/environment and determines whether there is data pending and requests data from the network coordinator.
The Protocol Stack
The network layer is responsible for:- Establishing a new network
- Providing the capability to join and leave a network
- Configuring a new device to carry out the necessary operations
- Synchronizing between devices in the network either through tracking beacons or polling
- Applying security policy for outgoing frames and removing security to terminating frames
- Routing frames to their destination
The application sublayer consists of:
- Application support (APS) sublayer
- maintains table for binding to match two devices based on their services and needs
- discovery to find devices
- forwarding of messages between bound service
- ZigBee device object (ZDO)
- defines the role of a device
- responds to binding requests
- establishes a secure relationship between network devices
- Manufacture-defined application objects, which implement the actual application according to ZigBee-defined application descriptions.
Power Conservation
The targeted sensor lifetime is between 2 – 5 years. A probe operating on one 1.5 V AA alkaline battery with an average power consumption in the range of 10 – 100 µwatts would last between 2 – 7 years. Sensor nodes must be asleep 99.9% of the time to ensure long battery life time. Examples of power management strategies:- Turn on transceiver only during active communication
- Shut down CPU between request processing
- Powering down the I/O subsystems when not in use
- Piggybacking acknowledgements in data messages
Power consumption can also be minimised by reducing and eliminating energy waste caused by:
- Idle listening: a node waits and listens for a packet to arrive.
- Over-emitting: a node sends a message and the destination node is not ready to receive it.
- Overhearing: a node listens for a message intended for another node.
- Collision: two nodes transmit at the same time, causing a collision and have to retransmit.
- Protocol overhead: i.e. frame headers and signalling required by a MAC protocol.
| ZigBee | Bluetooth |
| Small packets over large network | Larger packets over small network |
| Mostly static networks with many, infrequently used devices | Ad hoc networks with only a few devices |
| Rapid network join time | Long network join time |
Related link:
ZigBee Alliance
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