Highly reliable operation

ZigBee and IEEE 802.15.4 employ a range of techniques to ensure reliable communications between network nodes - that is, to ensure communications reach their destinations uncorrupted. Corruption could result, for example, from radio interference or poor transmission/reception conditions.

  • Data Coding: At a first level, a coding mechanism is applied to radio transmissions. The coding method employed in the 2400-MHz band uses QPSK (Quadrature Phase-Shift Keying) modulation with conversion of 4-bit data symbols to 32-bit chip sequences. This coding results in a high probability that a message reaches its destination intact, even if there are conflicting transmissions. (A conflicting transmission implies that more than one device transmits in the same frequency channel at the same time).

  • Listen Before Send: The transmission scheme also avoids transmitting data when there is activity on its chosen channel - this is known as Carrier Sense, Multiple Access with Collision Avoidance (CSMA-CA). Put simply, this means that before beginning a transmission, a node listens on the channel to check whether it is clear. If activity is detected on the channel, the node delays the transmission for a random amount of time and listens again - if the channel is now clear, the transmission can begin, otherwise the delay-and-listen cycle is repeated.

  • acknowledgments: Two systems of acknowledgments are available to ensure that messages reach their destinations:

    • End-to-End: When a message arrives at its final destination, the receiving device sends an acknowledgment to the source node to indicate that the message has been received. End-to-end acknowledgments are optional.

    • Next Hop: When a message is routed via intermediate nodes to reach its destination, the next routing node (or ‘next hop’ node) in the route sends an acknowledgment to the previous node to indicate that it has received the message. Next-hop acknowledgments are always implemented.

In both cases, if the sending device does not receive an acknowledgment within a certain time interval, it resends the original message (it can resend the message several times until the message has been acknowledged).

  • Frequency Agility: When a ZigBee network is initially set up, the ‘best’ channel in the relevant radio band is automatically chosen as the operating channel. The operating channel is normally the quietest channel detected in an energy scan across the band. However, it might not always remain the quietest channel if other networks that operate in the same channel are introduced nearby. For this reason, ZigBee includes an optional frequency agility facility. If the operating channel becomes too noisy, this feature allows the whole network to be moved to a better channel in the radio band.

  • Route Repair: Networks that employ a Mesh topology (see ZigBee PRO network topology) have built-in intelligence to ensure that messages reach their destinations. If the default route to the destination node is down, due to a failed intermediate node or link, the network can ‘discover’ and implement alternative routes for message delivery. ZigBee PRO is designed for Mesh networks and therefore incorporates “route repair” as a core feature.

The above reliability measures allow a ZigBee network to operate even when there are other ZigBee networks nearby operating in the same frequency band. Therefore, adjacent ZigBee networks do not interfere with each other. In addition, ZigBee networks can also operate in the neighborhood of networks based on other standards, such as Wi-Fi and Bluetooth, without any interference.

Parent topic:ZigBee overview