5: ZigBee Reliability and Security > Reliability Measures

Reliability Measures

ZigBee offers a range of techniques to ensure reliable communications. These are described below.

Listen Before Send

The transmission scheme used in ZigBee avoids transmitting data when there is activity on the 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.

Acknowledgements

An acknowledgement mechanism is built into ZigBee to ensure that messages reach their destinations.

When a message arrives at its destination, the receiving device sends an acknowledgement to say the message has been received. If the sending device does not receive an acknowledgement within a certain time interval, it resends the original message (it can resend the message several times until the message has been acknowledged).

Alternative Routes

In a Mesh topology, the network has 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.

Listen Before Send

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