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1: ZigBee Essential Facts and Features > Dependability
Dependability
ZigBee networks provide reliability and security mechanisms that make them highly dependable - that is, you can depend on ZigBee to provide a secure environment in which the network operates reliably (with minimal disruption). This page outlines the available reliability and security measures.
Reliable Operation
ZigBee employs a range of techniques to ensure reliable communications - that is, to ensure communications reach their destinations uncorrupted. Corruption could result, for example, from radio interference or poor transmission/reception conditions.
- Channel Selection: When a ZigBee network is initialised, the channels of the chosen RF band are assessed for activity. The results can be used to automatically select a quiet channel for the network to operate in.
- Listen Before Sending: To avoid conflicting transmissions (more than one device transmitting in the same frequency channel at the same time), before beginning a transmission, a node will listen on the relevant channel to check whether it is clear. It will only transmit if no activity is detected on the channel, otherwise the node waits for a random period of time before re-trying.
- Data Coding: ZigBee networks apply a coding mechanism to radio transmissions. Due to this coding, there is a higher probability that a message will get through to its destination intact, even if there are conflicting transmissions.
- Acknowledgements: A system of message acknowledgements is available in ZigBee to confirm that messages reach their destinations. When a message arrives at its destination, the receiving device sends an acknowledgement to say the message has arrived. 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).
- Route Discovery: 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 use alternative routes for message delivery.
The above reliability measures allow a ZigBee network to operate in the same space and RF band as other ZigBee networks. In addition, ZigBee networks can operate in the neighbourhood of networks based on other standards, such as Wi-Fi and Bluetooth.
Secure Operation
ZigBee networks are highly secure - they incorporate measures to prevent intrusion from potentially hostile parties and from neighbouring ZigBee networks. For this purpose, a “Security Toolbox” is included in a ZigBee network, offering:
- Access Control Lists: Only pre-defined “friendly" nodes can join a network.
- 128-bit AES-based Encryption: A very high-security key-based encryption system, preventing external agents from interpreting ZigBee network data.
- Message Freshness Timers: Timed-out messages are rejected, preventing message replay attacks on the network. An example of a replay attack would be a malicious individual recording the open command for a garage door opener, and then replaying it to gain entry.
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