The tested ZigBee devices all operate at 2.4GHz.

Each ZigBee device has one or more End Points, depending on its functionality. Each End Point is defined by one or more Clusters, in accordance with the ZigBee Alliance standard. The characteristics of each Cluster are defined as a set of Attributes & Commands.

The hub communicates with each device by exchanging frames. In effect, the hub is paired with each device. One device is not paired with any other device. This enables the hub to have direct control over the individual devices, while taking advantage of the extended range provided by the mesh network.

As in any other ZigBee based system, routers (ZRs) relay frames in both directions between the devices and the hub. The use of the routers in this way retains the main benefit of having a mesh network, which is to extended the network well beyond that achievable with a star configuration.

These routers are capable of controlling other devices, but this facility is not used in this control system. They merely act as a means of relaying frames within the network, as well as providing their main functionality, such as being a smart plug.

Although this places the hub at the centre of all device to device communications, the capability of its 1.2GHz quad core processor ensures that the system delivers a very fast response, especially in the context of normal human response times.

End devices are interrogated by the hub in the usual way. Battery powered devices are normally sleepy, so they only respond to any interrogation when they wake up. All devices report at predefined intervals, even if they have not been interrogated. The hub determines how often sleepy devices wake up and the data that they send.

One of the most important characteristics of these devices is that they are event driven, so the hub has to capture the frames as they are transmitted by each device. For example, the control system recognises that a door has opened, or has closed, and not that the door is open, or is closed.

The ZigBee standard defines the Application Layer and the Network Layer, or in simpler terms, the communication between the network chip attached to the hub and that attached to each device. It does not define the communication between the hub processor and the associated network chip, so this has to be configured in the hub software for each manufacturer’s network adapter.