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Troubleshooting in Zigbee networks

The Zigbee function protocol is used primarily in private buildings, and several articles by professionals in the field describe both its benefits and drawbacks. But what is the situation when Zigbee is used in non-residential buildings? DIAL provides the »Troubleshooting« service on site which draws on many years of experience in the field of building automation. The following concrete example of a lighting installation in a non-residential building illustrates the procedure when tracing errors (troubleshooting).

First of all let us take a brief look at the Zigbee automation standard: ZigBee is an automation standard in the 2.4 GHz radio network and is currently available in its 3.0 version. The radio requirements are defined in IEEE (Institute of Electrical and Electronics Engineers) Standard 802.15.4. The standard relates to the two lower layers of the OSI/ISO layer model, the »Physical Layer« and the »Data Link Layer«. At the application level Zigbee currently uses 13 profiles of which the most well-known are »Zigbee Light Link«, »Zigbee Home Automation« and »Zigbee Health Care«. The automation standard transmits and receives small volumes of data. In the background power generators are used which are also found in EnOcean.

In the 2.4 GHz band Zigbee uses 16 channels between 2.400GHz and 2.4835 GHz. The band width of the individual channels is 5 MHz because of the small message size. A maximum of 50 Zigbee devices can be imported in one zone and divided into 16 groups. Each zone has its own channel and is assigned by a coordinator. This allocates the channels to the individual Zigbee devices.

To protect from attacks or access from outside, the augmented procedure for CCM mode encryption is used. AES-128 is used as the encryption algorithm.

Potential obstacles and solutions in practice

Since Zigbee transmits within the open 2.4GHz radio network, there are existential problems. It is used by different radio protocols such as Bluetooth, EnOcean (2,4 GHz, Z-wave, DECT and Wi-Fi. The systems may only transmit after being addressed (listen before talk) or the device only transmit for 10% of the time during which it is active at a maximum transmission power of 100mW. In January 2015 the section for Wi-Fi devices in EN 300328 was adopted. Since many older devices, in accordance with the old standard, are still in use, these still emit signals permanently into the surroundings. This poses the threat that all communication may be lost.

Spectrum analyzer Zigbee overlay with WIFI

To prevent all automation being affected, Zigbee has taken certain measures. A telegram is repeated up to three times if a response is not received from the device which has been addressed. In a detached house with only one wi-fi network and only one channel in use this is a good solution. But what happens in an apartment building or in a non-residential building with many different wi-fi networks with different channels?

Here Zigbee uses the mesh topology form of devices in a group.

ZigBee Mesh network

A coordinator is responsible for the control, and Zigbee routers are used to distribute the telegrams to the individual devices via different paths. If a faulty connection or a disruption arises, another router forwards the telegram. This network is called »Full Mesh Network« where mainly router devices are used.

One reputable manufacturer has developed two types of Zigbee device for lighting control: firstly, a master device with daylight and motion sensors and integrated router and, secondly, an end device which transmits and receives telegrams but has no router of its own. The latter cannot forward telegrams but only receive and confirm.

Concrete example: Troubleshooting at DIAL

An industrial building has a production area which is 100 m long and 100 m wide. The lighting installations are mounted at a height of approx. 10 m. Device from one manufacturer and end devices have been installed together with a few Zigbee devices with integrated router. Therefore, there is no complete »Mesh Network« and the full benefits of Zigbee cannot be used. There are also different wi-fi networks which transmit on different channels. It is therefore possible for the whole installation to fail because the control parameters cannot be transmitted.

When troubleshooting, the following aspects are taken into consideration first of all: radio configuration in the .,4 GHz radio network, number of wi-fi channels and employment of other systems such as Bluetooth, EnOcean or Z-wave. Through radio configuration it is possible to identify whether a radio solution or a cable-connected automation solution would be suitable. The location of the luminaires and the number to be switched in one group are also relevant. In non-residential buildings there might be 30 or even 50 devices in one zone. The maximum figure of 50 devices for Zigbee might cause instability problems and is not to be recommended. During planning this figure should be restricted to 30 to 40 devices. Finally a »Full Mesh Network« should be created in order to take full advantage of the benefits of the Zigbee radio network. End devices might, for example, be switches. With luminaires it is necessary to check that the router functions in order to ensure that no telegrams are lost and that the network is stable.

Zigbee also provides the following benefit for customers: Energy is saved through the automatic switching and dimming of the luminaires. In addition, it is easier to supervise the lighting installation and the customer has access to the installation via a tablet. Making use of radio signals also saves extra installation costs. An extra benefit is that the lighting is switched on automatically when motion is detected , thus providing extra protection against break-ins.