Z-Wave is a wireless mesh protocol oriented to the residential control. Automation market and also suitable for light commercial applications. The Z-Wave technology offers a simple yet reliable method to wirelessly control lights, door locks, thermostats and a range of systems in residential and commercial environments. The Z-Wave protocol works in the unlicensed industrial, scientific, and medical (ISM) bands. The specific frequency band varies from region to region. And the frequency bands are defined in ITU-T G.9959.
The Z-Wave controllers build the Z-Wave network and give it a 4 byte “home ID” that makes it uniquely identifiable. “Z-Wave devices” are the endpoints that connect to the network (nodes). In a Z-Wave network, a maximum of 232 nodes may exist. A distinctive one-byte “node ID” is assigne to and used to identify Z-Wave devices. During the inclusion process, these IDs are sequentially increase and assigned to the joining device. The controller uses the “Command class” to monitor and control the end devices.
A node intended for the US region may use configuration 2 which provides two alternative communication channels. Based on the data rate of the transmitted packet, every Z-Wave device constantly use both channels for its regular communication. Since there are only two communication channels available, when more z-wave networks operating in the near-by wireless proximity would experience the issue of wireless interference. The wireless interference would result in packet loss, increased re-transmission, increased failure rate and other application level failures.
The interference issue would be much more evident in a warehouse/pairing-kitting environment, where there are several z-wave end devices that get paires with the controller & packes for shipment. At any given point in time, there could be several pairing processes being performes in parallel, and this implies that there would be several controllers operating in inclusion mode along with its corresponding end-device. Generally, more z-wave packets get exchanges between the controller and the joining device during the paring and command class interview process.The impacts identified are as follows;
- The controller fails to include a new device to the network.
- Nodes pairs with the near-by controller which is also in the inclusion mode.
- The controller & nodes cannot send/receive packets which results in the controller assuming the device is down.
- More wakeup beams without homeID hash results in the nodes of other networks also to stay awake, thus draining the battery faster.
Along with the impacts mentiones above, considering the case of these z-wave devices getting installes in the multi dwelling units, there could be a considerable amount of packet loss, re-transmissions and performance degradation where the z-wave devices of different networks coexist with each other.
What’s Possible Solutions:
Other wireless protocol standards such as ZigBee, WiFi operating in 2.4 GHz efficiently overcome these interference issues by taking the advantage of the standard supporting more number of operational wireless frequency/channels. To form the network, controllers select the best channel not used by other controllers in the vicinity. This enables seamless inter-operation of several ZigBee/WiFi networks without degradation of the performance.
Considering the fact that Z-Wave supports only two operating channels for the US region, the one possible solution to improve the performance of Z-Wave network while coexisting with several neighboring Z-Wave networks is that the device uses only the required optimal TX power for its packet transmission rather than max TX power.
The Z-Wave devices can select the optimal TX power in the following ways;
Static selection of TX Power:
- The Z-Wave devices and the controller shall be programmed to operate in two modes of TX power settings TX-Power-Normal & TX-Power-Low. The manufacturer shall select the best optimal value for these two parameters from -24dbm to +6dbm (min & max TX power for US region).
- Based on the button-press sequence, the device can switch to operate either in TX-Power-Normal or TX-Power-Low.
- The pairing team shall make the Z-Wave device & controller to operate in the low power during the pairing process. Thus enabling several pairing processes being performed in parallel due to reduction in the interference caused by near-by networks. Post the completion of pairing, through button-press sequence, the device shall be allowed to operate in the normal TX power.
Note: This method demands support from both the controller and Z-Wave device manufacturers to enable button-press based TX power selection.
Dynamic negotiation & selection of TX Power:
- This method enables devices to learn the best optimal TX power that would be required for the successful packet transmission to its peer devices.
- This learning process shall start during the device inclusion sequence immediately after assigning new node ID to the joining device. This process shall be named as TX Power negotiation sequence, where the newly joining device shall use NOP messages to learn the optimal TX power that would be required to reach its controller/immediate-peer device. The newly joining device shall initiate the learning process by sending NOP message with higher TX power and gradually decrease the TX power for the next NOP messages. This process of reducing the TX power shall stop when the peer device is not able to receive the transmitted message, thus selecting optimal power to reach the peer device.
- The controller/immediate-peer shall also initiate the same learning process with the newly joining node using NOP messages with TX power modulation, thus recording the optimal TX power to reach the newly joining device. The devices and the controllers shall use the the newly learnt TX power for all further transactions including the inclusion sequence.
- Periodically the devices shall re-initiate the learning process to keep the required optimal TX power updated. The device shall also re-initiate the learning process when there are transmission errors.
Note: This method demands review and approval of the suggested method from the Z-Wave alliance, spec ratification & support from manufacturers.
What applied by Rently:
Rently had opted to explore options on bringing in the changes to statically select TX power for its Z-Wave controller. This method requires changes in both Z-Wave controllers and from the devices manufacturers. However, considering these changes in controllers would help in reducing the interference caused by controller’s packets in the warehouse environment.
Z-Wave industry adapting the “Dynamic negotiation/selection of TX Power” would support warehouse scenario and also multi dwelling installations.
The process of Rently adapting the static selection of TX power is in preliminary stages of design. Results shall be provided in the upcoming blogs.
Dynamic selection of TX powers should also be applicable when the z-wave devices communicate within each other without controllers. Hence, the z-wave device should negotiate power not only with the controller but also with its neighbors. As part of upcoming explorations, we have to consider procedures for negotiation of power between neighbors.
 T-REC-G.9959-201501-I_MAC_PHY – 01/2015
 Z-Wave and Z-Wave Long Range Network Specification – 2021/08/27
Thanks for reading!!
To learn more about Engineering topics visit – https://engineering.rently.com/
Get to know about Rently at https://use.rently.com/