Bridging Unreachable Distances

Depending on the nature of the house, the Z-Wave signal between the transmitter and receiver can traverse a number of rooms. The range varies, depending on the version of the device and thickness of the walls to be crossed, and is usually around 30 to 100 feet. Z-Wave's proprietary mesh network allows greater distances to be bridged, however. In addition to their primary tasks as switches or sensors, many Z-Wave devices also act as routers for other signals that a controller wants to send to recipients that are out of range.

For this to happen, the controller tells the devices to locate and report their neighbors during initialization. If the controller then wants to send a message to one of these devices later on, but can't because of the limited range, it sends a packet with the routing information to the device that previously reported this neighbor. The device then forwards the package, on the basis of the received routing table, to the target, which can subsequently communicate back to the controller along the same path in the opposite direction (Figure 4).

Figure 4: The controller cannot reach the slave, but a third Z-Wave device acts as a router and enables a communication path.

Based on this procedure, Z-Wave builds a mesh network between the devices that can dynamically adjust its routes in case of an error. Each device on the network receives its own node ID; all the devices in a home are grouped under a home ID. This means that a particular device knows with certainty that a packet whizzing around in the ether is intended for it, and not for another node on the mesh network.

Power Supply

Z-Wave devices come in three different categories in terms of the power source and corresponding performance profiles. Devices with a steady power supply via a socket outlet permanently listen for wireless signals and process them immediately on receipt. They therefore often provide router functions for other devices in the mesh.

In the case of battery-powered devices (e.g., door locks), this is not easily possible if you want the battery to last for a reasonable amount of time. The device wakes up at regular intervals, checks for radio signals in the air, and then starts to decode and possibly process them.

The third category includes devices, such as remote controls, that do absolutely nothing in idle mode and only wake up and send signals at the push of a button. They are not usually reachable for a controller in the opposite direction.

Regional Differences

The release of radio spectra is subject to the control of national governments, so Z-Wave devices operate on different frequencies depending on the country. For example, Z-Wave devices in the United States send on 908.4MHz, whereas devices in Germany use 868.4MHz. Although a transmitter and a receiver from the same country will work elsewhere, the combination cannot be legally operated on an unapproved frequency. Instead, users need to purchase devices with the certified frequencies for their countries.