Z-Wave and Zigbee: A to Zzz

As we discuss popular hubs, switches and sensors in more detail, Z-Wave and Zigbee are going to come up more and more often. You might be wondering: What’s so great about these protocols, why do they seem to be everywhere in home automation? As the title of this posts implies, the answer might be a bit technical and boring for some people but I think it’s useful information, especially if you ever have to troubleshoot your smart home.

When we previously discussed wifi and its place in the smart home, I gave the analogy of a bartender at a crowded party with dozens of people shouting their order. The loud voices interfered with each other and the overworked bartender. Z-Wave and Zigbee are specifically designed to address these issues.
Instead of relying on communicating directly with the hub with powerful, battery-draining radios, they use low-powered radios to periodically check in with nearby devices which relay the information to and from other devices until information gets to and from the hub. This kind of network, where devices (also called nodes) relay information for other nodes, is called a mesh.

Back to our party analogy, Z-Wave is like having dozens of people standing in small groups and then quietly asking a nearby waiter to pick up a drink for them. The bartender gets their order hands the drink to the waiter to bring it back the one who ordered it. Everyone’s voices are calm and orderly and everyone gets served! Because they only need to reach the next node, the Z-wave radios can be much lower powered than wifi and thus, don’t interfere with each other.

Another interference-avoiding trick used by Zigbee meshes is that they aren’t constantly broadcasting. Each node checks in with its neighbors (or peers) periodically to see if there are any messages for it. This occasional use of low powered radios means that Zigbee is much more power efficient than always-on, high powered devices like wifi. As such, you’ll find that most tiny, battery powered sensors and switches run on the Zigbee protocol

Meshes also help make your system more reliable. If one node is unavailable, the information can still be passed on through other nearby nodes with minimal delay. Each node remembers who their neighbors are to speed up the time looking for someone to pass its message on to. Sometimes, these neighbor lists get out of sync and the devices waste time trying to talk to a neighbor that is offline or has been removed from the mesh. When this happens, the mesh can usually self-repair but most hubs provide a utility to rebuild the mesh. “Z-Wave repair” will cause each node to rediscover its neighbors and optimize the paths messages take from the hub to each of the nodes. Zigbee healing and repairing is a bit more kludgy and usually requires shutting down the hub for 20 minutes which causes the Zigbee nodes to panic and re-evaluate their life decisions. Then they find their nearest neighbors and rebuild the mesh.

One last thing to be aware of when buying and installing devices is how your mesh is formed. Our analogy called out waiters as the nodes that relay messages. In reality, these waiter nodes can be any node that is powered by your home’s mains power. Anything wired into your house like a switch, or plugged into an outlet like a smart plug or screwed into a fixture like a smart bulb can act as a waiter or in mesh terms, a repeater. Battery powered nodes usually cannot act as a repeater.  Remember, these are low-powered radios, so if you install a Z-Wave switch on the complete opposite side of your house from your hub, the signal might not reach it. You might have to add a smart plug halfway in between the hub and switch so it can repeat and extend the signal between them. That’s also why many folks recommend repairing your Z-Wave or Zigbee mesh after adding new devices so that the optimal path between repeaters can be established.

If you’re still with me, great! If not, no worries - I’ll mention the repair function again when we talk about adding devices.

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