The consumer IoT industry’s driving maxim seems to be: 'if it ain’t broke, connect it to the internet'. Goodbye switches; hello apps! As a result, people can now worry over whether their ceiling fan is too far away from their router, or whether they need separate apps to open the garage door and change the reading lamp’s color. The first of these problems—device range—arises for any network where devices are required to communicate directly with a controller. The second problem—interoperability—results from a lack of standardization at some layer of the device network (the radio, network, or application layers).

Z-Wave, a wireless standard designed specifically for home automation (controlling heat, lights, window blinds, etc.), promises to mitigate the problems of device range and interoperability. In this post, I want to provide an overview of Z-Wave, describe how Z-Wave networks work and go over Z-Wave’s advantages and disadvantages relative to its competitors.

Basic Overview of Z-Wave

You’re sold on the home automation concept—maybe you’d like to lock your doors from the car, or maybe you’re drawn in by the idea of turning your kitchen into a multi-colored disco from the comfort of your bed. You’ll need some devices (e.g., smart locks, smart bulbs), and some way of wirelessly controlling them. Opting for Z-Wave compatible products guarantees that all of your products will work with the same controller and with one another.

The Z-Wave networks consists of a hub (also called a master controller) and the devices and other controllers. The hub is responsible for connecting to the internet (through WiFi or LAN), configuring the Z-Wave network, and managing device inclusion/exclusion. Once the hub is running, adding devices is fairly simple: put the Z-Wave controller in inclusion mode and push a button on your device to add it to your network (or something more involved, depending on the security required by the device).

Who Is Behind Z-Wave?

Z-Wave originated with the Danish startup Zensys in the 1990s. After a series of acquisitions, it's now owned and developed by Silicon Labs. The standard is maintained and promoted by Silicon Labs and the Z-Wave Alliance—a consortium of over 450 vendors and manufacturers that bring Z-Wave compatible products to market. The radio layer (standard G.9959) and application layer (available at zwavepublic.com/) are now open and public standards, allowing anyone to experiment with creating a Z-Wave device.

What Is the Business Model?

While anyone can theoretically read the standards and create a Z-Wave device, the Z-Wave Alliance requires that devices go through a certification process in order to bear the Z-Wave label. Silicon Labs manufactures an application-specific integrated circuit (ASIC) that includes a Z-Wave compatible radio transceiver and a related development kit that uses precompiled libraries required for the certification process. Most Z-Wave devices use this ASIC, which allows device manufacturers to focus on just the application layer implementation to pass certification.

How Does Z-Wave Guarantee Interoperability?

The certification process is part of how Z-Wave can guarantee interoperability of Z-Wave devices. The Z-Wave standard defines how things need to work at the radio layer (mostly handled by Silicon Labs’ ASIC), the network layer, and the application layer. Recently, the Z-Wave Plus standard introduced even stricter application layer standards to improve interoperability with phone and tablet-based controllers. Because of this multi-layered standardization, any Z-Wave controller will know how to talk to any Z-Wave devices, and devices will know how to talk to one another.

While all Z-Wave products are guaranteed to work with one another, the certification process doesn't guarantee that all Z-Wave products will have all of the functions of future Z-Wave devices. So, if next year the world decides window blinds should not only open and close but whistle a tune, last year’s window blind controllers will still be able to open and close them, but they might not be able tell them to cut the Wagner and whistle Dixie.

What Do I Need to Know About the Radio?

Z-Wave operates in the sub GHz band (the exact bands vary by region. This allows Z-Wave devices to avoid interference in the heavily trafficked 2.4 and 5 GHz bands, providing a more reliable signal.

The effective range of a Z-Wave device is 100 feet in open air, but the range is closer to 30 feet in an indoor environment. Because of the way Z-Wave networks are designed, however, devices can be much further than 30 feet away from their controller.

How Are Z-Wave Networks Designed?

Z-Wave uses a source-routed mesh network topology. Devices on the network don’t need to communicate directly with the controller. They can relay messages through other Z-Wave devices. This allows you to daisy-chain devices further away from the hub than the radio signal alone would allow, and it provides more reliable connections through redundant routes to the hub.

The hub builds and maintains a routing table that contains the possible routes through the network. Devices can determine which nodes are in wireless range (their neighbors). When a device sends or receives a signal, it can tell the hub which devices are actually its neighbors, and the hub can update its routing table accordingly. Currently, a single Z-Wave network can support up to 250 devices.

How Does Z-Wave Secure the Network?

A home automation network faces two broad types of security challenges: Ensuring only authorized listeners can understand network traffic (e.g., you don’t want to broadcast that your doors are unlocked), and preventing unauthorized devices from creating network traffic (e.g., letting a stranger open your garage door).

This author’s solution is not to control his doors and lights over the internet. Z-Wave’s is to provide several authorization, authentication and encryption protocols to protect your network. When a device is initially included in the network, it can employ one of three basic authentication techniques:

• S2 Unauthenticated: The hub recognizes the device, and the user physically interacts with the device by pushing a button, powering it up, etc.
• Two-factor authentication: Devices report a unique ID to the hub, and the user must scan a QR code on the device
• S2 Access Control: The user must confirm a frequently changing ID on the device.
  

At the time of inclusion, the hub provides the device with an (encrypted) network encryption key. All communications to and from the device are encrypted with this key using the AES128 standard.

The initial inclusion prevents random third-party access to the network, while the subsequent encryption prevents both eavesdropping and access from third-party devices pretending to be authorized devices.

How Does Z-Wave Stack Up to the Competition?

Z-Wave’s closest competitor is Zigbee, which also provides a medium range mesh network standard. Zigbee’s data rates (40-250kbps) are higher than Z-Wave’s (10-100kbps). Zigbee is also currently the more popular of the two, meaning it has more devices from which to choose. But Zigbee operates on the congested 2.4GHz band, which means it's more likely to encounter interference than Z-Wave. Zigbee also has an overwhelming number of different specifications and standards, which means that it cannot match Z-Wave’s interoperability guarantee.

WiFi-based home networks offer several advantages: (1) they’re everywhere, (2) they can send a ton of data, and (3) they’re (relatively) cheap. In the realm of home automation, (1) and (2) can be disadvantages. We can find WiFi in almost every coffee shop, but just as often it’s unusable because of congestion. And the ability to send enough data to stream an HD movie comes at the price of power consumption. So, for home automation networks that need reliable connections to low power devices (e.g., remote controls or smoke detectors), Z-Wave offers a compelling alternative.

Conclusion

It remains unclear to me why most homes should be automated, but I understand that it’s a tide, and I’m powerless to stop it. Z-Wave’s interoperability guarantee makes it an extremely compelling choice for a home system. It makes home automation as stable as most other home upgrades. Just as one can reasonably expect their HVAC to last at least a decade without preventing their new door locks from working, buying into the Z-Wave ecosystem means one can expect the same from their home automation. And in a space that moves as quickly as IoT, that’s a major accomplishment.