Z-Wave overview

Z-Wave's design center is home and consumer lighting/automation. It is intended to use very low bandwidth for communication with sensors and switches. The design is based on the ITU-T G.9959 Standard at the PHY and MAC level. The ITU-T G.9959 is the International Telecommunications Union specification for short-range narrow-band radio-communication transceivers in the sub-1 GHz band.

There are several bands in the sub-1 GHz range that are used for Z-Wave depending on the country of origin. In the US, the 908.40 MHz center frequency is the standard. There are three data rates that Z-Wave is capable of with varying frequency spread for each:

• 100 Kbps: 916.0 MHz with a 400 KHz spread
• 40 Kbps: 916.0 MHz with a 300 KHz spread
• 9.6 Kbps: 908.4 MHz with a 300 KHz spread

Each band operates on a single channel. 

Modulation performed at the PHY level uses frequency shift keying for data rates at 9.6 Kbps and 40 Kbps. At the fast 100 Kbps rate, Gaussian frequency shift keying is used. The output power will be roughly 1mW at 0 dB.

Channel contention is managed using CSMA/CA, as described in other protocols previously covered. This is managed in the MAC layer of the stack. Nodes start in receive mode and wait a period of time before transmitting data if there is data being broadcast.

From a role and responsibility point of view, the Z-Wave network is composed of different nodes with specific functions:

• Controller device: This top-level device provides the routing table for the mesh network and is the host/master of the mesh under it. There are two fundamental types of controllers:
  • Primary controller: The primary controller is the master, and only a single master can exist in a network. It has the ability to maintain the network topology and hierarchy. It can also include or exclude nodes from the topology. It also has the duty of allocating node IDs.
  • Secondary controller: These nodes assist a primary controller with routing.
• Slave device/node: These devices perform actions based on commands they receive. These devices cannot communicate with neighbor slave nodes unless instructed to do so via a command. Slaves can store routing information but do not compute or update routing tables. Typically, they will act as a repeater in a mesh. 

Controllers can also be defined as portable and static. A portable controller is designed to move like a remote control. Once it has changed position, it will recalculate the fastest routes in the network. A static controller is intended to be fixed, such as a gateway plugged into a wall outlet. The static controller can always be "on" and receiving slave status messages. 

Controllers can also have different attributes within the network:

• Status update controller (SUC): The static controller also has the advantage of taking the role of status update controller. In this case, it will receive notifications from a primary controller regarding topology changes. It can also assist in the routing of slaves. 
• SUC ID server (SIS): An SUC can also assist in including and excluding slaves for the primary.
• Bridge controller: This essentially is a static controller that has the ability to act as a gateway between the Z-Wave mesh and other network systems (for example, WAN or Wi-Fi). The bridge is allowed to control up to 128 virtual slave nodes.  
• Installer controller: This is a portable controller that can assist in network management and quality-of-service analysis. 

Slaves also support different attributes:

• Routing slave: Fundamentally a slave node but with the ability to send unsolicited messages to other nodes in the mesh. Typically, slaves are not allowed to send a message to another node without the command of a primary controller. The node stores a set of static routes that it uses when a message is sent.
• Enhanced slave: These have the same abilities as a routing slave with the addition of a real-time clock and persistent storage for application data. An example might be a gas meter.