Understanding Z-Wave Networks, Nodes & Devices

UNDERSTANDING Z-WAVE NETWORKS, NODES, & DEVICES

Z-Wave technology complies of three layers. They are a radio layer, network layer and an application layer. These all work together so that communication between devices succeed, and also creates an effective network, so that numerous communications between different nodes and devices can occur simultaneously.

Radio Layer: This layer defines the way a signal is exchanged between a transmitter and a receiver. This includes issues like frequency, encoding, hardware access, etc.

Network Layer: This layer defines how real control data are exchanged between two communication partners. This includes issues like addressing, network organization, routing, etc.

Application Layer: This layer defines which messages need to be exchanged to specific applications such as switching a light or increasing the temperature of a heating device.

THE NETWORK LAYER

As well as radio waves, there is a network layer which can be broken up into three sub layers.

Media Access Layer: The MAC layer controls the usage of the wireless hardware and its functions are invisible to the end user.

Transport Layer: This function makes sure, that a message can be exchanged free of error between two wireless nodes. The end user cannot influence functions of this layer but the results of this layer are visible.

Routing Layer: This layer makes sure, that by utilising other nodes if needed, a message is passed between the original sender and the desired receiver. The functions of the routing layer are visible and can be optimised by the end user.

THE MEDIA ACCESS LAYER AND TRANSPORT LAYER EXPLAINED

Rather like sending a text message to a friend, you can’t see how the information transfers from your phone to theirs. It’s a given, that it will be received and read by the recipient. Likewise, many wireless communication networks, they involve communication between a sender and a receiver which is accomplished by simply sending a message over the air.

Occasionally, a message may get lost. In a mobile phone’s case, it could be due to poor reception. In terms of wireless home control, it could be due to interference or positioning the receiver too far away from the sender. When this happens, the sender does not get any feedback on whether the message was received or whether the receiver has been able to execute the command properly. This may result in stability problems, unless the installation was planned and tested correctly from the outset.

Z-Wave continues to be one of the more reliable forms of wireless communication. The system is far more durable in a home or work setting. Every Z-Wave command sent will be acknowledged by the receiver, so it will be clear whether the communication was successful or not. Every message on this system is sent like a registered letter, receiving a return receipt.

Even if you are informed of safe delivery, it doesn’t guarantee that this message will always be delivered correctly. However the sender will get an indication that a situation has changed, or an error has occurred.

The return receipt is called Acknowledge (ACK). A Z-Wave transceiver will try up to three times to send a message while waiting for an ACK. After three unsuccessful attempts the Z-Wave transceiver will give up and report a failure message to the user. The number of unsuccessful transmission attempts can be served as an indicator of the quality of wireless connection.

USING NODES FOR SUCCESSFUL COMMUNICATION

A network consists of at least two nodes which communicate with each other. To be able to communicate with each other, these nodes need to have access to a common media or need to have “something in common”.

In most cases this is a physical communication media like a cable. The communication media for radio is the air which is used by all kind of different users. Hence the communication protocol needs to define an identification which allows the different nodes of one network to identify each other and to exclude received messages from unknown or other radio sources.

Furthermore every node in a network must have an individual identification to distinguish him from other nodes within the network.

The Z-Wave protocol defines two identifications for the organisation of the network.

The Home ID is the common identification of all nodes belonging to one logical Z-Wave network. It has a length of 4 bytes = 32 bits.

The Node ID is the address of the single node in the network. The Node ID has a length of 1 byte = 8 bits.

Nodes with different home ID’s cannot communicate with each other, but they may have a similar Node ID. It is not possible to have two nodes with identical Node ID, which means you have total control of your own home system.

DEVICES

Z-Wave has two basic types of devices:

Controllers are Z-Wave devices which can control other Z-Wave devices;

Slaves are Z-Wave devices which are controlled by other Z-Wave devices.

Whilst controllers already have their own individual Home ID, Slaves do not have a Home ID. This is because controllers already have their own Home ID, which they can hand on to other Z-Wave devices and add them to their own Z-Wave network.

Z-Wave controllers exist in different forms: as a remote control, as PC software in conjunction with a Z-Wave transceiver connected in the PC (typically via USB), as a gateway or as a wall switch with special controller function.

The Home ID of a controller cannot be changed by the user and becomes the common Home ID for all devices, which were included with this unique controller.

The primary controller includes other nodes into the network by assigning them his own Home ID. If a node accepts the Home ID of the primary controller this node becomes part of the network. Together with assigning the Home ID the primary controller also assigns an individual Node ID to the new device, which is included. This process is referred to as Inclusion.

Definition

In the Controller

In the Slave

Home ID

The Home ID is the common identification of a Z-Wave network

The Home ID is already available at factory default.

No Home ID at factory default

Node ID

The Node ID is the individual identification (address) of a node within a common network

Controller has its own Node ID predefined (mostly 0x01)

Is assigned by the primary controller

Home ID versus Node ID

As you can see from the diagram below, these devices are available in factory default state. There are two controllers with a preset Home ID. Two other devices cannot operate as a controller (Slave) and, hence, have no own Home ID.

Depending on which of the controllers is used to build up a Z-Wave network, the network Home ID in this example will be either 0x00001111 or 0x00002222.

Both controllers have the same Node ID #1. The slave devices do not have any Node ID assigned. In theory this picture shows two networks with one node in each of them.

Because none of the node in the figure has any common Home ID, no communication can take place.

One of the two controllers is now selected as being the primary controller of the network. This controller assigns his Home ID to all the other devices (includes them) and also assigns them individual Node ID.

After successful Inclusion, all nodes have the same Home ID, i.e. they are connected in a network with each other. At the same time every node has a different individual Node ID. Only with this individual Node ID’s, they can be distinguished from each other and can communicate with each other. In a Z-Wave network several nodes having a common Home ID must not have the same Node ID ever.

In the network shown as an example there are two controllers. The controller whose Home ID, became the Home ID for all devices, is called the ‘primary controller.’ All other controllers become ‘secondary controllers.’

The primary controller can include further devices, whereas the secondary controller cannot. However, the primary and secondary controllers operate the same.

Because the nodes of different networks cannot communicate with each other due to the different Home ID, they can coexist and do not even “see” each other.

The 32 bit long Home ID allows to distinguish up to 4 billion (2^32) different Z-Wave to networks with a maximum number of 2^8 = 256 different nodes. Because some addresses of the network are allocated for the internal communication and special functions, maximum 232 different nodes can communicate in a network.

If Z-Wave nodes are deleted from a network, this is called Exclusion in the Z-Wave terminology. During the Exclusion process the Home ID and the Node ID are deleted in the device. The device is moved back in the factory default state (controllers have their own Home ID and slaves do not have any Home ID).

Meshing and Routing

In a typical wireless network the central controller has a direct wireless connection to all of the other networking nodes. This always requires a direct radio link. In case of disturbances the controller does not have any backup route to reach the nodes.

The radio network illustrated above is a non-routed network. Nodes two, three and four lie within the radio range of the controller which is labelled number 1. Node 5 lies beyond the radio range and cannot be reached from the controller.

However, Z-Wave is a wireless system that offers a very powerful mechanism to overcome this limitation. Z-Wave nodes can forward and repeat messages which are not in direct range of the controller. This gives greater flexibility as Z-Wave allows communication, even though there is no direct wireless connection or if a connection is temporarily not available, due to some change in the room or building.

The Z-Wave network with routing shows the controller with ‘Node ID 1’ can communicate directly to the nodes 2, 3 and node 4. Node 6 lies outside its radio range, however, it is within the radio range of node 2. Therefore the controller can communicate to node 6 via node 2. This way from the controller via node 2 to node 6 is called a “route”.

The other side of routing is that the diagram shows that direct communication between Node 1 and Node 2 is blocked, but there is still another option to communicate to node 6 via node 2, by using node 3 as another repeater of the signal. It is evident, that more nodes result in more different routing options for the controller and therefore in a more stable network.

What makes Z-Wave more appealing compared with other wireless communication networks, is that it can work around corners. This means the signal can detour around an obstacle in the direct communication path, as illustrated.

Z-Wave is able to route messages via up to four repeating nodes. This is a compromise between the network size and stability, and the maximum time a message is allowed to travel in the network.

Building Routes in a Z-Wave Network

Every node is able to determine which nodes are in its direct wireless range. These nodes are called neighbours. During inclusion and later on request, the node is able to inform the controller about its list of neighbours. Using this information, the controller is able to build a table which has all information about possible communication routes in a network. The routing table can be accessed by the user. There are several software solutions, typically called installer tools, which visualise the routing table to optimise the network setup.

The diagram shows an example of a Z-Wave meshed network, with one controller and five other nodes. The controller and is the primary controller with Node ID 1.It can communicate directly with node 2 and 3. There is no direct connection to node 4, 5 and 6. Communication to node 4 works either via node 2 or via node 3.

The rows of the table contain the source nodes and the columns contain the destination nodes. A “1” is a cell which indicates that the two nodes are direct neighbours.

The example shows the connection between Source Node 1 and destination Node 4. The cell between Node 1 and 4 is marked “0“. This means the nodes are not neighbours and cannot communicate directly. The route goes via Node 3 which is in direct range both from Node 1 and Node 4.

In the example Node 6 can only communicate with the rest of the network using Node 5 as repeater. Since the controller does not have a direct connection to Node 5, the controller need to use one of the following routes: 1 -> 3 -> 4 -> 5 -> 6 or 1 -> 2 -> 5 ->6.

A controller will always try first to transmit its message directly to the destination. If this is not possible it will use its routing table to find the next best way to the destination. The controller can select up to three alternative routes and will try to send the message via these routes. Only if all three routes fail (the controller does not receive an acknowledgement from the destination) the controller will report a failure.

Types of Network Nodes

A routing slave is a slave with some advanced functions regarding routing capabilities. Slaves are categorized further into standard slaves and routing slaves.

The three different node types have three main capabilities. The main difference between the three node types is their knowledge about the network routing table and subsequently their ability to send messages to the network:

	Neighbours	Route	Possible functions
Controller	Knows all neighbours	Has access to the complete routing table	Can communicate with every device in the network, if a route exists.
Slave	Knows all neighbours	Has no information about the routing table	Can only reply to the node which he has received the message from. Hence, can not send unsolicited messages
Routing Slave	Knows all his neighbours	Has partial knowledge about the routing table	Can reply to the node which he has received the message from and can send unsolicited messages to a number of predefined nodes he has a route too.

Properties of the Z-Wave device models

Slave

Fixed installed mains powered devices like wall switches, wall dimmers or Venetian blind controllers

Routing Slave

Battery-operated devices and mobile applicable devices as for example

sensors with battery operation,

wall plugs for Schuko and plug types,

thermostats and heaters with battery operation and all other slave applications

Typical applications for slaves

Challenges in typical network configurations

As a result of the routing functionality there are some typical network configurations with their individual challenges and requirements.

Z-Wave works by starting with a very small network and extending this network later on as and when you need. A very typical small network consists of a remote control and a couple of switches or dimmers. The remote control acts as primary controller and includes and controls the switches and dimmers.

During inclusion the dimmers and switches should be installed at their final location already, to make sure that a correct list of neighbours will be recognised and reported.

A network configuration like this works well as long as the remote control can reach all switches and dimmers directly (the node which is to be controlled is “in range”). In case the controlled node not in range, the user may experience delays, because the remote control needs to detect the network structure first before controlling the device.

In case a device was included and moved afterwards to a new position, this particular device can only be controlled by the remote control if it is in direct range. Otherwise the communication will fail, because the routing table entry for this particular device is wrong and the remote control is not able to do a network scan at the moment of operation.

Z-Wave Network with one static controller

Another typical network consists of a static controller - mostly PC software plus Z-Wave transceiver as a USB dongle or an IP gateway IP as well as a number of switches and dimmers.

The static controller is the primary controller, and includes all other devices.

Because a static controller is bound to a certain location, the other Z-Wave devices must be included while being in direct range with the static controller. They will typically be installed at their final location after inclusion.

Networks with multiple controllers

In a larger network several controllers will work together. A static controller – e.g. a PC – is used for the configuration and management of the system and one or several remote controls carry out certain functions in different places.

If a network has multiple controllers, the user needs to determine which of the controllers will be the primary controller.

Inclusion of a static controller is a challenge, if the devices need to be moved to their final location afterwards. A network re-organisation needs to be performed.

Static controllers are usually more reliable and cannot get lost easily. They typically offer backup functions to replace the hardware in case of severe damages.

Network with portable controller as a primary controller:

Remote controls are more vulnerable to damage and loss. Usually remote controls do not offer a backup function. If the primary controller was damaged or lost, a complete re-inclusion of the whole network would need to be performed. However, devices can be included after they were installed, which results in a much more stable network, and no need for network re-organisation.

The choice of the primary controller - static or portable - depends more on the personal preference of the user than on technical necessity.