Network Topology and Z-Wave Command Class PDF

Learning Outcomes Describe the two methods by which devices can be linked: o Twisted pair (TP) cables o Radio frequency (RF) signals State the benefits of RF control over TP cables Describe a topology as the way in which devices are connected to one another Identify the following commo...

Learning Outcomes Describe the two methods by which devices can be linked: o Twisted pair (TP) cables o Radio frequency (RF) signals State the benefits of RF control over TP cables Describe a topology as the way in which devices are connected to one another Identify the following common topologies for home automation: o Bus o Ring o Mesh o Star State the advantages and disadvantages of common topologies for home automation Describe a protocol as the rules used for communication between devices to send digital signals Understand that signals are classified into command classes Unit 2.3 | Network Topology and Z-Wave Command Class 95  2.3.1 Methods of Linking Devices Home automation devices are able to provide remote access and automation because they are able to communicate with one another through signals. For signals to travel from one device to another, however, the devices need to be linked or connected in some way. Devices can be linked or connected either through wired cables (typically twisted pair (TP)) or radio waves (also called radio frequency (RF) signals). These are described below in Table 2.3-1. Table 2.3-1: Wired cables and radio waves Twisted Pair (TP) Cables Radio Frequency (RF) Signals Fig. 2.3-1: Wired Cables Fig. 2.3-2: Radio waves TP cables consist of several pairs of insulated In the context of smart electronics, RF refers copper wires that are twisted together to wireless communication via radio waves. throughout their entire length. RF also refers to the range of frequencies from 3 kHz to 300 GHz, which correspond to the frequency of radio waves and the alternating currents that carry radio signals. [1G (or 1 giga) = 1,000,000,000] Unit 2.3 | Network Topology and Z-Wave Command Class 96  2.3.2 Benefits of RF Signals over TP Cables Table 2.3-2 states the advantages and disadvantages of TP cables and RF signals. Table 2.3-2: Advantages and disadvantages of TP cables and RF signals Type of Link TP Cables RF Signals Advantages Minimise the effects of noise or Do not require line of sight, unlike electromagnetic interference older technologies such as infrared Relatively inexpensive Highly portable and easy to install Readily available in existing Suitable for both indoors and buildings outdoors Disadvantages Low noise immunity Require a transmitter and a receiver Narrow bandwidth Has privacy and security issues The obvious benefits of RF signals over TP cables are: cost savings, due to not having to provide cables; and greater convenience, due to no longer having to manage the laying of physical cables and the ability to use wireless remote controllers. 2.3.3 Topology Topology refers to how a network is physically or logically laid out; in other words, the way in which the computers in the network are interconnected. The choice of topology depends on: cost; type and amount of equipment to be used; required response time; and required rate of data transfer. Unit 2.3 | Network Topology and Z-Wave Command Class 97  2.3.4 Common Topologies for Home Automation Four common topologies for home automation are the bus, ring, mesh and star topologies, which are described in Table 2.3-3. Table 2.3-3: Common topologies for home automation Type of Bus Ring Mesh Star Topology Layout A E B HUB D C Description All the devices All the devices Each device has All the devices are connected are connected a dedicated are connected to one bus in a closed-loop point-to- point to a central cable running circuit, with link to every device called a the length of each device other device; hub. the network; linked to the each link carries If one device the devices are next. traffic only wants to send connected like A signal is between the data to the stations along a passed along two devices it other device, it train line. the ring in one connects. has to first send The bus cable direction from A complete the data to the carries the one device to mesh hub, and then transmitted another until it connection has the hub will message along reaches its physical transmit that the cable. destination. channels to link data to the The message Each device in its devices. designated arrives at each the ring has a Each device in device. device, which repeater. the network checks the must have input destination and output address ports. contained in the message to see if it matches its own; the device does nothing if there is no match. The devices are connected to the bus cable by shorter cables called drop lines. Unit 2.3 | Network Topology and Z-Wave Command Class 98  Type of Bus Ring Mesh Star Topology Advantages It is easy to Each device has It provides It enables install, as the full access to many routes centralised bus cable can the speed of between the management of be laid along the ring. devices. the network the most through the use efficient path. of the hub, It uses less switch, router or cabling than a central mesh topology, computer. as each drop It is easy to add line only has to another device reach as far as to the network. the nearest If one device in point on the the network bus cable. fails, the Installation is remaining optimally devices in the efficient, as network devices can be continue to added to the function system at any normally. point along the bus cable; there is no need to connect additional devices to the source. Its cost is low. Unit 2.3 | Network Topology and Z-Wave Command Class 99  Type of Bus Ring Mesh Star Topology Disadvantages It is difficult to A faulty device The total It may be more reconfigure or can cause a branch length expensive to isolate faults. break in the can be implement, It is limited in ring and disable significantly especially when size by cable the entire greater than a switch or length and network. that of other router is used adding new Adapter cards topologies, as the central devices is are expensive. especially if the network device. difficult. network is fully The As the number connected. performance Signal reflection of devices at the ends of increases, the The cost may and the number the bus cable speed of the become too of devices the can cause network slows high to be network can degradation in down. justified. handle is quality. dependent on the central A fault or break network device. in the bus cable stops all If the central transmissions. network device fails, the entire As the number network will be of devices shut down, increases, the disconnecting speed of the all devices from network slows the network. down. Unit 2.3 | Network Topology and Z-Wave Command Class 100  2.3.5 Protocol Standard Two persons talking to each other must use a common language to understand each other. This is the same for two devices communicating with each other. The language used by devices is called a protocol standard. In a home automation system, this language can be expressed in the form of rules for how data or messages are transmitted and received. Z-Wave is an example of a protocol standard. 2.3.6 Z-Wave and Command Classes Command classes are groups of commands and responses related to a certain function of a device within the Z-Wave network. Fig. 2.3-3 shows some examples of different command classes. Fig. 2.3-3: Command classes A normal on/off switch is referred to as a binary switch as there are only two states, i.e., on or off. The command class for a binary switch allows us to determine the status of the switch via: the status request function the status report function The command class for a binary switch consists of three different functions: responses commands reports Unit 2.3 | Network Topology and Z-Wave Command Class 101  2.3.7 Basic Command Class The Basic command class allows Z-Wave devices to communicate with one another, even when each device does not know the specific function of the other. The Basic command class consists of two commands (SET and GET) and one response (REPORT): SET – sets a value between 0 and 255 GET – asks the device to report a value REPORT – responds to the GET command, reporting a value between 0 and 255. The following examples illustrate how devices interpret the Basic commands. Example 1 – Binary Switch SET – The controller sends “255” to the binary switch to turn it on. GET – The controller asks for the status of the binary switch; in this case it is “255”. REPORT – The binary switch responds to the request and sends “255” to update the controller about its status. SET = 255 The binary switch is turned on. Switch Controller Fig. 2.3-4: Binary switch Unit 2.3 | Network Topology and Z-Wave Command Class 102 Example 2 – Heater SET – The controller sends “0” to the heater to turn it off. GET – The controller asks for the status of the heater; in this case it is “0”. REPORT – The heater responds to the request and sends “0” to update the controller about its status. The heater is turned off. Heater Controller Fig. 2.3-5: Heater Example 3 – Dimmer SET – The controller sends “255” to the dimmer to set the brightness to 100%. GET – The controller asks for the status of the dimmer; in this case it is “255”. REPORT – The dimmer responds to the request and sends “255” to update the controller about its status. SET = 255 The dimmer is set to 100% brightness. Dimmer Controller Fig. 2.3-6: Dimmer Unit 2.3 | Network Topology and Z-Wave Command Class 103 Example 4 – Window Sensor A window sensor sends out a value of 0 when the window is closed and a value of 255 when it is open. The first SET command is ignored because the window sensor is an automatic sensor SET = 255 that is activated by detection only. REPORT = 255 When the window sensor Window sensor senses that the window is open, it reports a value of 255 back to the controller. Controller Fig. 2.3-7: Window sensor Unit 2.3 | Network Topology and Z-Wave Command Class 104