Forward-thinking – backwards compatible
Being well aware of the opportunities in the home installation market - with correspondingly long investment cycles - the Z-Wave Alliance is committed to the principle of maintaining the backwards compatibility of existing equipment- in contrast to most other manufacturers in the sector. Anyone who buys a Z-Wave product today has the confidence that they are buying a product based on a long-lived stable standard. And even in ten years time, new products in the market will still work with those that a consumer has had for years.
To enable this backward compatibility, Z-Wave has developed a meaningful and effective mechanism. In order to introduce new features into the existing standard with ease, each appliance must state what properties and options it has.
Colored LED lamps are one example of this. These were certainly not around when the Z-Wave standard was first released. Initially, LED lamps were treated as if they were normal incandescent lamps that were to be switched on, off, and also dimmed. The specification of a particular color was then added as a supplementary option.
There is a demand for color LEDs to light up white if no color is specified. This is in order to make it possible to continue using an older “pre-color” dimmer controller, even if the light to be dimmed is capable of more functions. When newer controllers are used, they can also select the color in the case of colored LEDs, as well as dim and switch older monochrome lights naturally.
Stability creates confidence
Nonetheless, the result is a universally usable, powerful, and yet stable wireless protocol which allows equipment from different manufacturers to be operated side by side in an installation. The opportunity to integrate new equipment from new manufacturers into the existing network without complications leads to confidence among end customers as well as distributors. It is this stability and future-ready capability that attracts even some of the larger and better-known companies such as Honeywell, Danfoss, FAKRO, MERTEN and Verizon.
Another challenge which Z-Wave has now successfully overcome is the problem with wireless frequencies: For home networking, only the unlicensed but usable 868 MHz and 2.4 GHz frequencies make sense. Because the 2.4 GHz band is already overpopulated by WLAN and Bluetooth, and these appliances transmit with high powers and occasionally very high bandwidth, all wireless protocols that can be taken seriously for home automation use the 868 MHz band- also referred to as the ISM (Industry Science Medicine) band.
However, the use of the 868 MHz band is heavily regulated: firstly, the maximum transmission power is limited to 10 mW, secondly equipment must also transmit for a maximum of 1% of a time interval (duty cycle = 1%). Despite this, there are millions of appliances in this frequency band which interfere with each other.
Therefore it is far from a trivial task when both the expansion and operation of a stable wireless network in a frequency band suffers from very heavy interference while also being limited on the time available. The data sent must be minimized while at the same time maintain stable wireless communication. Z-Wave uses a frequency modulation method for line coding and modulates signals on it with a birate of 40 kb/s.
Registration with acknowledgement of receipt
A very effective method for avoiding communication problems in wireless networks is meshing. Also, each appliance forwards messages from other appliances, which cannot themselves communicate directly with the desired communication partner. On the one hand, this increases a network's wireless range substantially. On the other hand, alternative routings arise if the direct path to the recipient is blocked.
This intrinsically simple method conceals a number of challenges behind its practical implementation. In an IP network, in which the router forwards the data, these routes are matched dynamically to the networking options by appropriate protocols such as OSPF. This dynamic switching of communication routes is one of the reasons for the amazing robustness of the internet. The price for this dynamic routing, however, is a substantial communication overhead. This is unacceptable in a wireless network for home automation, especially in the regulated 868 MHz band.
This is why Z-Wave uses static source routing. This means that all conceivable routes in the network are stored in the transmitter. In other words, even before transmission, an actual route is selected and coded accordingly. Only if this call is unsuccessful are the alternative routes used.
Appliances which transmit using Z-Wave technology always try to first reach the receiver on the direct wireless path. If this fails, alternative routes over the networks are calculated from the data in order to set up a call over an alternative path. This guarantees that, only if all connection attempts fail, will a failed call be reported to the application and thus to the user. The transmitter is therefore using virtual registration with acknowledgement of receipt. In this way, buildings, especially existing structures, can be networked sensibly by means of wireless communication alone.
Meanwhile, the volume and quality of products fitted with Z-Wave has reached a level which makes professional installations feasible, even for demanding clients. The price also means that in-house service skills can also be sold in addition to the actual hardware. The necessary integration and networking knowledge is therefore available.
The opportunities to extend freely programmable control components with proprietary solutions and scripts mean that there is sufficient scope for differentiation among one's competitors. Clients want energy efficiency and convenience in their homes, but the complexity of the solutions obscures their understanding. The idea is simple: be open to everything.
About the author
Christian Pätz is honorary professor for systems reliability at the Chemnitz Technical University and spokesperson of the Z-Wave Alliance in Europe.
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This article originally appeared on EE Times Europe.