KNX IP only A New Class of KNX Devices Dr.-Ing. Th. Weinzierl D-84558 Tyrlaching www.weinzierl.de KNX IP only means communication only over the Internet Protocol in a computer network. At first this may appear to be a replacement of the proven medium for building systems, but in fact it s an extension of the KNX system which will allow an entirely new class of KNX devices in the future. IP will be treated as a stand-alone media within KNX and will be considered equal to Twisted Pair, Powerline and Radio Frequency (RF). It opens doors to top level communication within a building (e.g. telecommunication, multimedia, etc.) and allows external access to a KNX installation. Figure 1: Ethernet Patch Wire Communication via KNXnet/IP The transmission of KNX messages via Ethernet is defined as part of the KNXnet/IP protocol and is already part of the KNX standard. The first application of the KNXnet/IP protocol was to utilize Ethernet for PC interfaces and routers. IP routers are comparable to line couplers and use Ethernet as the main line. In the meantime IP has been integrated as stand-alone medium within KNX and allows using the powerful KNX feature, like the configuration modes and interworking, also on IP. Configuration The proven configuration modes of the existing KNX media are also available for KNX IP devices. Since KNX IP devices are expected to be fairly complex, most of these devices will be limited to the configuration in System Mode via the ETS. KNX IP devices will be supported by ETS. 2008-09-08 Seite 1/5
The accustomed ETS presentation with group objects and parameters will of course be carried over to the KNX IP device class. In Easy Mode and even in Easy Push Button Mode interesting solutions based on KNXnet/IP can be expected. Interworking The interworking on KNXnet/IP is unchanged from the other KNX media. Therefore, KNX IP devices may utilize the proven performance of the KNX architecture including the group addressing. In addition, IP only devices can be plugged to other KNX media easily over IP- Router. Ethernet IP UDP KNXnet/IP Length Protocol Version Service Type Identifier Total Length Payload KNX-Telegram (cemi) MC 0x29 AddIL 0x00 Ctrl1 0xBC Ctrl2 Src hi Src lo Dst hi Dst lo L TPCI APCI Data... 0xE0 0x11 0x01 0x00 0x01 0x03 0x00 0x80... Figure 2: Message format KNXnet/IP Ethernet as Competitor to TP? With the growing importance of communication via IP and Ethernet, the question arises as to whether Ethernet will replace the proven KNX Media Twisted Pair and Powerline entirely. This question can clearly be answered, NO. The basic reason for this is the high cost of having to wire each end device with its own network cable. However, this is not a disadvantage if a device already has to have a network connection due to its application. For example, this will enable the use of buttons on a VoIP telephone for KNX functions (e.g. dimming of lights) if the manufacturer supports this in the firmware. This means that every device with a network connection could be turned into a KNX device without additional hardware costs through the integration of system software for KNX. The future therefore clearly belongs to hierarchical topologies: Ethernet will establish itself as the high performance backbone and connection for complex (KNX IP) devices. Twisted Pair, Powerline and RF will claim their importance in connecting distributed sensors and actuators. 2008-09-08 Page 2/5
LAN-Switch ETS Internet KNX IP Router KNX IP Router KNX IP only Device 1.1.0 2.1.0 5.1.1 KNX TP KNX TP 1.1.1 1.1.10 2.1.1 2.1.10 1.1.2 1.1.11 2.1.2 2.1.11 Figure 3: Mixed bus topology with IP and Twisted Pair Hardware Aspects for KNX IP The development of KNX IP devices does not require special KNX parts. An IP node basically consists of an Ethernet controller and a micro controller or a CPU. Ethernet controllers are available from different semiconductor manufacturers. Most of the time controllers with a data rate of 10 MBit/s are sufficient. The choice of the microcontroller basically depends on the required computing power of the device. In principle, the KNXnet/IP protocol could even be implemented on an 8-bit controller. Depending on the application higher performing controllers may be necessary. Numerous controllers already offer an interface for Ethernet on the chip, so that only the physical layer has to be added. PC platforms for may be interesting for higher level requirements to the performance. Figure 4: Platform for a KNX IP only device IP only devices will commonly have higher energy consumption than typical devices on KNX Twisted Pair. Depending on the application these will be equipped with dedicated power supply 2008-09-08 Page 3/5
(230 V) or external power adapter. The alternative power supply via Ethernet Standard Powerover-Ethernet (PoE, IEEE 802.3af) may be especially interesting. KNX IP devices can be supplied with power over the Ethernet cable of up to 15 W if the terminal device and the utilized network switch support this standard. System Software for KNX IP Devices The system software for a KNX IP device actually consists of two protocol stacks. An IP stack with UDP (User Datagram Protocol) is necessary for the communication over Ethernet because KNX IP is based on connectionless communication. Singlecast and Multicast message are utilized over UDP. There is a large choice of commercial and open source software providers for IP stacks. Many operating systems, for example Linux or Windows CE already contains the IPstack. The actual KNX stack is piled upon the IP/UDP stack. The KNX stack uses the IP/UPD stack as interface to the network. The embedding of KNX messages in UDP messages is defined by KNXnet/IP. The KNX stack not only contains protocol layers based on the OSI/ISO reference model but also the particular device model with corresponding management procedures. The device model 5705 was the first to be defined for KNX IP. It is similar to the device model 0705 from Twisted Pair. It allows up to 255 group objects and supports a loadable area for several thousand parameters. The application program in the device accesses the API (Application Programming Interface) of the KNX stack to communicate in the KNX network on the level of group objects. Application KNX Stack KNXnet/IP KNX Msg. IP/UDP Ethernet Figure 5: System software of a KNX IP device 2008-09-08 Page 4/5
Weinzierl Engineering GmbH (www.weinzierl.de) as system provider offers different solutions to efficiently develop and manufacture KNX IP devices. Based on detailed experience in the KNX development and certification new system software (communication stack) for KNXnet/IP was implemented. The bus monitor Net n Node now also supports the additional interfaces KNXnet/IP Tunneling and Routing. The KNX stack for KNX IP devices can be used with any IP stack, e.g. that of Linux. In addition an own IP stack is available which has been specifically developed for small platforms without an own operating system. Summary By the very definition of a KNX IP device, KNX now offers the complete set of media for building control. KNXnet/IP bridges digital communication systems with high band width like audio, Internet and telecommunication. KNXnet/IP offers companies which are already involved in KNX new possibilities for the development of trend setting products. The complete availability of hardware and software allows manufacturers a quick and cost effective integration of their devices into the world of KNX. The combination of two world standards - Ethernet and KNX - is an equally safe basis for manufacturers and installers for investments in new solutions. 2008-09-08 Page 5/5