KNX System Media
The core elements in the KNX system are the three media: Twisted Pair TP, Radio Frequency RF and the Internet Protocol IP. In addition, the well-defined interworking of various applications and the common software for commissioning, the ETS®. In addition, KNX security is becoming another key element of the system.
Since the beginning of KNX, the green cable with the red-black plug socket has symbolized the KNX bus, known as the twisted pair in the specification. Twisted pair is still the most widely used medium in KNX. The topology of the KNX system is very flexible and can be structured into areas and lines. Basis band signal coding with a data rate of 9600 baud is used for data transmission. This enables a very robust transfer of up to 50 telegrams per second. The bus voltage is 29 V⎓.
Despite its low bandwith, the KNX bus offers an ideal connection for distributed devices in the building, which generally only exchange relatively low amounts of data based on individual events. At the same time, the bus can supply a large number of devices with its bus voltage.
Radio Frequency RF is the wireless solution for the KNX standard. Since the introduction of ETS5, KNX RF has been fully integrated into the KNX system. A major use of KNX RF devices is the wireless extension of KNX systems. This has been further simplified with the introduction of the segment coupler function in ETS6. This means that a wireless coupler can not only be connected to a main or area line, but also to any TP line.
KNX RF uses frequency modulation (FSK) with a central frequency of 868.3 MHz. With data rates of 16,384 baud, a similar number of telegrams per second can be transmitted as with TP. The physical layer of KNX RF is equivalent to that of Wireless M-Bus (DIN EN 13757-4, S mode).
The KNX RF standard differs between (fully) bidirectional and semi-directional devices, which are only bidirectional for configuration and unidirectional in standard mode. This enables battery-operated sensors, as they do not have to be constantly ready for reception.
In the KNX system, the Internet Protocol IP is not only used as a fast backbone, but is also defined as an independent medium such as TP or RF. KNX devices which use the IP medium are also known as “KNX IP only devices”. The KNX IP medium is essentially the equivalent of the KNX routing protocol. This means that KNX IP only devices can communicate directly with KNX IP routers and therefore also with KNX TP via a network switch.
KNX IP includes all the typical features of KNX – such as interworking and configuration with the ETS® software. KNX IP provides access to top-level communication throughout the building (e.g. telecommunication, multimedia, etc.) and enables a completely new category of KNX devices.
Transmission of messages via KNX IP/Ethernet is defined as part of the KNXnet/IP protocol based on UDP (User Datagram Protocol).
Since IP networks are always subjected to a certain risk of unauthorized access, KNX IP communication should be secured. In addition to KNX Data Security, KNX IP Security is also available for the IP medium.
The common tool for KNX: The ETS®
The non-proprietary tool for commissioning KNX systems, the ETS software, is offered by the KNX Association in Brussels. For every product which is used in a project, the ETS requires a specific product data base. Most of the product data are available in the ETS online catalog.
A KNX device can be configured with device-specific parameters. It is linked to other devices via so-called communication objects with group addresses. The ETS supports all media of the KNX standard. Devices on the KNX bus can therefore be easily connected in a project with radio-based KNX devices (KNX RF) or with devices that are connected to the LAN network (KNX IP). The ETS supports KNX Security. The ETS also allows the mixing of secure and unsecure devices. Even within a device, it is possible for individual objects to be both secure and unsecure. This enables a step-by-step progression to fully protected solutions, as will be required in the future.
The most important target of security in KNX is surely to protect the installation from unauthorized manipulation. This is not just about hackers being able to switch the light on or off at their own will. Manipulation of heating or even access systems can lead to significant damage. The KNX installation itself can also become the target of an attack. If, for example, the programming of KNX devices is deleted without authorization, this can result that a building is temporarily no longer usable.
Another aim of security in KNX is to protect privacy. Consumer data should certainly not be freely accessible. Knowledge of user behavior or presence profiles can also facilitate criminal activities. This is why KNX Security includes data encryption.
Unfortunately, attacks on the digital infrastructure are now a daily routine. As a result, the topic of “cyber security” is becoming increasingly important for manufacturers and users. Official authorities such as the BSI in Germany (Federal Office for Information Security) and the EU are also concerned with current threats and possible solutions. It is already becoming foreseeable that all digital communication in buildings will have to be encrypted in future. This will be a major challenge for many systems and manufacturers, but also for system integrators.
Good news: KNX is prepared. KNX provides not only a solid specification for security, but also the corresponding tools, especially the ETS. With KNX Data Security, KNX has introduced end-to-end encryption. Numerous manufacturers already offer KNX devices with security. Weinzierl has been involved from the very beginning. Weinzierl has worked on the standard and expanded the stack implementations accordingly. In the meantime, many devices are already available in a secure version. The product range will be successively adapted in the direction of security.