Why 868 MHz?
Range
The success of mobile communications and its active use inspired the further development of radio communication protocols used for short distances, as well as for high-speed data exchange.
Examples of such protocols: Bluetooth and WLAN.
They gave impetus to the development of data transmission systems over a radio channel.
When the details of the KNX RF system implementation were being developed at the so-called physical layer (the first of seven layers in the open systems interconnection model), the experts in the task force chose the frequency of 868 MHz, since it was not used in ISM devices (Instrumentation, Science and Medicine).
One of the reasons for choosing this frequency was a law of nature.
With equal transmission powers, the propagation radius of radio waves is inversely proportional to the frequency.
The KNX RF system signal at a frequency of 868 MHz has an output power of 1 to 25 mW.
If the transmitter had a higher frequency, a more powerful emitter would have to be used to achieve the same coverage area.
The KNX RF system only communicates with the closest sensor, then the command is passed on.
At 868 MHz, the SRD mode (short range mode) is only used by 1%.
The KNX RF system does not require an increase in the data rate (currently 16 kbps), since this system does not use streaming data (voice, video and file transfer).
External environment
Another reason for choosing this frequency is the fact that when passing through obstacles (such as a concrete or stone wall, as well as furniture and even you and me), the radio signal can be partially or even significantly weakened. (Chicken in a microwave oven fries, absorbing radio radiation in the centimeter range.)
The space behind the obstacle, where the radio signal cannot reach, is called the «no-reception zone»
Zone of no reception
The radio signal spreads in all directions.
The choice of the 868 MHz frequency is optimal, since it ensures reliable data exchange, which does not require high speed.
This frequency is less absorbed by walls and other obstacles.
These two effects mentioned above extend the communication range to 30 m, taking into account the passage through two or three stone walls.
This is enough to cover one or several apartments (even when using only one repeater).
Large buildings may require additional repeaters.
The use of the KNX RF system guarantees the coverage radius, but this requires the use of highly sensitive receivers and sufficiently powerful transmitters.
Then the end devices will be able to receive even very weak signals.
Most devices manufactured for the KNX standard even exceed these minimum requirements.
Repeaters for KNX RF
To extend the transmission range, you can either increase the power of the emitters or install one or more repeaters.
The KNX RF protocol points to the second path, offering a repeater usage model that provides for the «smart» use of the data link layer protocol and source address information.
This will avoid multiple retransmissions to one address.
However, given the transmission radius for small and medium-sized KNX RF systems, it is possible to do without repeaters at all, which has been proven at real construction sites.
Other wireless communication technologies involve the use of state-of-the-art repeaters or their complex placement model.
The procedure for setting up WLAN-level networks is only available to a specialist, so they must provide self-configuration and self-healing functions.
Such protocols require some time to detect and resolve the problem.
And during this time, some signals may be lost. For example, a rain sensor powered by autonomous batteries sends a one-way telegram to close the window when it starts to rain.
