Gira 5110 00 Product Documentation page 37

Control of media access
When it wishes to transmit a KNX telegram, each KNX RF transmitter checks whether the radio
channel is already occupied by another RF transmitter (LBT: Listen Before Talk). If this is the
case, the RF transmitter waits with the required radio transmission. It transmits its own telegram
as soon as the radio channel is free again.
In addition, each RF transmitter waits for a short random interval on each transmission request,
before the radio telegram is actually transmitted. This random time is of different length for each
transmission operation. This suppresses to a great extent radio collisions between devices
which actually wish to transmit simultaneously (e.g. media couplers, which have received a
group telegram to be forwarded via the TP side), in combination with LBT.
The described transmission method for controlling media access generally prevents radio
collisions in a KNX RF environment, but cannot exclude them completely. For example, it may
occur that, in the case of a transmission between an RF transmitter (A) and an RF receiver (B),
there is an additional RF transmitter (C), which is located within the range of the RF receiver,
but cannot reach the other RF transmitter due to the spatial distance (Figure 8). In such a case,
the two RF transmitters are unable to detect when the other transmits radio signals (Hidden
Station Problem). In consequence, radio collisions can occur on the receiver located in the
range of the two RF transmitters.
Figure 8: Radio collisions on the receiver due to the transmitter being too far away
The described effect is system-dependent and can be a particular problem when there are two
or more media couplers in a KNX system. If the media couplers are out of the other's range,
then they cannot detect whether another media coupler is already transmitting a group
telegram. However, the KNX subscribers in the various RF lines can be located in such a way
that they are in the overlap areas of the RF domains of the couplers. In consequence, the
subscribers receive the colliding telegrams of multiple media couplers (Figure 19).
This circumstance must already be taken into account during the planning of a KNX RF system.
- If possible, position media couplers in such a way that they are within direct reception
range of each other.
- Structurally decouple RF domains in such a way that their subscribers are only positioned
in their own RF environment.
- Use repeaters instead of media couplers, if this is wise from a structural and topological
point of view. Repeaters should be located within the range of the media coupler and also
within the ranges of other repeaters.
i If complete separation of RF domains or the integration of media couplers into their radio
ranges cannot be guaranteed structurally, then it may be better to create only one radio
domain instead of multiple media couplers and to work with repeaters.
Order No. 5110 00
Software "Media coupler / repeater"
Parameters
Page 37 of 57