Function | Function as repeater
RF/TP media coupler/repeater | Order no. 5110 00 | 51103300
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Image 8: Application example 2: two repeaters in a strand configuration
Function in detail (expert knowledge)
To prevent radio collisions, according to applicable KNX specifications, after an RF
telegram has been received repeaters are permitted to retransmit more quickly than
normal RF devices without repeater function (telegram pause time of the repeater =
5...15 ms, normal RF device = 15...30 ms). As a result, repeated telegrams from re-
peaters persist temporally in the RF medium (as a result, they have a higher trans-
mission priority). Because of the listen-before-talk function, normal RF devices can
recognize that repeaters are transmitting repeated telegrams and delay their own
transmission requests until the radio channel is free again.
In addition, like all other RF devices, a repeater waits for a short random interval at
each transmission request before a radio telegram is actually transmitted and thereby
repeated. This random time is of different length for each transmission operation.
Consequently, radio collisions in combination with listen-before-talk are largely sup-
pressed to accommodate the possibility that there are several repeaters in one RF
domain which, so to speak, want to transmit at the same time. The principle of this
collision avoidance is only expedient when all repeaters in one RF domain are posi-
tioned within radio range of each other.
One repeater essentially repeats all RF telegrams from its own domain which it has
not already transmitted itself, in addition to always repeating all system broadcast
telegrams. Telegram repetitions are not forwarded to the TP side via a media coupler
if the coupler has already forwarded the original telegram previously. Moreover, re-
peated telegrams also do not appear in the group or bus monitor of the ETS when
telegram recording with a KNX RF USB data interface takes place. These specific
modes of action are made possible by extending the RF telegram structure and eval-
uating special telegram characteristics, as explained in the following:
