LL = Link Layer = Data Link Layer
A KNX device only processes the payload of a received telegram when it is also addressed by that telegram.
A device expects after sending a telegram, to receive a LL acknowledgement:
- Positive acknowledgement = LL_ACK: the sender of the telegrams does not repeat the telegram
- Negative acknowledgement = LL_NAK or LL_BUSY: the sender of the telegram will repeat the telegram 3 times
- No (reaction): the sender of the telegram will repeat the telegram 3 times
The LL acknowledgements can either come directly or indirectly from the destination device:
- Directly: sender and destination device are in the same line
- Indirectly: sender and destination device are not in the same line
Three different situations based on the number of couplers between sender and destination device can be distinguished:
One coupler
The sender and the destination device are on opposite sides of the coupler.
The sender will communicate with the coupler and will also receive the LL acknowledgments from the coupler.
The same coupler will communicate with the destination device (from the opposite side) and will also receive the LL acknowledgements from the destination device. And if required, repetitions are sent by the coupler.
Two couplers
The sender will communicate with the first coupler and will also receive the LL acknowledgments from the first coupler.
The first coupler will communicate with the second coupler.
The second coupler will communicate with the destination device and will also receive the LL acknowledgements from the destination device. And if required, repetitions are sent by the second coupler.
More than two couplers
The sender will communicate with the first coupler and will also receive the LL acknowledgements from the first coupler.
The first coupler will communicate with the second, the second with the third, etc.
The last coupler will communicate with the destination device and will also receive the LL acknowledgements from the destination device. And if required, repetitions are sent by the last coupler.