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.
For the 'indirect' case 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 communicate separately and independently from one another with the coupler.
Communication between sender and coupler
- The sender receives LL acknowledgments directly (and only) from the coupler.
- Possible repetitions are sent by the sender.
Communication between coupler and destination device
- The coupler receives LL acknowledgements directly (and only) from the destination device.
- Possible repetitions are sent by the coupler, the coupler will in this case use the individual address of the sender, the sender will never be aware of the repetitions.
Two couplers
The sender communicates with the first coupler.
The first coupler communicates with the second coupler.
The second coupler communicate with the destination device.
Communication between sender and the first coupler
- The sender receives LL acknowledgments directly (and only) from the first coupler.
- Possible repetitions are sent by the sender.
Communication between the second coupler and destination device
- The second coupler receives LL acknowledgements directly (and only) from the destination device.
- Possible repetitions are sent by the second coupler by using the individual address of the sender, the sender will never be aware of the repetitions.
More than two couplers
The sender communicate with the first coupler.
The first coupler communicates with the second, the second with the third, etc.
The last coupler communicate with the destination device.
Communication between sender and the first coupler
- The sender receives LL acknowledgments directly (and only) from the first coupler.
- Possible repetitions are sent by the sender.
Communication between the last coupler and destination device
- The last coupler receives LL acknowledgements directly (and only) from the destination device.
- Possible repetitions are sent by the last coupler by using the individual address of the sender, the sender will never be aware of the repetitions.