Create support for KNX-RF Retransmitter

[nanosonde]

Enable support for retransmitting received KNX-RF frames. This can be used to extend the range of KNX-RF.

See

knx/src/knx/rf_data_link_layer.cpp

Line 214 in cc1fc7d

// Frame duplication prevention based on LFN (see KKNX RF spec. 3.2.5 p.28)

See chapter 6.1.7 of 03.02.05 Communication Medium RF.

1) History List
We have to store the KNX serial number and the last received LFN (frame number) of this device in a list:

• minimum 3 entries
• maximum 7 entries or deletion mechanism after a timeout (e.g. 3s)
• only one entry for each KNX serial number

Then discard a received frame if its KNX serial number and LFN pair is already in the list.

2) Repetition Counter
Implement pseudo code:

if rf_repetition_counter(rec_Frame) > 0 and 
  rf_repetition_counter(rec_Frame) > rf_repetition_counter_limit(Repeater) 
    rf_repetetion_counter(rec_Frame)— 
else 
  discard(rec_Frame) 
endif 

rf_repetition_counter_limit should be read from PID_RF_REPEAT_COUNTER (PID = 74) in the Device Object (object_type = 0).

Additional remarks:

• The KNX serial number is only contained in RF frames where the destination address is a group address and it is not 0x0000 (broadcast).

Open questions:

• As the KNX serial number is only conained in frames that are used for normal group communication, does this mean that the RF retransmitter really only works on this single communication mode? So no RF retransmitting when configuring a device for example.