Question: In a TFO scenario for AMR speech codec in downlink, how does the TX Radio Subsystem in the BTS handle frames with `TX_TYPE = NO_DATA` when DTX in downlink is **not** requested?
Option 1: They are handled as usual, without any modification.
Option 2: They are replaced by `SID_FILLER` frames to maintain radio transmission during speech pauses.
Option 3: They trigger the error concealment mechanism in the Mobile Station (MS).
Option 4: They are ignored by the TX Radio Subsystem and not processed.
Explanation: To ensure continuous radio transmission in downlink even during speech pauses when DTX is not requested, the BTS's TX Radio Subsystem replaces `NO_DATA` frames with `SID_FILLER` frames. This approach allows the Mobile Station (MS) to generate Comfort Noise during these pauses, enhancing the user experience by maintaining a consistent audio presence.