Question: In the TX Radio Subsystem of an Adaptive Multi-Rate (AMR) speech codec, what triggers the transmission of a SID_UPDATE frame during speech pauses?
Option 1: The speaker's voice activity detection (VAD) signal
Option 2: Regular interval timers to update comfort noise on the RX side and improve link quality measurement
Option 3: Channel Encoder's request for more data to maintain transmission continuity
Option 4: Transcoder's output of frames marked with TX_TYPE = "NO_DATA"
Explanation: During speech pauses in AMR speech codec operation, the transmission is not completely halted but rather resumes at regular intervals to transmit one SID_UPDATE frame. This updates comfort noise on the receiving side for a natural call experience and enhances link quality measurement through Received Signal Strength (RSS). This action is triggered by the need for periodic updates.