Technical Details
SP flag = 0 indicates SID frame SP flag = 1 indicates speech frame
Speech frame = 260 bit
Transmit side:
TX DTX handle performs speech encoding, comfort noise computation, voice activity detection TX Radio Subsystem (RSS): Performs SP flag monitoring and Channel coding
Hangover period: After the transition from VAD=1 to VAD=0 it take N+1 consecutive frames to make a new updated SID frame available the bursts are directly passed to RSS with SP=1 this period is defined as Hangover period.
Noise Background spikes can often be confused with the speech frame and hence in order to nullify this issue a check list for SID computation is Nelapsed >23, old SID is utilized with VAD=0.
Once after the end of speech SID is computed it is continuously passed to the RSS marked with SP=0 as long as VAD=0.
If a SID (SP=0) is chosen for transmission is stolen for FACCH signaling than the subsequent frame is scheduled for transmission.
Receive side:
BFI=0 Meaningful information bit BFI=1 Not Meaningful information bit
Interestingly a FACCH frame in not considered as a meaningful information and should be transmitted with BFI=1
Traffic frames aligned with SACCH multi frame have TAF(Time alignment flag) =1
RX DTX handler performs speech decoding and comfort noise computation. RX Radio subsystem: Performs Error Correction and Detection and SID frame detection
Whenever a good speech frame is detected the RX DTX handler shall pass directly to speech decoder Whenever a lost speech or lost SID frames are detected the substitution or mutation shall be applied. Whenever a valid SID frame result in comfort noise generation. In case of invalid SID frame after consecutive Speech frames the last valid SID frame will be applicable
Read more about this topic: Discontinuous Transmission
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