05.093GPPLink adaptationRelease 1999TS
For an active AMR call without TFO, the codec mode is downgraded when the quality of the radio link decreases. In that case the codec bit rate is reduced so that more throughput is left on the radio interface to add protection. The principle when TFO is active is that as soon as one side detects the need to downgrade the codec rate, it informs the other side and the codec mode is immediately downgraded. There is no negotiation before downgrading.
C.1.1 Uplink downgrading
The following figure shows the uplink adaptation due to detection by the BTS1 of a bad quality in the uplink from the MS1. In that case the round trip delay between BTS1 and BTS2 has no impact, as the uplink codec adaptation is decided immediately by BTS1 and performed by the MS1.
Figure C.1.1: Uplink Downgrading
C.1.2 Downlink downgrading
The following figure C.1.2 shows the downlink adaptation due to detection by the MS1 of a bad quality of the downlink from the BTS1. The codec mode adaptation must be performed on the far end MS (MS2), where the source encoder is located. In that case the round trip delay before the downlink adaptation is effective has an high impact. If TFO would not be active, the round trip delay would be smaller, because the Codec Mode Request sent by the BTS1 to the TRAU1 would be immediately taken into account by the TRAU1 and the new codec mode applied in downlink: without TFO ongoing the (tandem) source encoder is located in TRAU1.
Figure C.1.2: Downlink Downgrading