05.093GPPLink adaptationRelease 1999TS
When the radio quality becomes better in one direction on both BSSs, then the signal on the radio interfaces needs less protection and consequently the useful speech information can be increased: the codec can be changed to a codec with a higher bit rate. The principle when TFO is active is that the decision to upgrade the codec is taken by the side that monitors the uplink quality. If a good quality has been detected in the uplink direction by this BTS2, then this BTS2 upgrades the Codec Mode, if the Code Mode Request from the peer MS1 allows that (when the downlink of the other side is also good).
C.2.1 Downlink upgrading
In the following figure, the downlink is monitored by MS1 and the uplink by BTS2.
Figure C.2.1: Downlink Upgrading
In that case the round trip delay from BTS1 to BTS2 before the codec adaptation is effective in downlink to MS1 has an high impact. The delay from the BTS2 to the MS2 before the codec adaptation is effective in uplink is small. The "effective" round trip delay may even be "indefinite" if either MS1 or BTS2 is not able to upgrade its codec mode.
In the figure above, if BTS2 had not measured a good uplink radio quality, then no adaptation would have been performed. The MS1 respectively BTS1 would go on sending continuously Codec Mode Request for the higher codec mode.
C.2.2 Uplink upgrading
In the following figure, the downlink is monitored by BTS 2.
Figure C.2.2: Uplink Upgrading
The round trip delay before the codec adaptation is effective has a low impact, if the peer MS is able to upgrade its downlink codec mode. The round trip delay may even be indefinite if the peer MS is not able to upgrade its downlink codec mode.
In the figure above, if MS2 had not measured a good downlink radio quality, then no adaptation would have been performed.
Annex D (informative):
Transmission Phase of AMR Inband Information
Introduction of RATSCCH for AMR
Clarification of the identification of the codec modes within the active codec set
Version 8.0.0 for Release 1999
Clarification on the allowed transitions of the Codec Mode Indication
Clarification on decision thresholds
Alignment of ACS change with Layer 3 signalling
Clarification in Transmitter/Receiver Synchronisation
RATSCCH protocol: repetition of REQ messages and other corrections
CHANNEL MODE MODIFY, phase and RATSCCH
Re-definition of channel quality measure used for AMR link adaptation