Ericsson has 4 different thresholds for HR. Two for HR allocation(new assignments) and two for FR to HR adaption (existing calls). Two in each category correspond to AMR and Non-AMR. With thresholds set to even 100% there is always some FR traffic. Mainly there are three reasons:
1. Some handsets don’t support HR.
2. Congestion in Transcoder HR resources.
3. Quality considerations(DMQG and DMQB). Before converting a FR channel to HR RXQUAL of the channel is compared with thresholds DMQGAMR and DMQGNAMR and if RXQUAL is greater than thresholds, FR to HR adaption is not made. RXQUAL of HR channels is continuously compared with thresholds DMQBAMR and DMQBNAMR and if greater, channel is converted back to FR if there is a FR channel avaialable.