dio Preparations The radio link may be dropped in some rare cases when UE is moving from one cell to another and the radio conditions change very rapidly. The connection can be recovered with the re-establishment procedure which is illustrated in Figure 14.24. UE moves from Cell A towards Cell B in this example but the handover fails and the radio link drops during the mobility. UE then sends Re-establishment request to Cell B after the radio link time-out has been identified. Cell B does not have any earlier information about the UE. Therefore, Cell B sends Radio link failure indication to Cell A to fetch the UE context from Cell A. Cell B can then re-establish the connection and the radio link failure does not lead to call drop. There is a short break of a few seconds in the user plane while the radio link failure timer runs and the connection is re-established. The break duration depends on the timer values. The major benefit of re-establishment is that the VoLTE call can be maintained and an annoying call drop can be avoided. This feature is also called as mobility robustness in 3GPP and it was included in Release 9. 14.4.6 Voice Quality on VoLTE The voice quality depends heavily on the voice codec sampling rate and the resulting audio bandwidth. AMR Narrowband (NB) codec provides audio bandwidth of 80-3700 Hz while AMR Wideband (WB) extends the audio bandwidth to 50-7000 Hz. The CS connections can use either AMR-NB or AMR-WB while VoLTE in practice always uses AMR-WB. Also many OTT VoIP applications use similar wideband audio. The voice quality using the legacy CS switched voice services was mostly impacted by layers 1-3 in radio interface while the voice quality of VoLTE depends on robustness of all layers. LTE provides dedicated bearers for voice with negotiated QoS settings which allow separate prioritization of VoLTE packets based on packet loss and delay targets to help various network elements in resource allocation decisions. The current generation for MOS (mean opinion score) voice quality measuremen