Consequences for Handover Strategies

The auditory results help to answer the research questions raised at the beginning of this section:

1. The most important network characteristic is the packet loss rate. The second most important characteristic is the audio bandwidth. Switching the audio bandwidth is roughly equivalent to the quality degradation of 5-10% packet loss, in both narrowband and wideband conditions. The degradations due to network handover (make-before-break) alone - without codec switching - are negligible in comparison to packet loss and bandwidth switching.
2. Switching codecs is advantageous if the packet loss rate is high, and if the changeover helps to reduce the packet loss impact. Switching codecs in order to take profit of a larger audio bandwidth is advantageous only if a sufficiently long period of WB speech transmission remains. From the limited results of our tests, it seems that this minimal length is around 30 seconds. Unfortunately, it is not possible to know the remaining length of a call. As a workaround, handover strategies could consider the perceived quality and use this value to estimate multiple scenarios (using different remaining lengths), and then select the most beneficial for the user, assuming past and current conditions. An interaction could be observed between packet-loss and audio-bandwidth degradations:
   • If packet loss is high (low basic quality), the impact of audio bandwidth on perceived quality is low.
   • If packet loss is low (high basic quality), the impact of audio bandwidth on overall call quality is high.
3. Parametric quality prediction models like the E-model are not yet able to estimate the speech quality resulting from codec changeovers. Signal-based models like WB-PESQ do a better job, but do not always perform as well on in-scope data.

The results may help to design efficient network handover and codec changeover strategies. In bad network conditions (high packet loss rate), a handover should be made if the packet loss rate can be reduced by this step. In this situation, it is not important whether the audio bandwidth can be maintained or not; the reduction of packet loss should be the ultimate goal. In contrast to this, a handover can also be fruitful in good network conditions (low packet loss rate). In this case, switching to a higher audio bandwidth can help to significantly improve quality. The improvement is most effective if it occurs early in the call; the remaining call duration should be more than 30 seconds. Changing from WB to NB is always linked to a loss in quality; the pure network handover without codec changeover, however, does not significantly impact the perceived quality.