higher than in VoLTE case. In conclusion, native VoLTE clients are very well optimized to minimize standby impact on UE's battery life and network load, while some of the OTT VoIP and third-party SIP clients may generate significantly higher load and negatively impact the battery consumption and network capacity, especially if very aggressive keep-alive settings are used by default. 16.7.4 Impact of Network Loading and Radio Quality As shown in previous sections, OTT VoIP applications, although less efficient in terms of data and battery consumption than VoLTE, can still provide very good voice quality under good radio conditions and low load. However, in a real network, the radio conditions can get bad and other users will generate additional traffic in the cell. One essential difference between OTT VoIP and VoLTE is that the latter gets a dedicated QCI1 bearer to transport the voice packets, which receives high priority in the radio interface and transport, while OTT VoIP generally will be transferred using the default data bearer mixed with other low-priority traffic. The effect of this difference in how voice packets are delivered can be clearly seen in Figures 16.35 and 16.36, where MOS and mouth to ear delay for one OTT VoIP application and VoLTE are shown for good and bad radio conditions and different levels of background data load. In this case, load was generated with 19 UEs doing continuous uplink and downlink file transfer protocol (FTP) transfers. Higher effective loads were achieved by increasing the OTT VoIP-good OTT VoIP-bad VoLTE-good VoLTE-bad OTT VoIP OTT VoIP 65% DCR ~100% DCR Effective non-GBR load (users) Figure 16.35 MOS (POLQA SWB) versus non-GBR load for VoLTE and OTT VoIP in good and bad radio conditions LTE Small Cell Optimization OTT VoIP-good OTT VoIP-bad VoLTE-good VoLTE-bad Effective non-GBR load (users) Figure 16.36 Mouth to ear delay versus non-GBR load for VoLTE and OTT VoIP in good and bad radio conditions relative weight of background data compared to the OTT VoIP users. For ex