ocessors to run the VoLTE stack. However, a UE with a chipset-integrated VoLTE implementation and optimized DRX settings can achieve a clearly lower power con- sumption. Figure 16.33 shows the power consumption of integrated VoLTE implementation with different DRX settings illustrating that VoLTE can obtain the same power consumption as 3G CS voice. OTT VoIP without DRX may consume up to two times more current. Silence Silence LISTEN Silence LISTEN Silence Figure 16.32 Voice pattern in power consumption testing LTE Small Cell Optimization Power consumption AMR-NB AMR-WB No DRX DRX 20 DRX 40 DRX 40 ms / on 6 ms / on 6 ms / on 4 3G CS VoLTE Figure 16.33 VoLTE power consumption measurements with integrated VoLTE client 16.7.3 Standby Performance Another important aspect of VoLTE performance is the behaviour during standby periods when the application is in background without performing any calls. During this period, different applications usually do some kind of activity to ensure connectivity is maintained either with IMS system or similar proprietary solution in case of OTT VoIP. Figure 16.34 shows the number of radio signalling messages generated when using different VoIP applications. Native VoLTE service was the one generating the least number of messages, Radio signalling Stand by current SIP 9 S SIP 100 S OTT AP1 OTT AP3 VoLTE Figure 16.34 Radio signalling and current consumption during standby Smartphone Optimization which were pretty much the minimum load that the smartphone generated anyway with basic configuration. OTT VoIP applications generated between 50% and 200% more radio signalling than VoLTE. The third-party SIP clients tested included a configurable timer for sending keep- alive messages, which by default were configured between 9 and 600 seconds. The cases with 9 and 100 seconds keep-alive interval are presented in the chart, showing the high impact both the number of messages generated during 1 hour standby time and the average current consumption, which in the worst case grew up to four times 