 is established between customers in two LTE access networks, between LTE and PSTN, or between LTE and 3G mobile network. KPI targets for VoLTE services for various scenarios (LTE - LTE, LTE - 3G, or LTE - PSTN voice calls) are given in Table 7.5. As one may note, the delay threshold is set to 400 ms (which is the maximum value for terrestrial voice connections, according to ITU-T G.114 [1]) because the PSTN/PLMN delay threshold at 150 ms is impossible in IP-based mobile networks for voice services such as VoLTE due to large delay budget due to packetization of voice (waiting more voice samples to be gathered in 7 Broadband QoS Parameters, KPIs, and Measurements Table 7.4 LTE QCI for VoLTE. Resource Priority Packet Packet error level delay (ms) loss rate Service Conversational voice Non-GBR IMS signaling GBR, Guaranteed Bit Rate; QCI, Quality Classification Identifier; IMS, IP Multimedia Subsystem. Table 7.5 KPI targets for Voice over LTE (VoLTE) services. VoLTE VoLTE VoLTE VoLTE between inter- in the same LTE connected LTE network networks VoLTE - 3G VoLTE - PSTN Registration 99.9% 99.9% 99.9% 99.9% success rate Service availability Post dialing LTE-LTE: 3.5 S LTE-LTE: 4s 4.5 S Delay (PDD) CSFB: 6s CSFB: 6s CSFB: not CSFB: not defined defined Voice QoE 4 (HD voice) 3.8 (HD voice) (MOS grade) 2.8 (otherwise) 2.8 (otherwise) Mouth-to-ear 400 ms 400 ms 400 ms 400 ms delay (Note 1) (Note 1) (Note 1) (Note 1) Call drop rate CSFB, Circuit Switched Fall Back; MOS, Mean Opinion Score. Note 1: For the best experience for voice the delay is targeted to a preferred maximum value at 150 ms [1]; however, it is impossible to reach for VoLTE although some network operators are able to provide national VoLTE calls with delays below 250 ms. In all terrestrial voice delivery cases the one-way delay should be kept below maximum value at 400 ms. single IP payload for higher utilization of resources), buffering of voice packets, rerout- ing of voice packets at handovers from old to new (target) base station, as well as playout buffe