 voice is in the order of 250 ms, as illustrated in Figure 10.5. Assuming that the delay for the core network is approximately 100 ms, the tolerable delay for MAC buffering/scheduling and detection should be strictly below 150 ms. Hence, assuming that both end users are (E-)UTRAN users, the tolerable one-way delay for MAC buffering and scheduling should be under 80 ms, which is the used air interface delay for HSPA. To improve voice quality further on LTE, the tolerable air interface delay was reduced to 50 ms for LTE [11]. For LTE FDD, the average HARQ RTT equals 8 ms, implying that at most 6 HARQ retransmissions are allowed for a VoIP packet when air-interface delay of 50 ms is used. For HSPA the HARQ RTT would be 12 ms in downlink and 16 ms in uplink with 2 ms TTI, which means at most 6 HARQ retransmissions in downlink and 4 HARQ retransmissions in uplink with 80 ms HSPA delay budget. Similarly, at most 1 HARQ retransmission could be allowed in the uplink, corresponding to 40 ms HARQ RTT with 10 ms TTI. Users Very Satisfied Users Satisfied Some Users Dissatisfied Many Users Dissatisfied Nearly All Users Dissatisfied Mouth-to-Ear Delay/ms Figure 10.5 E-model rating as a function of mouth-to-ear delay (ms) [42]. (Source: ITU-T Recommendation G. 114. "One way transmission time." With permission)) © 2009 by Taylor & Francis Group, LLC Radio Access Network VolP Optimization 10.3.3 Quality Criteria Considering the nature of radio communication, it is not practical to aim for 100% reception of all the VoIP packets in time. Instead, a certain degree of missing pack- ets can be tolerated without notably affecting the QoS perceived by the users. For VoIP traffic, the system capacity is measured as the maximum number of users who could be supported without exceeding 5% outage. During the standardization phases of HSPA and LTE in 3GPP, slightly different outage criteria were used when evaluating VoIP system performance. Used outage criteria are defined according to the system as follows: For HSPA, a user is in outage if m