ITU-T Recommendation G.114 sets a 150ms one-way delay ceiling for 'essentially transparent' conversational interactivity, with 400ms as the outer limit for network planning

ITU-T G.114 ("One-way transmission time," 05/2003) is the standards-body recommendation governing acceptable one-way ("mouth-to-ear") delay in telephone connections. It sets two distinct thresholds rather than a single number. Below 150ms, delay is treated as effectively unnoticeable: "if delays can be kept below this figure, most applications, both speech and non-speech, will experience essentially transparent interactivity." Above that, quality degrades along a curve, but network planning is still permitted up to a hard outer bound: "it is recommended to not exceed a one-way delay of 400 ms for general network planning... a value that allows flexibility in deploying global networks, without making an excessive number of user experiences unacceptable."

Between 150ms and 400ms, the Recommendation's companion E-model (ITU-T G.107) supplies a delay-to-quality curve — mapping one-way delay to a Transmission Rating R and from there to categorical user-acceptance bands, from "very satisfied" near 0ms degrading toward "nearly all users dissatisfied" as delay approaches and exceeds ~400–500ms — assuming echo is controlled and no other impairments are present.

This is a network-engineering standard for acceptable transmission delay, distinct from Stivers et al.'s measurement of human conversational-turn timing, but the two compete for the same budget in voice-AI system design: a spoken-dialogue system's total mouth-to-ear latency (speech-to-text + inference + text-to-speech + network) has to clear the ~150–400ms window in which G.114 says human listeners judge a delay as natural rather than degraded. See claim-voice-ai-sub-second-conversational-latency-budget for the architecture-level argument this bears on.