II.2 Achieving satisfactory delay
inter-regional routes
For many intra-regional (e.g., within Africa, Europe, North America) routes in the range of 5000 km or less, users of VoIP connections are likely to experience mouth-to-ear delays <150 ms.
Appendix III/Y.1541 illustrates this calculation using reference terminals with a total of 50 ms mean delay (10 ms packets).
The calculation shows that the 100 ms objective of Y.1541 Class 0 can be met with a well-engineered access network (with a T1 or E1 rate or larger as Y.1541 requires) and with as many as 12 network routers. Appendix X/Y.1541 shows that similar speech quality can be maintained with reference terminals contributing a total delay of a less stringent 80 ms (using 20 ms packets and robust packet loss concealment).
inter-regional routes
For inter-regional routes covered terrestrially, even those traversing the 27 500 km of the ITU's traditional worst-case Hypothetical Reference Connection, a VoIP mouth-to-ear path is likely to see a delay of just over 300 ms.
This assumes terminals contributing a total of 80 ms delay
(20 ms packets), a well-engineered access network and supporting IP network paths encountering 20 or fewer network routers (as per Appendix III/Y.1541).
Of course, it is extremely unlikely that the worst case of 27 500 km will be encountered by many calls. For the much more frequent inter-regional calls of, for example, 10 000 km or less, the corresponding delays would be
approximately 225 ms; certainly not as low (or as desirable) as 150 ms, but still quite satisfactory for the vast majority of users.
Whilst delays in the mid-200 ms range may not be a serious problem for long inter-regional calls, where users expect calls to be somewhat different from regional calls, it is critical that network planners do not allow local and regional calls to encounter such delays because user expectations
are that such calls be completely delay-transparent.
ITU-TG.114规定，对于高质量语音可接受的时延是300ms。
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