"Delivering Content to Handhelds"
"Buyer's Guide: Choosing a VoIP PBX"
If network latency is high, voice conversations can be painful at best.
The
ITU-TG.114 standard for one-way transmission time recommends
approximately 150
ms for maximum one-way latency to maintain voice quality. In some
implementations, as much as 400-ms delay can be acceptable. That is, if
the bad
guys of VoIP (voice over IP)--echo, jitter and packet loss--don't
rear their
ugly heads.
Echo: Delays in the round trip between caller and receiver can
lead to
echoes on the line. Echoes occur when voice signals are reflected back
to the
speaker and are caused by impedance and data conversion between the
telephone
set and the network. If the round-trip delay exceeds 30 to 50 ms, echo
may
reduce the system's usability. Echo cancelers can ameliorate the
problem, but
they're not foolproof. A canceler has a limited amount of memory to
compare a
received voice pattern with the current voice pattern. When they match,
the
canceler cancels the duplicate as an echo. But where the network delay
between
the caller and the receiver is excessive, a canceler may not have
sufficient
memory to capture, compare and cancel a potential echo.
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