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Packet loss concealment (PLC)
Some packet loss can be dealt with by attempting to conceal the loss by generating packets to
take the place of the missing packets. ITU standards G.711 Annex I and the G.729 standard
define methods by which packet loss concealment can be provided. Excessive packet loss
cannot be disguised so, ultimately, PLC gives way to comfort noise generation (CNG) if too
many packets are lost in succession.
Ramping down to silence is a typical way that PLC is performed. Loss of six consecutive
packets is considered to be the maximum number of packets over which PLC can be sensibly
applied.
Echo
The two main types of talker echo are acoustic echo and electrical echo caused by hybrid
impedance mismatch.
Acoustic echo occurs when the talker's voice traverses through the airpath from the receiver
back to the microphone of the remote terminal. This effect depends on the properties of the
remote room; for example, large room size, hard walls.
Electrical echo is due to an impedance mismatch between four-wire and two wire systems or in
the conversion between a headset and its adapter.
The user's perception of echo increases with delay. In practice, most echo received by the ear
within 30 ms is ignored. But if the level of the received echo signal is extremely high, even a
couple of milliseconds will cause echo perception. Echo received after 30 ms may be perceived
as an annoyance. Usually, only the speaker hears an echo but the receiver does not. Because
of the end-to-end latency in some IP Telephony implementations exceeds the latency in some
circuit-switched systems, the perception of echo can be greater in the IP Telephony system.
One strategy for dealing with echo is through the use of echo cancellers. Echo cancellers, which
have varying amounts of memory, store incoming voice streams in digital form in a buffer and
compare the received voice with the previously transmitted voice patterns stored in memory. If
the patterns match, the canceller removes the echo. Echo cancellers are not perfect, however.
There will be a residual level left even in optimal operating conditions. Echo cancellers operate
properly only if the one-way trip delay between the echo canceller and the echo source (for
example, the acoustic airpath at the telephone set or electrical hybrid) is larger than the echo
canceller can handle, the echo canceller will not find a pattern to cancel.
Avaya's G350 and G700 Medial Gateways, the Avaya TN2302 Media Processor (in the G650
Media Gateways), the Avaya IP SoftPhone, and the Avaya 4600 Series IP Telephone all
incorporate echo cancellers designed for IP Telephony to improve voice quality.
Issue 3.4.1 June 2005
Echo
209
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