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Silence suppression/VAD
Besides low bit rate, Voice Activity Detection (VAD) or silence suppression can also be used to
save bandwidth. During a conversation, because only one party is speaking at any given time,
more than 40% of the transmission is silence. Voice Activity Detection (VAD) in the IP telephone
monitors the locally produced voice signal for voice activity. When no activity is detected for the
configured period of time, the Avaya software informs the Packet Voice Protocol. This prevents
the encoder output from being transported across the network when there is silence, resulting in
bandwidth savings. When silence suppression is enabled, the remote end is instructed to
generate "comfort noise" when no voice is present to make the call sound more natural to
users. The trade-off with silence suppression lies with the silence detection algorithm. If it is too
aggressive, the beginnings and ends of words can be "clipped." If not aggressive enough, no
bandwidth is saved. Silence suppression is built into G.729B. It can be enabled for other codecs
from within Communication Manager. Because of voice quality concerns with respect to
clipping, silence suppression is generally not used (with the exception of G.729B).
The following Avaya products employ silence suppression to preserve bandwidth:
Avaya Communication Manager software (for control)
Avaya 4600 series IP Telephone
Avaya IP SoftPhone
Avaya Media Gateways
For procedures to administer QoS parameters, refer to Administration for Network Connectivity
(555-233-504).
Transcoding/tandeming
Transcoding or tandeming describes a voice signal that has been passed through multiple
codecs, such as can be the case when call coverage is applied on a branch office system back
to a centralized voice mail system, the calls may experience multiple transcodings (this could
include G.729 across the WAN and G.723.1 into the voice mailbox). Each transcoding action
results in degradation of voice quality. These problems may be minimized by the use of the
Communication Manager feature called DCS with Rerouting (Path Replacement). This feature
detects that the call coming through the main ECS has been routed from one tandem ECS,
through the main, and back out to a third switch. In these cases, the system then re-routes the
call directly, thus replacing the path through the main system with a more direct connection.
Avaya products minimize transcoding while non-Avaya products may cause slight to excessive
transcoding. "Shuffling" and "Hairpinning" also reduce transcoding.
Silence suppression/VAD
Issue 3.4.1 June 2005
213
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