Nortel 1000 Description, Installation And Operation Manual page 221

voice packets are sent until the silence period is broken. There are two
parameters that control Silence Suppression, as follows:
1 Idle noise level. This is set at a default level of –65 dBm0.
2 Voice activity detection threshold. This is set at a default of 0dB. Voice
packets are formed when the audio level exceeds the idle noise level by
this threshold value.
These default parameters are suitable for most office environments.
Increasing either of these two parameters lowers the amount of IP traffic
generated, but increases clipping and dropped packets.
Disable Silence Suppression at tandem nodes
Silence Suppression introduces a different concept of half-duplex or
full-duplex at the voice message layer that results in a kind of statistical
multiplexing of voice messages over the WAN.
When a system equipped with an IP Trunk 3.01 (and later) node serves as a
tandem switch in a network where some circuit-switched trunk facilities have
an excessively low audio level, Silence Suppression, if enabled, degrades the
quality of service by causing choppiness of speech.
Under tandem switching conditions where loss level cannot compensate,
disable Silence Suppression using the OTM ITG ISDN Trunk Node
Properties DSP profile tab codec options sub-tab. See Step 8 on
Disabling Silence Suppression approximately doubles LAN/WAN
bandwidth use. Disabling Silence Suppression consumes more real-time on
the IP trunk card.
Table 18 on page 152
Suppression is disabled. This does not impact the data rate for fax, since fax
does not have Silence Suppression enabled.
ITG engineering guidelines
shows the bandwidth requirement when Silence
IP Trunk Description, Installation, and Operation
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page 328.