3Com SuperStack 3 NBX Administrator's Manual page 431

Connection Considerations 431
Most people notice latency when the end-to-end delay is above 200 ms.
(The round-trip delay is typically no greater than 400 ms.) Conversations
sound most natural when latency is below this range. Network latency
can be measured by "pinging" the network connection, but the network
connection is only part of the delay. The entire end-to-end delay also
includes the H.323 gateway, firewall or router, and the LAN itself. System
administrators can control some local device delays by controlling the
system load and by upgrading system components as needed.
Jitter
Momentary transmission delays can affect the pace of a conversation
and, if severe, cause the voice to "break up." This is known as "jitter."
All voice-over-internet devices have a "jitter buffer" at the receiving end
whose purpose is to absorb jitter. It does this by delaying the first packets
that arrive by some significant amount (from 50 to 200 ms). This delay
creates a window of time for receiving the next group of related samples
which are then forwarded to a callee at a regular rate. However, if some
packets are too late, and exceed the jitter buffer capacity, those packets
are lost and there are gaps in the audio.
Quality of NBX systems address Quality of Service (QoS) issues using methods that
Service Control are discussed in this section.
Silence Suppression
NBX Business and Basic Telephones generate voice frames at regular
intervals for the duration of a connection. These frames normally
continue when no one is speaking, which creates unnecessary LAN
traffic.
The NBX system has a "silence suppression" mode that sends a "silence
indicator" when the NBX Telephone senses the start of a silent period.
When another NBX device receives this indicator, it inserts "white noise"
until it receives the next frame that contains real voice. All subsequent
"voiceless" frames are suppressed during the silent period.
This type of silence suppression applies to Layer 2 Ethernet transfers. At
Layer 3, the ConneXtions software achieves a similar result by not
sending empty packets during a silent period. The receiving ConneXtions
gateway generates a silence indicator or sends frames filled with silence,
depending on the silence suppression mode.