3Com 3C10402B Administrator's Manual page 477

Connection Considerations 477
Networks differ in the age of their equipment and in the quality of their
service. Traffic can form a bottleneck if network loads force a wide area
service provider to route traffic through old equipment.
Congestion
Users notice congestion when audio "breaks up" during a call.
Congestion can occur anywhere on the network, for example, at an
overloaded LAN (local or remote), at an overloaded router or firewall, or
within an overloaded internet. Because voice packets are only significant
during a conversation, IP networks respond to congestion by discarding
data packets they cannot accommodate. Resending or delaying packets is
not an effective solution.
At the local level, congestion symptoms can be subtle. For example,
routers from different vendors can respond differently to congestion
because of the way they prioritize their response to packet congestion.
When considering communications problems, it is important to maintain
reserve capacity and to use a systematic approach that considers the
entire, end-to-end, connection.
You can reduce NBX system's bandwidth requirements by enabling
"silence suppression," but doing so compromises audio quality. 3Com
telephones generate voice frames at regular intervals for the duration of a
connection. These frames normally continue when no one is speaking.
When you enable silence suppression, the system sends a "silence
indicator" when the telephone senses the start of a silent period. When
another 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. However,
most telephone users will notice the difference between genuine silence
and generated silence.
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.
Connections
Sometimes packet loss is caused by a poor physical connection. This type
of packet loss is more likely to occur in a LAN than in a WAN. Typical