3Com NBX 100 Administrator's Manual page 420

420 A D: C X H.323 G
PPENDIX ONNE TIONS ATEWAY
Telephones generate voice frames at regular intervals for the duration of
a connection. These frames normally continue when no one is speaking.
When silence suppression is enabled, the NBX system 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. 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
causes are faulty wiring, connectors, and termination. High-bandwidth
LANs (100BASE-T) are more likely to have termination problems than
10BASE-T LANs.
Packet Delay
Latency and jitter delays affect the Quality of Service.
Latency
Latency is the sum of all the fixed delays in an end-to-end connection.
Latency prevents a caller from responding immediately to another caller's
remarks.
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.