3Com NBX 100 Administrator's Manual page 419

Connection Considerations 419
The frequency response, dynamic range, and noise of a voice
conversation depend on the voice representation. If all data packets reach
their destination, the system provides voice of a specified quality.
The H.323 standard accommodates alternative voice compression
standards that allow users to trade some voice quality for bandwidth by
selecting a different compression standard (G.711 or G.723).
Consequently, packet loss and delay are crucial to the Quality of Service.
Packet Loss
Packet loss can occur for reasons discussed in Bandwidth, Congestion,
and Connections, next.
Bandwidth
Bandwidth is the capacity to carry information. By using H.323, the same
bandwidth that supports one uncompressed G.711 voice connection can,
instead, support several compressed G.723 conversations with little
noticeable difference in quality.
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. NBX