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406 APPENDIX D: CONNEXTIONS H.323 GATEWAY
Quality of
Service Control
NBX systems address Quality of Service (QoS) issues using methods that
are discussed in this section.
Adaptive Jitter Buffering
All IP network devices use buffers to retime the packets that they receive
from a network. Retiming allows these devices to compensate for the
variable delays that occur as the packets pass through an IP network.
H.323 calls take different paths through a network so the ConneXtions
gateway uses an adaptive “jitter buffer” to minimize delay variability.
Initially, the jitter buffer delays the entire packet stream by 50 ms, an
amount that is too small to be noticed in conversation, but large enough
to account for the variability.
If the packet delays are too variable, packets may not arrive in time to be
useful. This can result in lost packets and gaps in the conversation. When
ConneXtions detects the gaps caused by late-arriving packets, it
automatically extends the jitter buffer delay to match the delay so similar
packets are not lost. ConneXtions can extend the jitter buffer delay up to
its 200 ms limit.
Reconstruction
NBX Business and Basic Telephones expect to receive voice packets at
regular intervals, but unanticipated network delays can cause lost packets
and gaps in the conversation. Reconstruction makes these gaps less
noticeable with “best guess” substitutes based on the preceding and
following samples.
If your network is not optimized for voice, the quality of voice can be
affected.
Priority Schemes
Packet-based voice systems depend on the speedy and consistent delivery
of voice packets for good voice quality. This dependency presents an
obstacle to H.323 communication on the Internet because it was
designed to treat all packets alike with respect to time. By treating
packets that carry e-mail with the same priority as packets that carry
real-time voice, the Internet ignores the important differences between
these applications.