Nortel BCM50 2.0 Installation Manual page 48

48 Chapter 6 IP telephone overview
This is how the jitter buffer works:
Assume a jitter buffer setting of five frames.
• The IP telephone firmware places the first five arriving frames in the jitter buffer.
• When frame six arrives, the IP telephone firmware places it in the buffer, and sends frame one
to the handset speaker.
• When frame seven arrives, the IP telephone buffers it, and sends frame two to the handset
speaker.
The net effect of using a jitter buffer is that the arriving packets are delayed slightly in order to
ensure a constant rate of arriving frames at the handset speaker.
This delaying of packets can provide somewhat of a communications challenge, as speech is
delayed by the number of frames in the buffer. For one-sided conversations, there are no issues.
However, for two-sided conversations, where one party tries to interrupt the other speaking party,
it can be annoying. In this second situation, by the time the voice of the interrupter reaches the
interruptee, the interruptee has spoken (2*jitter size) frames past the intended point of interruption.
In cases where very large jitter sizes are used, some users revert to saying OVER when they wish
the other party to speak.
Possible jitter buffer settings, and corresponding voice packet latency (delay) for the BCM50 2.0
system IP telephones are:
• None
• Small (G.711/G.729: 0.05 seconds)
• Medium (G.711/G.729: 0.09 seconds)
• Large (G.711/G.729: 0.15 seconds)
QoS routing
To minimize voice jitter over low bandwidth connections, the BCM50 2.0 programming assigns
specific DiffServ Marking in the IPv4 header of the data packets sent from IP telephones and from
IP trunks.
The DiffServ Code point (DSCP) is contained in the second byte of the IPv4 header. DSCP is used
by the router to determine how the packets will be separated for Per Hop Behavior (PHB). The
DSCP is contained within the DiffServ field, which was known as the ToS field in older versions.
The BCM50 2.0 assigns Expedited Forwarding (EF) PHB for voice media packets. On the BCM50
2.0, these assignments cannot be adjusted.
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