Quality of Service
322
9.5
QoS parameters for Voice over IP applica-
BAT54-Rail/F..
Release
7.54
06/08
D Time of processing (packeting, coding and compression by the sender
and the addressee), duration of handing over the packet from applica-
tion to the interface (serialization), and the time for transmitting via the
WAN distance (propagation) contribute to the fixed part of delay.
D The variable part is determined by the jitter resp. by the setting of the
jitter buffer.
These two parts together compose a delay, which should ideally not
exceed 150 ms.
D Apart from the general loss by network transmission, the packet loss is
significantly influenced by the jitter buffer. If packets arrive with a larger
delay than it can be balanced by the jitter buffer, the packets will be dis-
carded and will increase the packet loss. The larger the jitter buffer, the
smaller is the loss. Conversely, the entire delay will increase with the jitter
buffer size. That means for configuration, that the jitter buffer should be
selected as small as the quality can be considered still as sufficient.
In detail, delay is determined especially by the codec used, the resulting
packet size and the available bandwidth:
D The time for processing is determined by the used codec. For a sampling
time of 20 ms, exactly each 20 ms a new packet is generated. Times for
compression can mostly be neglected.
Delay < 150 ms!
Processing
Serialization
Propagation
Processing
150ms
In comparison: satellite quality
Processing Serialization Propagation
Jitter buffer
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