Jitter Buffer; Quality Of Service - Nortel 1110 Installation And Operation Manual

Page 218 of 630

Jitter buffer

Quality of Service

553-3001-368 Standard 26.00
Nortel IP Softphone 2050
Use the default value sent from the TPS (the value configured in TM– [Nortel
recommends that you use the default value]) to configure the IP Softphone
2050 jitter buffer.
The jitter buffer has a desired size and a maximum allowable size. If the jitter
buffer exceeds its maximum allowable size, sufficient frames are discarded
to reduce the contents of the jitter buffer to the desired setting. If the jitter
buffer underruns, frames are held in the jitter buffer until it fills to the desired
level. Both underrun and overrun result in a discontinuity in the audio.
For codecs that support silence suppression, the jitter buffer is resynchronized
at the beginning of each talk spurt.
A combination of codec selection, jitter buffer and packet time, and the use
of the DiffServ Code Point (DSCP) of the network contributes to the end-to-
end Quality of Service (QoS).
However, the IP Softphone 2050 is an application within the context of the
PC operating system, so the operating system has an effect on the end-to-end
QoS for the IP Softphone 2050. Functionality, which is commonly handled in
DSP hardware (such as, the codec packetization implementation from within
the Voice Gateway Media Card) is implemented in software for the IP
Softphone 2050. It runs as part of the application code on the PC CPU. If the
CPU is busy with other tasks, voice quality can be negatively affected.
The number of buffers used to buffer audio data between the application and
PC audio hardware device driver is adjustable from the Settings > Sound
Devices window. Using fewer buffers reduces the audio path delay but
increases the chances of dropouts and choppy speech, depending on the speed
and utilization of the PC CPU.
This system-wide registry key setting affects other applications and operating
system components but is only effective if Windows QoS Packet Scheduler
is installed. For more information about Windows QoS Packet Scheduler, see
"Windows QoS Packet Scheduler" on page 232.
August 2007