Nortel 2001 Installation And Operation Manual page 154

Page 154 of 302
553-3001-368 Standard 4.00
Nortel Networks IP Softphone 2050
Jitter buffer
The jitter buffer of the IP Softphone 2050 is configurable. Set the jitter buffer
in one of the following ways:
• using the default value sent from the TPS (the value configured in OTM
— It is recommended that the default value be used.)
• using the Configuration Utility
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.
QoS
A combination of codec selection, jitter buffer and packet time, and the use
of the network's DiffServ code point all contribute to the end-to-end QoS.
However, the IP Softphone 2050 is an application within the context of the
PC OS, so the OS has an effect on the end-to-end QoS for the IP
Softphone 2050. The DSP functionality (such as codec packetization)
implemented in DSP hardware on the IP Softphone 2050 and Voice Gateway
Media Card runs as part of the application code on the PC's 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 Configuration Tool
(see "Select Sound Devices tab" on
the audio path delay but increases the chances of dropouts and choppy speech,
depending on the speed and utilization of the PC's CPU.
DiffSERV (DSCP)
The IP Softphone 2050 uses DSCP settings assigned by the TPS. The IP
Softphone 2050 supports DSCP on Window 98, Windows 98 SE, Windows
2000 Professional, and Windows XP.
February 2005
page 163). Using fewer buffers reduces