Clock Synchronization; Jitter Buffer; Qos - Avaya 1110 Fundamentals

Avaya 2050 IP Softphone
Because the Avaya 2050 IP Softphone does not provide an echo canceller, a slight echo from
acoustic coupling on the headset can occur in some call situations.

Clock synchronization

Buffer underruns and overruns can occur since no sample clock is at the receiving end of an
IP audio stream synchronized to the transmitting clock. The buffer overruns and underruns are
corrected by two mechanisms, both of which apply to the IP Phones and the DSPs on the Voice
Gateway Media Card.

Jitter buffer

Use the default value sent from the TPS (the value configured in TM– [Avaya recommends
that you use the default value]) to configure the Avaya 2050 IP Softphone 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.

QoS

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 2050 IP Softphone 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 2050 IP
Softphone. 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 2050 IP Softphone. 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.
142 IP Deskphones Fundamentals
Comments? infodev@avaya.com
February 2013