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Display characteristics 217

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 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” (page 253).

For Windows 2000 the Windows QoS Packet Scheduler is not installed by

default and the EnablePriorityBoost registry setting is not created.

For Windows XP the Windows QoS Packet Scheduler is installed by

default and the EnablePriorityBoost registry setting is created. The default

setting is 1 (enable QoS).

Windows 2000 and Windows XP require a system-wide registry key to

enable QoS capabilities. You must have Administrator privileges to create

or modify the following value:

HKEY_LOCAL_MACHINE/SYSTEM/CurrentControlSet/Services/Qossp/

EnablePriorityBoost

Value 0—do not enable QoS

Value1—enable QoS

Trace utilities can be used to verify QoS settings. See “Ethereal traces”

(page 219).

Nortel Communication Server 1000

IP Phones Fundamentals

NN43001-368 02.07

5 February 2009