Avaya IP Telephony Guide de configuration télécharger pdf (Page 23)

Chapter 1 Introduction 23

IP Telephony Configuration Guide

If you experience poor voice quality, and suspect it is due to heavy network traffic, you can get

better voice quality by configuring the IP telephone to use a G.729 CODEC.

Jitter Buffer

Voice frames are transmitted at a fixed rate, because the time interval between frames is constant.

If the frames arrive at the other end at the same rate, voice quality is perceived as good. In many

cases, however, some frames can arrive slightly faster or slower than the other frames. This is

called jitter, and degrades the perceived voice quality. To minimize this problem, configure the IP

telephone with a jitter buffer for arriving frames.

This is how the jitter buffer works:

Assume a jitter buffer setting of five frames.

• The IP telephone firmware places the first five arriving frames in the jitter buffer.

• When frame six arrives, the IP telephone firmware places it in the buffer, and sends frame one

to the handset speaker.

• When frame seven arrives, the IP telephone buffers it, and sends frame two to the handset

speaker.

The net effect of using a jitter buffer is that the arriving packets are delayed slightly in order to

ensure a constant rate of arriving frames at the handset speaker.

This delaying of packets can provide somewhat of a communications challenge, as speech is

delayed by the number of frames in the buffer. For one-sided conversations, there are no issues.

However, for two-sided conversations, where one party tries to interrupt the other speaking party,

it can be annoying. In this second situation, by the time the voice of the interrupter reaches the

interruptee, the interruptee has spoken (2*jitter size) frames past the intended point of interruption.

In cases where very large jitter sizes are used, some users revert to saying OVER when they wish

the other party to speak.

Possible jitter buffer settings, and corresponding voice packet latency (delay) for the Business

Communications Manager 2.5 system IP telephones are:

• None

• Small (.06 seconds)

• Medium (.12 seconds)

• Large (.18 seconds)

QoS routing

When it sends a voice frame onto the network, the IP telephone firmware places some header

information on the frame.

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Avaya IP Telephony Guide de configuration Page 23