
Chapter 3 Installing IP Telephones 33
IP Telephony Configuration Guide
6 From the Codec menu, select a default Codec, or leave the Default Codec at Auto. This is the
Codec that is used if an IP telephone has not been configured with a preferred codec. For
information on this, see “Choosing a codec” on page 33.
7 From the Jitter Buffer menu, select a Jitter Buffer level. For information on choosing a Jitter
Buffer, see “Choosing a Jitter Buffer” on page 33.
Choosing a codec
The default codec is used when an IP client has not been configured to use a preferred Codec (see
the next section for individual IP client Codec settings). If the default Codec is set to AUTO, the
Business Communications Manager will choose the appropriate CODEC when an IP client goes
on a call. For example, if both endpoints of the call are I20XX telephones on the same subnet, the
Business Communications Manager chooses G.711 for maximum voice quality. If the telephones
are on different subnets, the Business Communications Manager will choose G.729 to minimize
network bandwidth consumption by voice data packets.
For IP telephones, the Business Communications Manager supports both A and MU law variants
of the G.711 CODEC, as well as the G.729 and G.723 CODECS.
• The G.711 CODEC samples the voice stream at a rate of 64Kbps (Kilo bits per second), and is
the CODEC to use for maximum voice quality.
• The G.729 CODEC samples the voice stream at 8Kbps. The voice quality is slightly lower
using a G.729 but it reduces network traffic by approximately 80%.
• The G.723 CODEC should be used only with third party devices that do not support G.729 or
G.711.
Choosing a Jitter Buffer
A jitter buffer is used to prevent the jitter associated with arriving (Rx) voice packets at the IP
telephones. The jitter is caused by packets arriving out of order due to having used different
network paths, and varying arrival rates of consecutive voice packets.The greater the size of the
jitter buffer, the "better sounding" the received voice is. However, voice latency (delay) also
increases. Latency is very problematic for telephone calls, as it increases the time between when
one user speaks and the user at the other end hears the voice. The administrator can adjust the
default jitter buffer size to the following values:
• NONE: Minimal latency, best for short-haul networks with good bandwidth.
• AUTO: Business Communications Manager will dynamically adjust the size.
• SMALL: Business Communications Manager will adjust the buffer size, depending on
CODEC type and number of frames per packet to introduce a 60 millisecond
delay.
• MEDIUM: 120 millisecond delay
• LARGE: 180 millisecond delay
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