Configuring Voice Codecs - Cisco SPA303 Administration Manual

Configuring Security, Quality, and Network Features

Configuring Voice Codecs

Configuring Voice Codecs
Cisco Small Business SPA300 Series, SPA500 Series, and WIP310 IP Phone Administration Guide
Jitter Buffer Adjustment—Controls how the jitter buffer should be
adjusted. Select the appropriate setting: up and down, up only, down
only, or disable. Defaults to up and down.
• Echo—Impedance mismatch between the telephone and the IP Telephony
gateway phone port can lead to near-end echo. Cisco IP phones have a
near-end echo canceller with at least 8 ms tail length to compensate for
impedance match. Cisco IP phones implement an echo suppressor with
comfort noise generator (CNG) so that any residual echo is not noticeable.
• Hardware noise—Certain levels of noise can be coupled into the
conversational audio signals because of the hardware design. The source
can be ambient noise or 60 Hz noise from the power adaptor. The Cisco
hardware design minimizes noise coupling.
• End-to-end delay—End-to-end delay does not affect voice quality directly
but is an important factor in determining whether IP phone subscribers can
interact normally in a conversation. A reasonable delay should be about 50–
100 ms. End-to-end delay larger than 300 ms is unacceptable to most
callers. Cisco IP phones support end-to-end delays well within acceptable
thresholds.
• Adjustable Audio Frames Per Packet—Allows you to set the number of
audio frames contained in one RTP packet. Packets can be adjusted to
contain from 1–10 audio frames. Increasing the number of frames
decreases the bandwidth utilized, but it also increases delay and can affect
voice quality.
A codec resource is considered allocated if it has been included in the SDP codec
list of an active call, even though it eventually might not be chosen for the
connection. If the G.729a codec is enabled and included in the codec list, that
resource is tied up until the end of the call whether or not the call actually uses
G.729a. If the G729a resource is already allocated (and since only one G.729a
resource is allowed per IP phone), no other low-bit-rate codec can be allocated for
subsequent calls. The only choices are G711a and G711u.
Since two G.723. 1 /G.726 resources are available per IP phone, you should disable
the use of G.729a to guarantee support for two simultaneous G.723/G.726
codecs.
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