Configuring Security, Quality, and Network Features
Ensuring Voice Quality
Cisco Small Business SPA300 Series, SPA500 Series, and WIP310 IP Phone Administration Guide 132
5
Ensuring Voice Quality
Voice quality perceived by the subscribers of the IP Telephony service should be
indistinguishable from that of the PSTN. Cisco IP phones support several codecs.
See:
• “Supported Codecs” section on page 132
• “Bandwidth Requirements” section on page 133
• “Factors Affecting Voice Quality” section on page 134
Supported Codecs
Negotiation of the optimal voice codec sometimes depends on the ability of the
Cisco IP Phone to “match” a codec name with the far-end device/gateway codec
name. Cisco IP phones allow the network administrator to individually name the
various codecs that are supported such that the correct codec successfully
negotiates with the far-end equipment.
Note that Cisco IP phones support voice codec priority. You can select up to three
preferred codecs.
The administrator can select the low-bit-rate codec used for each line. G.711a and
G.711u are always enabled. The following table shows the codecs supported by
Cisco IP phones. The third column shows the voice quality Mean Opinion Score
(MOS), with a scale of 1–5, in which higher is better.
Codec (Voice
Compression
Algorithm)
Complexity and Description MOS Score
G.711 (A-law
and u-law)
Very low complexity. Supports uncompressed 64
kbps digitized voice transmission at one through ten 5
ms voice frames per packet. This codec provides the
highest voice quality and uses the most bandwidth of
any of the available codecs.
4.5
Highest
voice quality
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