Cisco Systems OL-14619-01 manual Audio Codecs, How Codecs Affect the File Size of Voice Messages

Chapter 3 Network Infrastructure Requirements

Audio Codecs

In some cases you might find that an existing voice messaging system has more ports than the current maximum number of voice ports on a Cisco Unity system. In this case, the customer can purchase additional Cisco Unity servers and connect them by using Cisco Unity Digital Networking.

Audio Codecs

An audio codec is an algorithm that encodes and decodes (and compresses or decompresses) audio data. Cisco Unity uses audio codecs with streaming (live conversation) and for WAV-file voice messages. Cisco Unity supports the following audio codecs:

•G.711 Mu-Law (the default codec)

•G.711 A-Law

•G.729a

•Intel Dialogic OKI ADPCM 8 kHz

•Intel Dialogic OKI ADPCM 6 kHz

•GSM 6.10

•G.726 codec (supported only when using VPIM to communicate with another voice-messaging system)

In general, choose an audio codec based on available storage capacity for voice messages and on available network bandwidth. We recommend using G.711 whenever possible because of its excellent quality for recording and playback. However, if other considerations prevent the customer from using G.711, it is fine to use G.729a, which is also a voice-quality codec. (A large-scale or multiserver deployment may require the use of more than one codec.)

The rest of this section discusses some of the issues to consider when choosing a codec. For a more thorough analysis, see the Audio Codecs and Cisco Unity white paper at http://www.cisco.com/en/US/products/sw/voicesw/ps2237/prod_white_papers_list.html.

For information on how codec choice affects Active Directory, see the applicable Active Directory Capacity Planning white paper at http://www.cisco.com/en/US/products/sw/voicesw/ps2237/prod_white_papers_list.html.

How Codecs Affect the File Size of Voice Messages

Stored voice messages can consume considerable amounts of disk space. The amount of storage a WAV file uses depends on what kind of compression the codec uses, if any. The higher the compression, the smaller the file and the smaller the disk-space impact on the database and at the subscriber workstation. However, some sound-quality reduction and some CPU processing overhead may result when compressing messages during recording and decompressing at playback.

As illustrated in Table 3-1, voice messages that are recorded in G.711 Mu-Law and A-Law require the most disk space to store, while G.729a offers the smallest file size.

Note Audio quality ratings are subjective, and are provided here only for basic comparison between the different fully supported codecs.

Design Guide for Cisco Unity Release 5.x

OL-14619-01 3-5