Codec is a contraction of coding and decoding digital data. This is the format in which the audio stream is stored. It includes both the number of bit rate (bits/sec) and compression that is used.
One of the following codecs is used by the Unified Messaging server to encode the messages:
• Windows Media Audio (WMA)—16-bit compressed
• GSM 06.10 (GSM)—8-bit compressed
• G.711 PCM Linear (G711)—16-bit uncompressed
• Mpeg Audio Layer 3 (MP3)—16-bit compressed
The Exchange 2010 unified messaging default is MP3. This is a change from Exchange 2007 where the default was WMA. Although using WMA results in slightly smaller file sizes, most people prefer the universal nature of MP3. This enables a much larger number of mobile devices to play voicemail messages. The Audio Codec setting is configured on the UM dial plan on the Settings tab.
A dirty little secret is that the digital compression results in loss of data. When the data is compressed and decompressed, information is almost always lost. That is, bits of the conversation or message can be lost. This is a trade-off that the codec makes to save space. This is why the G.711 codec is available, which doesn’t compress data and doesn’t lose data but at a heavy cost in storage.
These are stored in the message as attachments using the following formats:
• Windows Media Audio (.wma)—For the WMA codec
• RIFF/WAV (.wav)—For GSM or G.711 codecs
• Mpeg Audio Layer 3 (.mp3)—For the MP3 codec
The choice of the audio codec affects the audio quality and the size of the attached file. Table 20.1 shows the approximate size of data in the file attachment for each codec.
The G.711 audio codec setting results in a greater than 10:1 storage penalty when compared to the WMA audio codec setting. Although the GSM audio codec setting results in approximately the same storage as the WMA codec setting, this comes at a cost of a 50% reduction in audio quality. MP3 provides similar audio quality to WMA at an acceptable file size. The ubiquitous nature of the MP3 codec makes it the preferred choice for Exchange 2010.
The .wma file format has a larger header (about 7 KB) than the .wav format (about 0.1 KB). For small messages, GSM files are smaller. However, after messages exceed 15 seconds, WMA files are smaller than the GSM files.