Excessive Speech Transcoding - Mitel MiVoice Business Voice Quality And Troubleshooting Manual

EXCESSIVE SPEECH TRANSCODING

When you place a VoIP call, your analog voice signals are converted to digital data, compressed
into packets, and then transported over the network. The conversion and
compression/decompression process is called "transcoding" and is performed by
coder/decoders (CODECs).
Note: The terms "transcoding" and "compression" are often used interchangeably, but
this usage is incorrect. Transcoding is the direct digital-to-digital conversion of one voice
digitization standard to another. It occurs when voice information is changed from one
CODEC type to another. Compression simply refers to a reduction in the amount of data.
The amount of voice compression applied by the CODEC determines the quality of the voice
signals. Voice compression works by removing a certain amount of information that is deemed
non-essential (for example, not sending data during silent periods and sending only the main
voice frequency elements rather than the full bandwidth). As a result, some voice information
is lost. The quality of compressed voice is never as good as uncompressed voice, but
intelligibility will typically be maintained.
There are several commonly used ITU-T standards used by CODECs for voice digitization:
• G.711 is the default standard for Mitel communication platforms and for the PSTN. This
standard digitizes voice into a 64 Kbps data stream. The G.711 standard does not compress
voice. In a LAN environment where bandwidth is plentiful, G.711 should be used.
• G.729a is also supported by Mitel communication platforms for compressed voice operating
at 8 Kbps, with 8 to 1 compression. With voice quality just below that of G.711, it is the
second most commonly implemented standard. In a WAN environment, where access
bandwidth is limited, G.729a can be used to increase the amount of voice traffic supported
on a particular link. However, the recommended approach is to use G.711 with higher voice
quality and a lower amount of voice traffic.
• G.722.1 operates at 64 Kbps, but offers high-fidelity speech. Whereas the two previously
described standards deliver an analog sound range of 3.4 kHz, G.722 delivers 7 kHz. This
standard is supported on MCD Release 5.0 and later platforms.
Compression affects a number of call connection types. These include:
• IP phone to IP phone
• IP phone to TDM and vice versa
• IP phone at a remote site back to TDM or IP
• IP connection across an IP trunk route
Compression affects other aspects of the MiVoice Business system as well. These include IP
phones, the MiVoice Business platform, MiVoice Business devices, IP applications, IP
networking routes and trunk routes, and licenses. Voice subjected to excessive compression
or too many transcoding hops (for example, from G.711 standard to G.729.a and back to G.711)
sounds like the voice is coming through a drain pipe.
IP Network Issues
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