Hey Mac! What vocoder you got?

There are a great many statistics that you need to look into to measure the quality of your IP telephony. But there is another factor that you should not forget: the type of vocoder you have. Knowing the characteristics of your vocoder can help you to assess call quality accurately, and also may point to problem fixes and quality improvements.

A vocoder is used to encode, decode, compress and decompress analog speech so that it can be transmitted efficiently as binary data. There are several standard vocoders that vary in the amount of compression provided and number of voice frames per packet. For example, G.711 digitizes and compresses analog voice so that speech energy can be transmitted over a 64-Kbps channel and uses a 20-ms sample size. G. 723 digitizes and compresses analog voice so that speech energy can be transmitted over a 5.3- or 6.3-Kbps channel and uses a 30-ms same size.

However, voice-signal compression naturally results in signal distortion and hence degrades the voice quality, and the greater the compression, the greater the distortion. That means that different vocoders start at different levels of the quality spectrum compared to PSTN "toll quality." For example, ITU G1.07 is an international standard guide for the voice quality rating of codecs and end-to-end transmission quality that results from the combinations of impairments occurring on VoIP networks. It assesses a "0" impairment or degradation factor for G.711 while assessing impairment

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factors of "19" and "15" to 5.3-Kbps channel and 6.3-Kbps channel G.723 vocoders, respectively, without taking network impairments into consideration. Thus, some amount of packet loss may be acceptable if G.711 is used while any packet loss may push G.723 implementations over the call-quality cliff.

This circumstance is exacerbated by the fact that there is also more voice energy per packet in G.723 than there is with G.711 (see above). Thus more signal is lost and more degradation occurs per packet loss with G.723 versus G.711.

In sum, stats alone don't tell the story in managing a VoIP network. Managers need to know the vocoder type and the frame size as well. Management tools must be able to identify and correlate these parameters into the call-quality statistics or they will lead you down a path of false positives and disguised negatives.

Bob Massad is a VP at Telchemy, Inc., a provider of voice quality management solutions.

This was first published in February 2003

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