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Assuring VoIP Quality: Not There Yet

It's a simple concept that we've all come to accept: VoIP will eventually be the way enterprise users (and the rest of us for that matter) make phone calls. The problem is that most users have a penchant for applications and services that actually work, and VoIP isn't quite there yet. The art of delivering high-quality voice--so well-perfected in the TDM world--remains immature in the IP world.

Vendors know it and for two years have been working on providing the standards and tools necessary to deliver TDM-grade voice quality. Over the next six months, those efforts will start to materialize through a variety of product introductions and enhancements, both in endpoint devices and gateways, as well as in test and measurement devices. Much of this new technology will borrow from decades-old work done by the telecom industry.

The bad news is that the resulting data from telco-style voice quality tests is so different from that for our existing IP networks that the industry is just now figuring out how to reconcile the two performance metrics into actionable network management information. This means that managing voice quality, particularly on a heterogeneous IP network, isn't possible today. Enterprise network architects must instead rely on good network design and hope that VoIP trouble spots don't appear. The good news is that help may be on the way. Voice quality management standards are beginning to gain acceptance among IP network and VoIP equipment vendors, bringing IP voice a step closer in the right direction.


Before jumping into the emerging standards for VoIP quality management, it's important to understand how the telecom industry measures quality. Telecom architects long ago realized that they needed a universal way to assess the voice quality of phone calls. For that purpose, they came up with the Mean Opinion Score (MOS). In its original form, MOS called for gathering a diverse group of people and asking them to listen to and rate a number of speech samples played over a phone circuit. Each listener rated the quality on a scale from one to five, with one being unacceptable and five being excellent. A MOS rating of 4.0 is generally considered "toll quality." The ITU Telecommunication Standardization Sector (ITU-T) defines rules for conducting these tests (ITU-T Recommendation P.800) so that when SBC runs a MOS test, it gets comparable results to what British Telecom might find.

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