Proprietary VoIP encoding falls short

RT24 voice encoding delivers inferior quality for business usage.

By Charles Bruno
Network World, 10/02/00

Engineers testing a proprietary method of encoding voice traffic for transmission at low data rates have learned that the technology does not deliver voice quality acceptable for business applications.

Testers at Tolly Research, which publishes ITclarity research reports, says that despite significant bandwidth savings, the RT24 voice encoding technique delivers inferior quality audio and should be used between remote sites only when no other voice communications are available.

Tolly Research measured the voice quality of Lucent Technologies' ultra-low-bit-rate RT24 vocoder using Lucent's Voice over IP gateway products. RT24 is a Lucent-proprietary algorithm for ultra-low-bit-rate transmissions at 2.4K bit/sec.

Voice quality was quantified objectively using the Perceptual Speech Quality Measure (PSQM), which provides, according to the ITU-T, the most accurate estimate of the subjective quality of coded speech (ITU-T Recommendation P.861).

Voice-quality measurements were conducted for two different signaling methods, Integrated Services Digital Network (ISDN) Primary Rate Interface (PRI) and T-1 robbed-bit, to determine the impact on perceived voice quality. ISDN PRI uses a single out-of-band common channel for signaling separate information channels. In contrast, T-1 robbed-bit signaling uses selective in-band bits to provide signaling, thereby reducing the 64K bit/sec bandwidth available for voice to 56K bit/sec. Each of these signaling methods was tested in combination with the proprietary vocoder we examined.

The question we were seeking to answer is whether or not an ultra-low-bit-rate vocoder, which delivers a compression ratio greater than 25:1 compared to a standard G.711 vocoder for uncompressed voice, provides acceptable voice quality for business applications.

The RT24 vocoder, as implemented in Lucent's MAX VoIP gateway products, does not come close to providing voice quality on par with other vocoders Tolly Research has tested. There was essentially no difference in voice quality when using Common Channel Signaling or T-1 robbed-bit signaling in conjunction with the RT24 vocoder. And, unlike any of the PSQM results for the vocoders tested in previous tests, there basically is no difference between PSQM scores for male and female voices when using the RT24 vocoder. In general, the voice quality for RT24 is unacceptable for business use.

The above information is derived from an ITclarity research track report. To purchase a subscription to Tolly Research's ITclarity service, click here.

About Tolly Research

Tolly Research, provider of ITclarity, is a technology research firm based in Brielle, N.J., that specializes in testing-based research. Tolly Research provides accurate test results, insightful analysis, and practical recommendations for leading-edge technologies required by network professionals. While many businesses have specific implementation issues, there is a broad base of technology research that is applicable to all organizations. Tolly Research offers a more cost-effective method of gathering data than committing internal resources to tackle each new technology. For more information on Tolly Research, point your browser to www.tollyresearch.com, send e-mail to itclarity@tollyresearch.com or call 732-292-9411.

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