Low-bit-rate voice encoder gets VoIP nod

Tests reveal Lucent gateway generates acceptable voice quality even at low-speed rates.

By CHARLES BRUNO
ITclarity, 08/14/00

Network executives faced with the decision to deploy voice over IP technology in their enterprise networks must take solace from the fact that the products they acquire are capable of providing a degree of voice quality acceptable for business use.

Researchers for ITclarity, the industry's leading source of hands-on testing of emerging technologies, recently tested a standards-based voice encoder (vocoder) for transmission of multimedia over extremely low-bit-rate connections that still provides acceptable quality audio for business applications.

Engineers measured the voice quality of Lucent Technologies' implementation of a G.723 vocoder; G.723 is an ITU-T recommendation that specifies algorithms for a dual-rate vocoder originally designed for low bit-rate multimedia transmissions at either 6.3K bit/sec or 5.3K bit/sec. Both rates employ a Code-Excited Linear Prediction (CELP) algorithm; the difference between the rates is based on the excitation technique (Multipulse Maximum Likelihood Quantization, or MP-MLQ, for 6.3K bit/sec and an algebraic code for 5.3K bit/sec).

Voice-quality measurements were conducted for two different signaling methods, 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. Each of these signaling methods was tested in combination with the two vocoders we examined.

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).

Engineers determined that the G.723.1 vocoder, as implemented in Lucent's MAX VoIP gateway products, does not provide voice quality on par with vocoders that support the G.728 and G.729 specifications for encoding speed for transmission over 16K bit/sec and 8K bit/sec connections. And, voice quality was notably different than earlier tests of a G.711 vocoder, which handles uncompressed voice over a 64K bit/sec circuit. However, voice quality over the low-bit-rate vocoders is still acceptable for business use.

Moreover, engineers found there is virtually no difference in voice quality when directing the encoder traffic over ISDN PRI lines or T-1 circuits that employ robbed-bit signaling.

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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