Tests yield impressive voice quality
Lucent VoIP gateways deliver 'excellent' voice quality using G.711 vocoder.
 
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By CHARLES BRUNO
Engineers tested Lucent Technologies Inc.'s MAX 6000 and MAX TNT VoIP gateways, both of which employ a G.711 vocoder for encoding voice files. G.711 is an ITU Telecommunications Standardization Sector (ITU-T) Recommendation (1988) that utilizes Pulse Code Modulation (PCM) to encode/decode a voice signal using eight bits per sample at a rate of 8 kHz, yielding a "data" rate of 64K bit/sec. G.711 does not support compression.
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). PSQM is measured between 0 and 6.5. The best possible PSQM score is 0, which means that the output, or speech file, is identical to the input or source file.
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, thereby reducing the 64-K bit/sec bandwidth available for voice to 56K bit/sec.
The PSQM scores for all G.711 tests were less than 0.5, which indicates a relatively small difference in perceived quality for uncompressed speech before encoding and after decoding. Common Channel Signaling (e.g., ISDN PRI) achieved a better PSQM rating than T1 robbed-bit signaling. However, this is not too surprising since robbed-bit signaling reduces the information/voice bandwidth available to the user from 64K bit/sec to 56K bit/sec. However, the differences are audibly indiscernible.
"For uncompressed voice, we recommend robbed-bit signaling since it provides good voice quality and the maximum number of simultaneous voice calls," says Scott Hamilton, chief operating officer of Tolly Research, which publishes the ITclarity reports. "But we also expect that recommendation may change once we begin testing compressed voice over these gateways."
The above information is derived from an ITclarity research track report. To purchase a subscription to Tolly Research's ITclarity service, click here.
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In an independent assessment of voice quality delivered over IP networks, Tolly Research has found that a pair of Lucent Technologies' voice over IP gateways deliver excellent quality audio when handling uncompressed voice traffic.
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