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  5. One Useful Way To Test VoIP Quality: Page 4 of 5
Networking

One Useful Way To Test VoIP Quality: Page 4 of 5

Sage Instruments' neat little 935AT portable unit performs end-to-end call tests to assess overall speech quality.
November 23, 2004

The PVIT test measures speech clipping (useful for verifying voice activity detectors), comfort noise, frame slips, and noise floor performance. You run this test for 15 minutes and it produces the following measurements:

  • Frame slip - Voice Packet Jitter/Jitter Buffer Resizing: Jitter occurs when a network jitter buffer dynamically readjusts the buffer size. Buffer resizing balances the conflicting aims for less frame loss and shorter end-to-end transmission delay. A positive (contracting) slip occurs when a voice frame is deleted. A negative (expansive) slip occurs when a filler frame is inserted. PVIT time stamps each voice frame slip and measures its duration.
  • Voice Clip-Speech Clipping/Silence Suppression: PVIT provides detailed diagnostic information on key packet network characteristics that impact voice clarity. It reports a voice clip when the signal corruption is at the leading edge of a voice segment. Each voice clip event is measured for duration and time stamped. A running calculation also presents the average duration of all voice clip events.
  • Noise hit-Comfort Noise: If, during any individual silence period during the test, the noise level exceeds -45 dBm (45 dBrnC), a Noise Hit event will be reported by PVIT.

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