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Chapter 12 The Service Configuration Screens
User’s Guide
152
12.2.1.3 Voice Coding
A codec (coder/decoder) codes analog voice signals into digital signals and
decodes the digital signals back into voice signals. The WiMAX Device supports the
following codecs.
• G.711 is a Pulse Code Modulation (PCM) waveform codec. PCM measures
analog signal amplitudes at regular time intervals (sampling) and converts them
into digital bits (quantization). Quantization “reads” the analog signal and then
“writes” it to the nearest digital value. For this reason, a digital sample is usually
slightly different from its analog original (this difference is known as
“quantization noise”). G.711 provides excellent sound quality but requires
64kbps of bandwidth.
• G.723 is an Adaptive Differential Pulse Code Modulation (ADPCM) waveform
codec. Differential (or Delta) PCM is similar to PCM, but encodes the audio signal
based on the difference between one sample and a prediction based on previous
samples, rather than encoding the sample’s actual quantized value. Many
thousands of samples are taken each second, and the differences between
consecutive samples are usually quite small, so this saves space and reduces
the bandwidth necessary.
However, DPCM produces a high quality signal (high signal-to-noise ratio or
SNR) for high difference signals (where the actual signal is very different from
what was predicted) but a poor quality signal (low SNR) for low difference
signals (where the actual signal is very similar to what was predicted). This is
because the level of quantization noise is the same at all signal levels. Adaptive
DPCM solves this problem by adapting the difference signal’s level of
quantization according to the audio signal’s strength. A low difference signal is
given a higher quantization level, increasing its signal-to-noise ratio. This
provides a similar sound quality at all signal levels. G.723 provides high quality
sound and requires 20 or 40 kbps.
• G.729 is an Analysis-by-Synthesis (AbS) hybrid waveform codec. It uses a filter
based on information about how the human vocal tract produces sounds. The
codec analyzes the incoming voice signal and attempts to synthesize it using its
list of voice elements. It tests the synthesized signal against the original and, if
it is acceptable, transmits details of the voice elements it used to make the
synthesis. Because the codec at the receiving end has the same list, it can
exactly recreate the synthesized audio signal.G.729 provides good sound quality
and reduces the required bandwidth to 8kbps.
12.2.1.4 MWI (Message Waiting Indication)
Enable Message Waiting Indication (MWI) enables your phone to give you a
message–waiting (beeping) dial tone when you have one or more voice messages.
Your VoIP service provider must have a messaging system that sends message-
waiting-status SIP packets as defined in RFC 3842.