ZyXEL Communications MAX-306M1 User Manual

Chapter 12 The Service Configuration Screens

User’s Guide

• 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.

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.