Zyxel IES-1000 EE [109/368] Pstn call setup signaling

Chapter 17 VoIP

VOP1224-61 User’s Guide

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• G.726 is an Adaptive Differential Pulse Code Modulation (ADPCM) waveform

codec that uses a lower bitrate than standard PCM conversion.

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 difference level. 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.726 operates at 16, 24, 32 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.

17.1.7 PSTN Call Setup Signaling

PSTNs (Public Switched Telephone Networks) use DTMF or pulse dialing to set up

telephone calls.

Dual-Tone Multi-Frequency (DTMF) signaling uses pairs of frequencies (one lower

frequency and one higher frequency) to set up calls. It is also known as Touch

Tone®. Each of the keys on a DTMF telephone corresponds to a different pair of

frequencies.

Pulse dialing sends a series of clicks to the local phone office in order to dial

numbers.

17.1.7.1 VoIP VLAN

Virtual Local Area Network (VLAN) allows a physical network to be partitioned into

multiple logical networks. Only stations within the same VLAN can communicate

with each other.

2. The VOP supports DTMF at the time of writing.