ZyXEL Communications Prestige 2602R Series, User Manual

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Prestige 2602R Series User’s Guide
Chapter 7 Introduction to VoIP 107
7.3 SIP ALG
The Prestige is a SIP Application Layer Gateway (ALG). A SIP ALG allows VoIP calls to
pass through NAT by examining and translating IP addresses embedded in the data stream.
When a VoIP device behind the Prestige registers with the SIP register server, the Prestige
translates the device’s private IP address inside the SIP data stream to a public IP address. You
do not need to use STUN with a VoIP device that is behind the Prestige.
7.4 Pulse Code Modulation
Pulse Code Modulation (PCM) measures analog signal amplitudes at regular time intervals
and converts them into bits.
7.5 Voice Coding
A codec (coder/decoder) codes analog voice signals into digital signals and decodes the digital
signals back into voice signals. The Prestige supports the following codecs.
7.5.1 G.711
G.711 is a Pulse Code Modulation (PCM) waveform codec. G.711 provides very good sound
quality but requires 64kbps of bandwidth.
7.5.2 G.729
G.729 is an Analysis-by-Synthesis (AbS) hybrid waveform codec that uses a filter based on
information about how the human vocal tract produces sounds. G.729 provides good sound
quality and reduces the required bandwidth to 8kbps.
7.6 PSTN Call Setup Signaling
Dual-Tone MultiFrequency (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.
1
1. The Prestige does not support pulse dialing at the time of writing.