ZyXEL Communications 2002 Series - Manual (Page 46)

Prestige 2002 Series User’s Guide
46 Chapter 5 Introduction to VoIP
5.4 SIP ALG
Some NAT routers may include 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 the Prestige registers with the SIP register server, the SIP ALG translates
the Prestige’s private IP address inside the SIP data stream to a public IP address. You do not
need to use STUN if your Prestige is behind a SIP ALG.
5.5 Pulse Code Modulation
Pulse Code Modulation (PCM) measures analog signal amplitudes at regular time intervals
and converts them into bits.
5.6 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.
5.6.1 G.711
G.711 is a Pulse Code Modulation (PCM) waveform codec. G.711 provides very good sound
quality but requires 64kbps of bandwidth.
5.6.2 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.
5.7 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.