ZyXEL Communications P-2608HWL-D1, User Manual

Page: 156 / 451

P-2608HWL-Dx Series User’s Guide
156 Chapter 11 SIP
Figure 87 STUN
11.1.7.4 Outbound Proxy
Your VoIP service provider may host a SIP outbound proxy server to handle all of the ZyXEL
Device’s VoIP traffic. This allows the ZyXEL Device to work with any type of NAT router
and eliminates the need for STUN or a SIP ALG. Turn off a SIP ALG on a NAT router in front
of the ZyXEL Device to keep it from retranslating the IP address (since this is already handled
by the outbound proxy server).
11.1.8 Voice Coding
A codec (coder/decoder) codes analog voice signals into digital signals and decodes the digital
signals back into voice signals. The ZyXEL Device supports the following codecs.
• G.711 is a Pulse Code Modulation (PCM) waveform codec. PCM measures analog signal
amplitudes at regular time intervals and converts them into bits. G.711 provides very
good sound quality but requires 64kbps of bandwidth.
• 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.
11.1.9 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.
1
1. The ZyXEL Device supports DTMF at the time of writing.