ZyXEL V630 User Guide - Page 131
Voice Coding
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Chapter 14 SIP Account Setup 14.1.6.1 STUN STUN (Simple Traversal of User Datagram Protocol (UDP) through Network Address Translators) allows the V630 to find the presence and types of NAT routers and/or firewalls between it and the public Internet. STUN also allows the V630 to find the public IP address that NAT assigned, so the V630 can embed it in the SIP data stream. STUN does not work with symmetric NAT routers or firewalls. See RFC 3489 for details on STUN. The following figure shows how STUN works. 1 The V630 (A) sends SIP packets to the STUN server (B). 2 The STUN server (B) finds the public IP address and port number that the NAT router used on the V630's SIP packets and sends them to the V630. 3 The V630 uses the public IP address and port number in the SIP packets that it sends to the SIP server (C). Figure 131 STUN 14.1.6.2 Outbound Proxy Your VoIP service provider may host a SIP outbound proxy server to handle all of the V630's VoIP traffic. This allows the V630 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 V630 to keep it from retranslating the IP address (since this is already handled by the outbound proxy server). 14.1.7 Voice Coding A codec (coder/decoder) codes analog voice signals into digital signals and decodes the digital signals back into voice signals. The V630 supports the following codecs. • 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.726 is an ADPCM waveform codec that uses a lower bit rate than standard PCM conversion. G.726 operates at 16, 24, 32 or 40 kbps. V630 User's Guide 131