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Chapter 9 SIP
P-2304R-P1 Series User’s Guide
111
9.1.7.2 Use NAT
If you know the NAT router’s public IP address and SIP port number, you can use the Use
NAT feature to manually configure the ZyXEL Device to use them in the SIP messages. This
eliminates the need for STUN or a SIP ALG.
You must also configure the NAT router to forward traffic with this port number to the ZyXEL Device.
9.1.7.3 STUN
STUN (Simple Traversal of User Datagram Protocol (UDP) through Network Address
Translators) allows the ZyXEL Device to find the presence and types of NAT routers and/or
firewalls between it and the public Internet. STUN also allows the ZyXEL Device to find the
public IP address that NAT assigned, so the ZyXEL Device 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 ZyXEL Device (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 ZyXEL Device’s SIP packets and sends them to the ZyXEL Device.
3 The ZyXEL Device uses the public IP address and port number in the SIP packets that it
sends to the SIP server (C).
Figure 57 STUN
9.1.7.4 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).
9.1.8 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. 7 11 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.