Figure 87 Stun - ZyXEL Communications V300 Series User Manual

Chapter 10 SIP Account Setup
Use STUN or outbound proxy to allow the V300 to list its public IP address in the
SIP messages.
STUN
STUN (Simple Traversal of User Datagram Protocol (UDP) through Network
Address Translators) allows the V300 to find the presence and types of NAT
routers and/or firewalls between it and the public Internet. STUN also allows the
V300 to find the public IP address that NAT assigned, so the V300 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.
The V300 (A) sends SIP packets to the STUN server (B).
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The STUN server (B) finds the public IP address and port number that the NAT
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router used on the V300's SIP packets and sends them to the V300.
The V300 uses the public IP address and port number in the SIP packets that it
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sends to the SIP server (C).

Figure 87 STUN

Outbound Proxy
Your VoIP service provider may host a SIP outbound proxy server to handle all of
the V300's VoIP traffic. This allows the V300 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 V300 to keep it from retranslating the IP address (since this is
already handled by the outbound proxy server).
Voice Coding
A codec (coder/decoder) codes analog voice signals into digital signals and
decodes the digital signals back into voice signals. The V300 supports the
following codecs.
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V300 Series User's Guide