Figure 86 Stun - ZyXEL Communications V300 Series User Manual

The V300 must register its public IP address with a SIP register server. If there is a NAT router
between the V300 and the SIP register server, the V300 probably has a private IP address. The
V300 lists its IP address in the SIP message that it sends to the SIP register server. NAT does
not translate this IP address in the SIP message. The SIP register server gets the V300's IP
address from inside the SIP message and maps it to your SIP identity. If the V300 has a private
IP address listed in the SIP message, the SIP server cannot map it to your SIP identity.
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.
1 The V300 (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 V300's SIP packets and sends them to the V300.
3 The V300 uses the public IP address and port number in the SIP packets that it sends to
the SIP server (C).

Figure 86 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.
V300 User's Guide
Chapter 10 SIP Account Setup
93