Figure 137 Stun - ZyXEL Communications V501-T1 User Manual

The V500 must register its public IP address with a SIP register server. If there is a NAT router
between the V500 and the SIP register server, the V500 probably has a private IP address. The
V500 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 V500's IP
address from inside the SIP message and maps it to your SIP identity. If the V500 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 V500 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 V500 to find the presence and types of NAT routers and/or firewalls
between it and the public Internet. STUN also allows the V500 to find the public IP address
that NAT assigned, so the V500 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 V500 (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 V500's SIP packets and sends them to the V500.
3 The V500 uses the public IP address and port number in the SIP packets that it sends to
the SIP server (C).

Figure 137 STUN

Outbound Proxy
Your VoIP service provider may host a SIP outbound proxy server to handle all of the V500's
VoIP traffic. This allows the V500 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 V500 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 V500 supports the following codecs.
V500 Series User's Guide
Chapter 11 SIP Account Setup
189