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P-2002 Series User’s Guide
54 Chapter 5 Introduction to VoIP
5.4.3 STUN
STUN (Simple Traversal of User Datagram Protocol (UDP) through Network Address
Translators) allows the Prestige to find the presence and types of NAT routers and/or firewalls
between it and the public Internet. STUN also allows the Prestige to find the public IP address
that NAT assigned, so the Prestige can embed it in the SIP data stream. STUN does not work
with symmetric NAT routers (see Section 5.3.2.4 on page 52) or firewalls. See RFC 3489 for
details on STUN.
The following figure shows how STUN works.
1 The Prestige (A) sends SIP packets to the STUN server.
2 The STUN server finds the public IP address and port number that the NAT router used
on the Prestige’s SIP packets and sends them to the Prestige.
3 The Prestige uses the public IP address and port number in the SIP packets that it sends to
the SIP server.
Figure 23 STUN
5.4.4 Outbound Proxy
Your VoIP service provider may host a SIP outbound proxy server to handle all of the
Prestige’s VoIP traffic. This allows the Prestige 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 Prestige to keep it from retranslating the IP address (since this is already handled by the
outbound proxy server).
5.5 Pulse Code Modulation
Pulse Code Modulation (PCM) measures analog signal amplitudes at regular time intervals
and converts them into bits.
5.6 Voice Coding
A codec (coder/decoder) codes analog voice signals into digital signals and decodes the digital
signals back into voice signals. The Prestige supports the following codecs.