ZyXEL Communications MAX-306M1 User Manual

 Chapter 12 The Service Configuration Screens

User’s Guide

This section describes the features of the Advanced SIP settings screen.

STUN (Simple Traversal of User Datagram Protocol (UDP) through Network 
Address Translators) allows the WiMAX Modem to find the presence and types of 
NAT routers and/or firewalls between it and the public Internet. STUN also allows 
the WiMAX Modem to find the public IP address that NAT assigned, so the WiMAX 
Modem 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 WiMAX Modem (A) sends SIP packets to the STUN server (B).

The STUN server (B) finds the public IP address and port number that the NAT 
router used on the WiMAX Modem’s SIP packets and sends them to the WiMAX 
Modem.

The WiMAX Modem uses the public IP address and port number in the SIP packets 
that it sends to the SIP server (C).

Figure 57   STUN Example

Your VoIP service provider may host a SIP outbound proxy server to handle all of 
the WiMAX Modem’s VoIP traffic. This allows the WiMAX Modem 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 WiMAX Modem to keep it from re-translating 
the IP address (since this is already handled by the outbound proxy server).

A codec (coder/decoder) codes analog voice signals into digital signals and 
decodes the digital signals back into voice signals. The WiMAX Modem supports 
the following codecs.