ZyXEL Communications MAX-207HW2, User Manual

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Chapter 10 The Service Configuration Screens
User’s Guide
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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.
1
The WiMAX Modem (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 WiMAX Modem’s SIP packets and sends them to the WiMAX
Modem.
3
The WiMAX Modem uses the public IP address and port number in the SIP packets
that it sends to the SIP server (C).
Figure 45 STUN
10.2.1.2 Outbound Proxy
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).
10.2.1.3 Voice Coding
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.
• G.711 is a Pulse Code Modulation (PCM) waveform codec. PCM measures
analog signal amplitudes at regular time intervals (sampling) and converts them
into digital bits (quantization). Quantization “reads” the analog signal and then
“writes” it to the nearest digital value. For this reason, a digital sample is usually
slightly different from its analog original (this difference is known as
“quantization noise”). G.711 provides excellent sound quality but requires
64kbps of bandwidth.
A
B
C
NAT
1