User's Manual Part 2

Chapter 10 The VoIP Account Screens

BM2022w User’s Guide

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STUN

STUN (Simple Traversal of User Datagram Protocol (UDP) through Network Address Translators)

allows the BM2022w to find the presence and types of NAT routers and/or firewalls between it and

the public Internet. STUN also allows the BM2022w to find the public IP address that NAT assigned,

so the BM2022w 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 BM2022w (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

BM2022w’s SIP packets and sends them to the BM2022w.

3 The BM2022w uses the public IP address and port number in the SIP packets that it sends to the

SIP server (C).

Figure 93 STUN

Outbound Proxy

Your VoIP service provider may host a SIP outbound proxy server to handle all of the BM2022w’s

VoIP traffic. This allows the BM2022w 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 BM2022w to keep it from

retranslating the IP address (since this is already handled by the outbound proxy server).

NAT and SIP

The BM2022w must register its public IP address with a SIP register server. If there is a NAT router

between the BM2022w and the SIP register server, the BM2022w probably has a private IP address.

The BM2022w 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 BM2022w’s

IP address from inside the SIP message and maps it to your SIP identity. If the BM2022w has a

private IP address listed in the SIP message, the SIP server cannot map it to your SIP identity.

Use a SIP ALG (Application Layer Gateway), STUN, or outbound proxy to allow the BM2022w to list

its public IP address in the SIP messages.

DTMF

Dual-Tone Multi-Frequency (DTMF) telephone call signaling uses pairs of frequencies (one lower

frequency and one higher frequency) to set up calls. It is also known as Touch Tone. Each of the

keys on a DTMF telephone corresponds to a different pair of frequencies.

NAT