Chapter 10 The VoIP Account Screens

SIP User Agent

A SIP user agent can make and receive VoIP telephone calls. This means that SIP can be used for peer-to-peer communications even though it is a client-server protocol. In the following figure, either A or B can act as a SIP user agent client to initiate a call. A and B can also both act as a SIP user agent to receive the call.

Figure 89 SIP User Agent

AB

SIP Proxy Server

A SIP proxy server receives requests from clients and forwards them to another server.

In the following example, you want to use client device A to call someone who is using client device C.

1The client device (A in the figure) sends a call invitation to the SIP proxy server (B).

2The SIP proxy server forwards the call invitation to C. Figure 90 SIP Proxy Server

1

B

2

AC

STUN

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

1The WiMAX Device (A) sends SIP packets to the STUN server (B).

2The STUN server (B) finds the public IP address and port number that the NAT router used on the WiMAX Device’s SIP packets and sends them to the WiMAX Device.

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WiMAX Device Configuration User’s Guide