ZyXEL Communications VMG8825-B Series User Manual page 247

In the following example, you want to use client device A to call someone who is using client device C.
Client device A sends a call invitation for C to the SIP redirect server (B).
1
The SIP redirect server sends the invitation back to A with C's IP address (or domain name).
2
Client device A then sends the call invitation to client device C.
3
Figure 150 SIP Redirect Server
SIP Register Server
A SIP register server maintains a database of SIP identity-to-IP address (or domain name) mapping. The
register server checks your user name and password when you register.
RTP
When you make a VoIP call using SIP, the RTP (Real time Transport Protocol) is used to handle voice data
transfer. See RFC 1889 for details on RTP.
Pulse Code Modulation
Pulse Code Modulation (PCM) measures analog signal amplitudes at regular time intervals and converts
them into bits.
SIP Call Progression
The following figure displays the basic steps in the setup and tear down of a SIP call. A calls B.
Table 112 SIP Call Progression
A
1. INVITE
Chapter 23 Voice
B
2. Ringing
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