User's Manual

Chapter 21 Voice 185
3 C
lient device A then sends the call invitation to client device C.
Figure 115 SI
P Redirect Server
SIP Register Server
A SIP register server maintains a database of SIP identity-t
o-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 T
ransport Protocol) is used to handle voice
data transfer. See RFC 1889 for details on RTP.
Pulse Code Modulation
Pulse Code Modulation (PCM) measur
es 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 104 SIP Call
Progression
A B
1. INVITE
2. Ringing