ZyXEL Communications ADSL VoIP IAD with 802.11g Wireless 2602HW Series - Manual (Page 135)

Prestige 2602HW Series User’s Guide
Chapter 9 Introduction to VoIP 135
Figure 49 SIP Redirect Server
9.2.3.4 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.
9.2.4 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.
9.3 SIP ALG
The Prestige 2602HW is a SIP Application Layer Gateway (ALG). A SIP ALG allows VoIP
calls to pass through NAT by examining and translating IP addresses embedded in the data
stream. When a VoIP device behind the Prestige registers with the SIP register server, the
Prestige translates the device’s private IP address inside the SIP data stream to a public IP
address. You do not need to use STUN with a VoIP device that is behind the Prestige
2602HW.
9.4 Pulse Code Modulation
Pulse Code Modulation (PCM) measures analog signal amplitudes at regular time intervals
and converts them into bits.