ZyXEL Communications V300 - Manual (Page 108)

Chapter 10 SIP Account Setup
V300 User’s Guide
108
Figure 120 SIP Redirect Server
10.1.5.4 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.
10.1.6 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.
10.1.7 NAT and SIP
NAT (Network Address Translation - NAT, RFC 1631) is the translation of the IP address of a
host in a packet, for example, the source address of an outgoing packet, used within one
network to a different IP address known within another network.
The V300 must register its public IP address with a SIP register server. If there is a NAT router
between the V300 and the SIP register server, the V300 probably has a private IP address. The
V300 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 V300’s IP
address from inside the SIP message and maps it to your SIP identity. If the V300 has a private
IP address listed in the SIP message, the SIP server cannot map it to your SIP identity.
Use STUN or outbound proxy to allow the V300 to list its public IP address in the SIP
messages.