ZyXEL Communications 2002 User Manual
P-2002 Series User’s Guide
48
Chapter 5 Introduction to VoIP
Figure 16 SIP Redirect Server
5.2.3.4 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.
mapping. The register server checks your user name and password when you register.
5.2.4 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.
handle voice data transfer. See RFC 1889 for details on RTP.
5.3 NAT
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 is changed to a different IP address known within another network.
host in a packet. For example, the source address of an outgoing packet, used within one
network is changed to a different IP address known within another network.
In the simplest form, NAT changes the source IP address of a packet received from a device to
another IP address before forwarding the packet towards the destination. When the response
comes back, NAT translates the destination address back to the device’s IP address and
forwards it to the device.
another IP address before forwarding the packet towards the destination. When the response
comes back, NAT translates the destination address back to the device’s IP address and
forwards it to the device.
NAT routers are commonly used to translate private (or internal) IP addresses in packet
headers to public (or external) IP addresses and vice versa. A NAT router maps a private IP
address and port pair to a public IP address and port, and whenever the NAT router receives a
packet with that public IP address and port, it knows how to reroute the packet back to the
private IP address and port.
headers to public (or external) IP addresses and vice versa. A NAT router maps a private IP
address and port pair to a public IP address and port, and whenever the NAT router receives a
packet with that public IP address and port, it knows how to reroute the packet back to the
private IP address and port.