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communication using the V-SIPGW16 card may deteriorate depending on the network conditions.
DNS (Domain Name System)
A DNS server normally provides the name resolution service for your computer. As domain names are
alphabetic, they are easier to remember. The Internet, however, is based on IP addresses. Therefore, every
time a domain name is used, a DNS server must translate the name into the corresponding IP address, and
vice versa. For example, the domain name www.example.com may be translated to 192.0.34.166. If one DNS
server does not know how to translate a particular domain name, it asks another one, and so on, until the
correct IP address is returned.
NAT (Network Address Translation) Traversal
When NAT/NAPT (Network Address Port Translation) is enabled, the router translates a local IP address from
the PBX into a global IP address. However, the router with NAT enabled does not translate local IP addresses
stored in SIP messages into global IP addresses.
Therefore, the address which the SIP Server recognises as the destination IP address to reply to is actually
the local IP address of the PBX, not the global IP address of the router. Therefore, if the SIP server receives
a SIP message from the PBX and sends a message back to the PBX using the address stored in the SIP
message, the packet information will not reach the PBX.
STUN Servers function to solve the global IP address problem under certain NAT conditions, for example, in
case of full duplex communication. A STUN Server, used alongside the SIP Server, finds out the global IP
address of the router with NAT enabled. With the STUN feature enabled, the packet information sent by the
SIP Server is able to "traverse" NAT and reach the PBX.
The settings can be configured to specify whether to enable the NAT Traversal feature for each ISP/ITSP. In
addition, the NAT Traversal method can be selected from "STUN" and "Fixed IP Address" (refer to
3.3.1 Main, NAT Traversal).
The V-SIPGW16 card may require the NAT Traversal feature to be enabled to connect to the WAN via a router.
The following diagram illustrates how VoIP communication is enabled between the V-SIPGW16 card and the
SIP Server (SIP Receiver) via a router with NAT enabled.
Programming Manual for Virtual SIP Trunk Card 5
1.1 Information about IP Telephony Service