• You need to subscribe with an ITSP for telephone connection. The ISP and ITSP may be part of the samecompany.Note• VoIP communication using the V-SIPGW16 card may deteriorate depending on the ITSP being used.• VoIP 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 PC. As domain names are alphabetic,they are easier to remember. The Internet, however, is based on IP addresses. Therefore, every time a domainname is used, a DNS server must translate the name into the corresponding IP address, and vice versa. Forexample, the domain name www.example.com may be translated to 192.0.34.166. If one DNS server doesnot know how to translate a particular domain name, it asks another one, and so on, until the correct IP addressis returned.NAT TraversalWhen NAT/NAPT (Network Address Port Translation) is enabled, the router translates a local IP address fromthe PBX into a global IP address. However, the router with NAT enabled does not translate local IP addressesstored in SIP messages into global IP addresses.Therefore, the address which the SIP Server recognizes as the destination IP address to reply to is actuallythe local IP address of the PBX, not the global IP address of the router. Therefore, if the SIP Server receivesa SIP message from the PBX and sends a message back to the PBX using the address stored in the SIPmessage, the packet information will not reach the PBX.STUN Servers function to solve the global IP address problem under certain NAT conditions, for example, incase of full duplex communication. A STUN Server, used alongside the SIP Server, finds out the global IPaddress of the router with NAT enabled. With the STUN feature enabled, the packet information sent by theSIP 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. Inaddition, the NAT Traversal method can be selected from "STUN" and "Fixed IP Address". For details, referto "9.9 PBX Configuration—[1-1] Configuration—Slot—V-SIPGW—Shelf Property" in the PC ProgrammingManual.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 theSIP Server (SIP Receiver) via a router with NAT enabled.Document Version 2016-03 Installation Manual 3278.3.1 IP Telephony Service
