How VoIP Operates

This is how the VoIP feature operates:

1. When a user dials a number, the number is looked up in the NAC/NAS table.



• If you have configured only a DNA for the number in the NAC/NAS table, the call is routed over the frame relay network using PANL. There is one exception. If the DNA begins with 000000, the call is selected for VoIP (the 000000 indicates to the Passport that this is a false DNA). Using the 000000 prefix in the DNA allows NAC/NAS to dynamically route calls over VoIP.
• If you have configured both a DNA and an IP address in NAC/NAS, then the preferences for call routing is checked.
2. The call routing preferences that you have selected determines how the call is routed.



• If you have defined a user preference for the dialed number, either for VoIP or for VoFR, that preference is used to route the call.
• If you have not defined a user preference for the dialed number, then the general preference (VoIP or VoFR) is used to route the call.
3. Once VoIP is selected as the call routing method, the originating Passport establishes a TCP connection on port 1490 with the remote Passport. This connection is used to set up the call.



4. When the call is established and the remote end goes off-hook (the call is answered), voice packets are transmitted between the two Passport units using UDP port 490.



5. When the call ends (back on-hook), the call management connection on TCP port 1490 is used to take down the call.

Note: If a call is selected for routing as VoIP, it remains VoIP across the network. The IP packets are encapsulated if they are traversing a frame relay segment (PANL). There is no gateway function (VoIP packets cannot be converted to VoFR).