Panasonic PABX TDA Series VoIP Operation Across NAT

Basics of NAT
NAT, Network Address Translation, is a network service that is designed to preserve the limited IP addresses in the IP-v4 scheme. It hides internal Network IP addresses of a network and exposes only one (or a few) external IP address(es) to the outside world.

Small Private IP Network comprising of KX-TDA200 PBX with TDA0480 IP-GW card and two PCs – connected to the outside Public IP Network (Example: Internet) using a NAT device. Please note that in this scenario – the only “valid” Public IP address is that of the outside interface on the NAT device 204.35.123.55. This is the only address that the outside devices “see” no matter which internal system – TDA, or PCs generates IP traffic that goes to the outside IP Cloud. Hence using NAT a company only needs a very limited set of publicly addressable IP addresses.


H.323 VoIP Packets
Difference b/w Normal Data Packets Vs H.323 Packets over IP
The main difference between a normal data packet and an H.323 VoIP packet is that a normal data packet carries its own IP address in its IP header and its source and destination port values in the TCP header. Whereas in an H.323 VoIP packet – the source IP address and source IP port is also carried in the H.323 payload body of the H.323 packet. This H.323 data payload is called Protocol Data Unit (PDU).

NAT Setup and VoIP Configurations
To understand what is involved in TDA VoIP set-up and configurations we take an example scenario consisting of two TDA sites – each connected over the IP data network via H.323 Aware NAT devices

Requirements:
NAT (a) and NAT (b) must both be H.323 Aware (i.e. NAT devices must have ability to look inside the H.323 Message Structure called Protocol Data Unit (PDU) and change Source IP address and Source Port values. Further, it should be able to properly route UDP audio packets (RTP) to internal H.323 gateways).

VoIP Configuration:
When Configuring IP-GW using MCS protocol – please make sure:

Programming at Site A• Configure IP-GW (A) at site A normally. See TDA0480 Programming Guide for details
• Configure IP-GW (B) using external IP address of NAT (b) (as if IP-GW (B) is at NAT (b)’s external IP address). This is because when Party (A) is calling Party (B) - if H.323 Call Setup message is sent from 20.0.0.3 to 192.168.1.230 – it would fail, as 192.168.1.230 is a private destination IP address. So the destination IP address has to be NAT (b)’s public IP address.
• Upload and transfer configuration and DNS files only to IP-GW (A)

Programming at Site B• Configure IP-GW (B) at site B normally. See TDA0480 Programming Guide for details
• Configure IP-GW (A) using external IP address of NAT (a) (as if IP-GW (A) is at NAT (a)’s external IP address). This is because when Party (B) is calling Party (A) - if H.323 Call Set-up message is sent from 192.168.1.230 to 20.0.0.3 – it would fail, as 20.0.0.3 is a private destination IP address. So the destination IP address has to be NAT (a)’s public IP address.
• Upload and transfer configuration and DNS files only to IP-GW (B)

NAT Configurations
When Configuring NAT (a) and NAT (b) – please make sure to:
• Configure NAT (a) to accept any H.323 packets it receives at 80.33.155.41 and then forward it internally to 20.0.0.3.
• Configure NAT (b) to accept any H.323 packets it receives at 81.35.250.170 and then forward it internally to 192.168.1.230.