User guide

Image 11: The Telecentre and the Training Centre are equipped with one PBX each. The PBX's are

Interconnected via a VSAT satellite link.

9.1 Typical satellite connection issues

The Internet connection at the Telecentre has a very limited bandwidth (128/64 Kbps) so our solution

needs to optimize the bandwidth usage. In the next example we propose to link two PBX's using

Asterisk and IAX2. To ensure good bandwidth usage we propose to use a high compression codec

such as G.729 or Speex. Finally, we will enable call trunking to ensure that simultaneous phone calls

can be bundled together to reduce the headers' overhead.

A typical scenario in a VSAT Internet connection is that the satellite border router is connected to the

Internet with very few public IP addresses. If we do not have any spare public IP address for our

Asterisk box we need to ensure that the IAX2 communication port 4569 is correctly mapped to the

internal PBX. The mechanism to create a port mapping depends on the type of product installed as the

border router.

• If you have a Cisco router running NAT you will need to issue something like:

#ip nat inside source static udp 192.168.46.1 4569 interface fastEthernet 0/0 4569

• If your border router is running Linux, you need to forward all connections to port 4569 to the

internal machine (192.168.46.1) using iptables

#/sbin/iptables -t nat -A PREROUTING -p udp --dport 4569 -i eth0 -j DNAT --to-destination

192.168.46.1:4569

The important thing to remember is that the PBX's need to be reachable from the public Internet by

UDP port 4569.

9.2 Interconnecting of two Asterisk servers

9.2.1 Telecentre

The iax.conf configuration file in the Telecentre looks like the following:

[general]

bindaddr = 0.0.0.0

41. Port 4569 is the default communication port for IAX2.

Page 41 TRICALCAR | www.wilac.net/tricalcar – Version: February 2008