Switching to VoIP

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Fax and Modems

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the sound encoding somewhere along the fax machine’s call path, possibly crippling

it.

There are three ways to ensure faxes can traverse your VoIP network:

• Guarantee that G.711, and only G.711, will be used for all possible fax call

paths, and eliminate all jitter.

• Implement T.38 servers at locations with fax machines.

• Get a T.37-compatible fax machine or connect fax machines to T.37 servers.

There’s only one way to positively ensure modem traffic can traverse your VoIP net-

work:

• Guarantee that G.711, and only G.711, will be used for all possible modem call

paths, and eliminate all jitter.

To make fax and modem communications over VoIP more reliable,

decrease the baud rate of the fax or modem connection to 9600 or

lower. While this will slow the connection down, it will allow you to

overcome minor jitters, which will at least keep the connection intact.

Project 14.1. Turn Your Linux Box into a Fax Machine

What you need for this project:

• Asterisk running on Linux

• A POTS line connected to a Zaptel-compatible interface card

Asterisk offers a built-in fax-detection mechanism. This allows you to handle faxes

that are sent to your Asterisk PBX on a POTS line. Asterisk’s

Answer( ) command

triggers the fax detection. If an incoming fax is detected, Asterisk automatically ...