Faxes are an rather old method for communication, but in Germany they have one advantage: as opposed to emails, faxes are legally binding in the same way as letters are. So, from time to time, sending out a fax is useful. In addition, I have a telephone flatrate included in my data plan, so I don’t have to pay the postage for letters any more. The plan: use the USB port on my router to attach a modem and send out faxes using the mgetty package.

For 3 Euro I acquired an old ELSA Microlink 56k USB modem. I did some research, this modem is one of the few ones supported by the Linux usb-cdc kernel drivers. In addition it can be powered from USB only, so I don’t need another power supply. A week after winning an auction on ebay it arrived today.

My router runs OpenWRT. I installed the following packages using the opkg system:

kmod-usb2
kmod-usb-uhci
kmod-usb-net
kmod-usb-core
kmod-usb-acm
mgetty

mgetty contains a program called “sendfax”, which can be used to dial a number and send the data. The latter needs to be in G3 format. Since I write all my letters using LaTeX, I integrated a “fax” target into my Makefile for LaTeX documents. This target reads the PDF output of LaTeX and converts it to G3 using Ghostscript. The essential command is:

gs -dSAFER -dBATCH -dNOPAUSE -sDEVICE=dfaxhigh -sOutputFile=fax.g3.%d -q main.pdf

This creates a bunch of fax.g3.* files, one for each page within the PDF file. Then, I simply copy these files to my router and issue the sendfax command, for example:

sendfax #fax number# fax.g3.1 fax.g3.2

The command dials the fax number and sends the specified g3 files:

root@nibbler:~# sendfax 032121xyzxyz fax.g3.1 fax.g3.2
Trying fax device '/dev/ttyACM0'... OK.
Dialing 032121xyzxyz... OK.
sending 'fax.g3.1'...
sending 'fax.g3.2'...

If an error occurs, the command will fail and tell you about it. The mgetty package also contains various scripts for building a fax spooler, but for my personal usage, I will stick to raw sendfax. Please note that it is also possible to receive faxes, but I did not install these components so far.

No more postage for me. Yay!

Picture: CC-BY-SA-NC vtengr4047, thanks!