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Last change: 2007-10-15

FreeISDN

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Mission Goals

Wonderful German Telekom recently "proved" that their telephony rates are among the lowest in the world.. Well, I can imagime some countries having higher rates, yes...

Three months ago I found a copy of the CCITT Q.931 standard. That was a very interesting piece of literature (written in Winword, but aesthetics was not the important aspect here), it is documenting (part of) the DSS1 protocol, also known as EURO-ISDN.

After reading it (and understanding the standardese ;), I wondered wether it would be possible to transfer some data, without making an connection, and thus, without paying for it. And yes, it's possible, and, of course, it's also illegal.

Mission Design Principles

I needed direct access to the D-Channel.. I also needed a decent developing system, decent tools... in one word: Linux. First I modified the hisax driver in isdn4linux to be able to send and receive DSS1 packets. The patch is very simple, small, unclean, dirty. But it works.

Then I wrote a usermode program for the protocol. After a little thought, I decided to implement my own tcp-like protocol. Although tcp already has quite advanced recovery/retry features (depending on the implementation.. Windoze95 doesn't exactly show off here), the low - very low bitrates of my medium (30-300 baud) demanded specialised retry/timeout features and a reliable protocol.

As a side effect, it's possible to run almost everything on the connection that can talk to a tty. pppd is only one example of a useful program for this purpose ;)

Mission Status

Simple messages can already be exchanged (i.e. udp-like protocols work). I also had a ppp connection over isdn (ping RTT 16.9 seconds ;), but after two minutes FreeISDN segfaulted, so I guess I have to do a little more work until it's finished ;)

Apart from that (and some minor problems like other isdn-devices rejecting the data packets (protocol conformance? where?)), it seems that the approach basically works...

Mission Resources

;-> Well, since there is not much point in using this software, I didn't develop it further (1999), but if you are interested in how I did it (the soruce really is very, very bad) you can download the sourcecode for version 0.1.0.

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