+The program 'datenklo' is an emulator for the original "CCC modem", a building instruction released by the Chaos Computer Club in 1985.
+The heart of this software is the AM7911 modem chip emulation.
+It modulates digital levels to FSK tones and demodulates FSK tones to digital levels.
+A software UART (serial transceiver) converts characters (bytes) into levels and leves into bytes.
+The modem chip emulation is linked to sound card via ALSA driver.
+The UART is linked to a tty emulation.
+The tty emulation uses FUSE, so that the linux kernel provides a virtual serial interface. (/dev/ttyDATENKLO0 by default)
+
+Eurosignal (a.k.a Europäischer Funkrufdienst EFuRD) was a paging service in Europe.
+It was introduced in 1974 in Germany, 1975 in France and 1985 in Switzerland.
+The system was shut down in France and Switzerland in 31. December 1997, in Germany on 1. April 1998.
+It did not support any message forwarding.
+An incoming call was indicated as a beep (and/or vibration) by the pager.
+But up to 4 paging IDs could be assigned to one single device.
+The device showed which ID was paged.
+
+
+
+Paging a subscriber was done via telephone dial up.
+The prefix was assigned to large areas having multiple transmitters with the same frequency/channel.
+(In Germany were three areas: North, Middle, South)
+The suffix (the pager ID) was then encoded into tones and transmitted with up to 2 KW of power.
+The pager device decoded the tones and checked if they match with the assigned ID.
+Inside the pager were jumpers and later a chip with diodes to match the received tone sequence with the assigned ID.
+Up to four numbers were assigned to one pager.
+
+
+
+One may remember the sound at the beginning of the FM broadcast band:
+
+
+
Listen to the base station's paging sequences:
+
+
+
+Check out oebl.de for much more detailed description in German.
+
+
+
+
+How it works
+
+
+
+4 frequencies were used by the system:
+
+
+
+
Channel A: 87,340 MHz
+
Channel B: 87,365 MHz
+
Channel C: 87,390 MHz
+
Channel D: 87,415 MHz
+
+
+
+The caller calls a prefix followed by a 6 digits number.
+The system puts the number into a queue, so that simultaneous calls can be handled and paging is performed in series.
+
+
+
+A train of paging messages are transmitted without interruption.
+There are two types of messages: A paging message and an idle message.
+Each message was about 8.2 seconds long, divided into a sequence of tones.
+
+
+
+This is how a paging message looks like:
+
+
+
++---------------+-------+-------+-------+-------+-------+-------+
+| Idle | Digit | Digit | Digit | Digit | Digit | Digit |
+| Tone | 1 | 2 | 3 | 4 | 5 | 6 |
++---------------+-------+-------+-------+-------+-------+-------+
+ 220 ms 100 ms 100 ms 100 ms 100 ms 100 ms 100 ms
+
+
+
+The digit clock is retrieved from the change of tones.
+If two subsequent digits are the same, the second digit was replaced by a 'Repeat' digit, so that there is always a change of tone between digits.
+The Idle tone resets the receiver register and indicates a new message.
+A pager ID '122233' would be transmitted as 1-2-R-2-3-R.
+A message is repeated multiple times, depending on the load of the system. (2-5 times ???)
+If there is no message to be transmitted, the following idle message is sent.
+
+
+
+This is how an idle message looks like:
+
+
+
+
++---------------+-------+---------------------------------------+
+| Idle | Digit | Idle |
+| Tone | R | Tone |
++---------------+-------+---------------------------------------+
+ 220 ms 100 ms 500 ms
+
+
+
+
+Here is a list of all frequencies used.
+The distance between frequencies increases on higher frequencies, allowing simple RC filters to be used to detect the tones.
+
+The message is amplitude modulated.
+The modulation index is 92 %.
+
+
+
+Alternatively the signal can be transmitted frequency modulated, to reduce interference with TV stations.
+The frequency is shifted by 7.5 KHz above or below the actual channel frequency.
+The frequency deviation causes a change of amplitude in the demodulator of the pager, so it works as well.
+
+
+
+
+Setup of a base station
+
+
+
+Find out what channel your pager can receive and select the same channel on the transmitting side.
+Assume that your pager is set to channel B, select channel B:
+
+
+
+
+# src/eurosignal/eurosignal -k B
+
+...
+eurosignal.c:290 notice : Created 'Kanal' B
+Base station for channel B ready, please tune transmitter and/or receiver to 87.3650 MHz
+...
+
+
+
+
+If you don't have a FM transmitter instead of an AM transmitter, tune it 7.5 KHz below or above the indicated channel.
+I suggest to use SDR, so check out the SDR page.
+Then enter the ID of your pager. You will find it printed on the device and/or on the 18 pin chip inside.
+If your pager's ID is 123456, enter it and press 'd' to start paging:
+
+
+
+
+on-hook: 123456 (press d=dial)
+mncc_console.c:358 info : Outgoing call to '123456'
+call.c:699 info : Received MNCC call from fixed network '' to mobile '123456'
+call.c:706 info : Indicate MNCC call confirm towards fixed network
+call.c:717 info : Outgoing call from '' to '123456'
+eurosignal.c:318 info : Creating call instance to page ID '123456'.
+call.c:478 info : Call has been answered by '123456'
+call.c:466 info : Indicate MNCC answer towards fixed network
+mncc_console.c:206 info : Call connected to '123456'
+eurosignal.c:605 info : Station acknowledges, playing announcement.
+eurosignal.c:387 notice : Transmitting ID '123456'.
+eurosignal.c:387 notice : Transmitting ID '123456'.
+eurosignal.c:387 notice : Transmitting ID '123456'.
+eurosignal.c:387 notice : Transmitting ID '123456'.
+mncc_console.c:388 info : Call hangup
+call.c:774 info : Received MNCC release from fixed network with cause 16
+call.c:778 info : Call released toward mobile network
+eurosignal.c:760 info : Call has been disconnected by network.
+
+
+
+
+In order to receive a paging message, use -R (--rx).
+Then the software will decode the received IDs and display them.
+
