Connect to dialup modems over VoIP using SIP, no modem hardware required.
https://www.aon.com/cyber-solutions/aon_cyber_labs/introducing-d-modem-a-software-sip-modem/
- Increased data rates up to full 56k (tested with Cisco 2951 with PVDM2 digital modems and clock synced to GPS using icE1usb at the other end, direct SIP between D-Modem and Cisco)
- Highly improved connection stability (connections lasting days instead of minutes)
- Audio output of modem tones using the PJSIP audio output
- Anonymous calls without credentials (broken)
- RTP- and SIP-ports randomized, allowing multiple instances on the same system
ATX3(no dialtone required) is the default- Running as non-root directly supported (and encouraged!)
- Various bug fixes
- d-modem can now be called and signals the pty terminal to allow answering
You'll need Linux and a working 32-bit development environment (gcc -m32 needs to work, Debian-based systems can install: git gcc build-essential libc6-dev-i386 gcc-multilib pkg-config), along with PJSIP's dependencies (OpenSSL). Then run 'make' from the top-level directory.
Traditional “controller-based” modems generally used a microcontroller and a DSP to handle all aspects of modem communication on the device itself. Later, so-called “Winmodems” were introduced that allowed for field-programmable DSPs and moved the controller and other functionality into software running on the host PC. This was followed by “pure software” modems that moved DSP functionality to the host as well. The physical hardware of these softmodems was only used to connect to the phone network, and all processing was done in software.
D-Modem replaces a softmodem’s physical hardware with a SIP stack. Instead of passing audio to and from the software DSP over an analog phone line, audio travels via the RTP (or SRTP) media streams of a SIP VoIP call.
The repository contains two applications:
slmodemd – A stripped down and patched version of Debian’s sl-modem-daemon package. All kernel driver code has been replaced with socket-based communication, allowing external applications to manage audio streams.
d-modem – External application that interfaces with slmodemd to manage SIP calls and their associated audio streams.
After they have been built, you can configure SIP account information in the SIP_LOGIN environment variable for calls over a SIP proxy:
# export SIP_LOGIN=username:password@sip.example.com
Next, run slmodemd, passing the path to d-modem in the -e option. Use -d for debug logging.
# ./slmodemd/slmodemd -d9 -e ./d-modem
SmartLink Soft Modem: version 2.9.11 Oct 28 2021 16:51:30
symbolic link `/dev/ttySL0' -> `/dev/pts/3' created.
modem `slamr0' created. TTY is `/dev/pts/3'
Use `/dev/ttySL0' as modem device, Ctrl+C for termination.
In another terminal, connect to the newly created serial device at 115200 bps:
# screen /dev/ttySL0 115200
You can now interact with this terminal (almost) as you would with a normal modem using standard AT commands. A similar modem’s manual provides a more complete list.
To successfully connect, you might need to manually select a modulation and data rate. In our testing, V.32bis (14.4kbps) and below might be more reliable, though V.34 (up to 33.6kbps) and V.90 (up to 56k) connections are usually successful. For example, the following command selects V.32bis with a data rate of 4800 – 9600 bps. Refer to the manual for further details.
at+ms=132,0,4800,9600
OK
Finally, dial the number of the target system. Below shows a connection to the NIST atomic clock:
atd303-494-4774
CONNECT 9600
National Institute of Standards and Technology
Telephone Time Service, Generator 1b
Enter the question mark character for HELP
D L
MJD YR MO DA HH MM SS ST S UT1 msADV <OTM>
59515 21-10-28 21:40:18 11 0 -.1 045.0 UTC(NIST) *
59515 21-10-28 21:40:19 11 0 -.1 045.0 UTC(NIST) *
59515 21-10-28 21:40:20 11 0 -.1 045.0 UTC(NIST) *
59515 21-10-28 21:40:21 11 0 -.1 045.0 UTC(NIST) *
59515 21-10-28 21:40:22 11 0 -.1 045.0 UTC(NIST) *
59515 21-10-28 21:40:23 11 0 -.1 045.0 UTC(NIST) *
If you want to initiate a direct call to a SIP endpoint without credentials, use ATDTendpoint@sip.domain:
ATDT1234@192.168.0.1
CONNECT 37333
Login:
For local two-instance testing without Asterisk/SIP registration, set a unique DMODEM_SELFTEST_ID for each slmodemd process. This mode lets one instance dial the other directly by ID.
Example for debugging mixed V.8 implementations (open stub vs proprietary):
# terminal 1 (caller side)
DMODEM_SELFTEST_ID=1001 SLMODEMD_V8_OPEN_STUB=1 \
./slmodemd/slmodemd -d9 -e ./d-modem /dev/slamr0
# terminal 2 (answer side)
DMODEM_SELFTEST_ID=1002 SLMODEMD_V8_OPEN_STUB=0 \
./slmodemd/slmodemd -d9 -e ./d-modem /dev/slamr1
Then open both PTYs (for example with minicom) and dial from the caller to the answerer:
ATD1002
On the other side, wait for RING and answer:
ATA
Optional: set DMODEM_SELFTEST_DIR to choose where local UNIX sockets are created (default /tmp).
For network/SIP testing:
You should be running Asterisk or some other sip service.
Run slmodemd from 2 terminals and specifying different modem devices. Export sip accounts per slmodem:
# ./slmodemd/slmodemd -d2 -e ./d-modem /dev/slamr0
# ./slmodemd/slmodemd -d2 -e ./d-modem /dev/slamr1
In 2 other terminals, connect to the newly created serial devices:
# minicom -D /dev/ttySL0
# minicom -D /dev/ttySL1
To successfully connect, you might need to manually select a modulation and data rate:
at+ms=132,1,,14400
OK
On one of the terminals, dial the number of the second system.
atd5123
On the other terminal, it should indicate RING. Use the AT Answer command to answer.
ata
Now modems are connected and can interact with each other:
CONNECT 33600
To stop data transmission, first escape from on-line mode (+++), then hang up:
+++
ath
Commands to pass modem terminal through to a VM (eg Windows) from an LXC container (running SLModem). VM will need serial device.
;on proxmox/qemu host
# socat -d -d TCP-L:[TCP PORT] UNIX-CLIENT:/var/run/qemu-server/[guest id].serial0
;on slmodem container or machine
# socat -d -d TCP:[IP OF QEMU]:[TCP PORT] /dev/ttySL0[1,2...],b[BAUD RATE such as 115200],raw,echo=0
In the VM, open a terminal program and dial out to another modem using the serial port.
- Additional logging/error handling is needed
- The serial interface could be replaced with stdio or a socket, and common AT configuration options could be exposed as command line options
- d-modem can now recieve calls but it is buggy
- call handling needs work. might not make another call after it has made one.
Copyright 2021 Aon plc