29 Feb
2024
29 Feb
'24
3:12 a.m.
Hello fellow retrotechnologists,
I am looking for any existing software implementations of V.xxx
modulations, those used by analog modems over POTS. Specifically, I
am looking for software that can grok and generate streams of G.711
PCMU or PCMA samples, whether it be ISDN/T1/E1 or RTP, and thereby
digitally grok and generate voiceband modem waveforms that can
interoperate with a real V.xxx modem on the other end of the link. I
was previously given an impression that some free sw offerings already
exist that do what I just described, but I don't remember any specific
names, nor do I recall ever being given any direct links. Would
anyone here happen to know about such software?
In my corner of USA I still have a real POTS landline, served out of a
real 5ESS switch. That switch turns this POTS line into a 64 kbit/s
G.711 PCMU sample stream, and using SIP trunk providers like BulkVS, I
can access those PCMU sample streams (which were, once upon a time,
telco-internal, unless you had ISDN which was super-rare in USA) when
making and receiving calls on USA PSTN. I have a US Robotics Courier
"V.Everything" modem connected to this POTS line, and my idea is to
run some modem-emulating software on G.711 PCMU RTP streams on the SIP
end - and aim for working interoperation between the real POTS modem
on one end and a software emulation of modem waveforms in G.711 PCMU
on the other end.
In this lovely country of USA we also still have some public
(govt-operated) service numbers where an analog modem answers when you
call:
https://www.nist.gov/pml/time-and-frequency-division/time-distribution/auto…
I can call that NIST ACTS number from any voice phone and hear the
answering modem screeching at me. Back when GSM/2G service of T-Mobile
USA was in much better condition (as recently as 2018), I was able to
make CSD calls to that ACTS number from my FreeCalypso GSM MS devices,
and I used to get reliable connections, receiving the time code in
ASCII each time - thus T-Mobile must have had a working IWF gateway
that turned CSD into analog modem emulation, the exact same emulation
I am currently after. That sweet CSD IWF seems to be broken now, but
last night I tried a long-distance call from my POTS line (with my USR
Courier modem) to that ACTS number in Colorado, and on the second try
I got a successful connection at 9600 baud. Thus NIST ACTS itself
appears to still be up and functional, and if I can find some working
modem emulation software, I want to try connecting to ACTS with a
modem-emulating G.711 PCMU RTP stream.
Given that I operate my own Osmocom-based GSM network, my eventual goal
is to build my own interworking gateway from GSM CSD to analog modem
emulation, replicating the functionality that was wrongfully taken away
by T-Mobile. However, before I bring GSM CSD into the mix, I seek to
test the PSTN side of the equation by itself, just the leg between a
software modem emulator in G.711 RTP and a real POTS modem on the
other end. Toward that end, I would appreciate any pointers to suitable
software implementations of V.xxx modulation standards.
M~