retronet-discuss February 2024

discussion@lists.retronetworking.org

5 participants
2 discussions

Software to emulate V.xxx modulations in PCM sample stream?
by Mychaela Falconia 01 Mar '24

01 Mar '24

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~

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Suggestion for documentation clarification Osmo-v5
by Jens Müller 16 Feb '24

16 Feb '24

Regarding https://osmocom.org/projects/retronetworking/wiki/Osmo-v5 I'm somewhat confused how this software interacts with other systems hardware-wise. Maybe you can add this to that documentation page for clarity? I understand this provides the network side of the E1 connection, but where does this run/how does it get connected to for example a Nokia EKSOS from a hardware perspective? Thanks for your work, Jens

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retronet-discuss February 2024