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This is 20 year old memories so I may have some of the details wrong, but the gist was the school had rolled out an early IP telephony solution and the landline

This is 20 year old memories so I may have some of the details wrong, but the gist was the school had rolled out an early IP telephony solution and the landlines were just rj-11 patch cables into a monolithic IP telephony box that assigned each line a number and shouldn't have been capable of handling the dial up connection through that

Waterluvian 3y ago
Sounds like an interesting mystery. If a human can talk, a modem can talk. It’s analog after all. Maybe the IP abstraction meant, “there’s quantization happening that makes it technically impossible for a modem to talk at any sensible baud rate. There’s still some circuit switched stuff going on here that we shouldn’t be paying for.” Now I’m curious to see what baud rate can be achieved over today’s VOIP lines!
- deleted 3y ago
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- b112 3y ago
  I've used analog fax over SIP in the last few years, with ulaw, usually very successfully. Not sure if it linked at 9600 or what, didn't pay attention.
  - deleted 3y ago
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- epcoa 3y ago
  > If a human can talk, a modem can talk. It’s analog after all. That’s an oversimplification, especially for 56k where the signal isn’t analog in the sense of the slower speeds and uses PAM (pulse amplitude modulation) that is trained at the beginning of the call (it can be retrained but that is long and drawn out). Central offices had to be upgraded and basic u-law quantization meant 56k was a no go even on most classic POTS lines. There’s a bit more to it than just “noise.” For VoIP, Amplitude distortion and quantization error (from multiple sources) means you will never get 56k over any voip system. Phase distortion and echo cancellation make other speeds frustrating. Doing better than 9600 is going to be difficult. My guess is the OP was not dealing with an IP system (sounds less likely for a school in 2000). In any event they noted they could not achieve 56k which is expected that they were not able to establish a PAM link over the PBX, but the channel was good enough for analog FSK modulation. That they could have achieved 28k pretty much rules out any IP system with an ATA of that vintage .
  - ghaff 3y ago
    Way back when my dad had an analog POTS connection at his rural Maine house. It worked fine for voice calls but it was flaky to non-existent for a modem connection, to which the telco basically went <shrug>.
    - toast0 3y ago
      If you go back in time, you want to test it with faxing, if it doesn't work for data modem, it probably doesn't work well for faxing, and if it doesn't work for faxing, the telco might actually fix it.
  - FrankPetrilli 3y ago
    > For VoIP, Amplitude distortion and quantization error (from multiple sources) means you will never get 56k over any voip system https://frank.petril.li/posts/dialup-adventures-1/ https://frank.petril.li/posts/dialup-adventures-1/ I've done V.90 over VoIP. In a very controlled environment that was tuned for it, but VoIP nonetheless. The main issue is that VoIP timing, even with deep buffers, is prone to more jitter than a PDH network and the phase drift will eventually cause enough errors to force a retrain every few minutes. Either way, "never" is a little too absolute IMHO. :)
    - epcoa 3y ago
      Fair enough. How about V.92? Have you had a setup where upstream PCM could be successfully established?
      - FrankPetrilli 3y ago
        Good question - it's been almost two years since this project so I don't recall whether V.92 came up at all, even on a pure T-1 setup. I still have all the gear, if I set it up and run it again I'll update this thread.
        simfree 3y ago

Waterluvian 3y ago

Sounds like an interesting mystery. If a human can talk, a modem can talk. It’s analog after all. Maybe the IP abstraction meant, “there’s quantization happening that makes it technically impossible for a modem to talk at any sensible baud rate. There’s still some circuit switched stuff going on here that we shouldn’t be paying for.” Now I’m curious to see what baud rate can be achieved over today’s VOIP lines!

deleted 3y ago
[deleted]
b112 3y ago
I've used analog fax over SIP in the last few years, with ulaw, usually very successfully. Not sure if it linked at 9600 or what, didn't pay attention.
- deleted 3y ago
  [deleted]
epcoa 3y ago
> If a human can talk, a modem can talk. It’s analog after all. That’s an oversimplification, especially for 56k where the signal isn’t analog in the sense of the slower speeds and uses PAM (pulse amplitude modulation) that is trained at the beginning of the call (it can be retrained but that is long and drawn out). Central offices had to be upgraded and basic u-law quantization meant 56k was a no go even on most classic POTS lines. There’s a bit more to it than just “noise.” For VoIP, Amplitude distortion and quantization error (from multiple sources) means you will never get 56k over any voip system. Phase distortion and echo cancellation make other speeds frustrating. Doing better than 9600 is going to be difficult. My guess is the OP was not dealing with an IP system (sounds less likely for a school in 2000). In any event they noted they could not achieve 56k which is expected that they were not able to establish a PAM link over the PBX, but the channel was good enough for analog FSK modulation. That they could have achieved 28k pretty much rules out any IP system with an ATA of that vintage .
- ghaff 3y ago
  Way back when my dad had an analog POTS connection at his rural Maine house. It worked fine for voice calls but it was flaky to non-existent for a modem connection, to which the telco basically went <shrug>.
  - toast0 3y ago
    If you go back in time, you want to test it with faxing, if it doesn't work for data modem, it probably doesn't work well for faxing, and if it doesn't work for faxing, the telco might actually fix it.
- FrankPetrilli 3y ago
  > For VoIP, Amplitude distortion and quantization error (from multiple sources) means you will never get 56k over any voip system https://frank.petril.li/posts/dialup-adventures-1/ https://frank.petril.li/posts/dialup-adventures-1/ I've done V.90 over VoIP. In a very controlled environment that was tuned for it, but VoIP nonetheless. The main issue is that VoIP timing, even with deep buffers, is prone to more jitter than a PDH network and the phase drift will eventually cause enough errors to force a retrain every few minutes. Either way, "never" is a little too absolute IMHO. :)
  - epcoa 3y ago
    Fair enough. How about V.92? Have you had a setup where upstream PCM could be successfully established?
    - FrankPetrilli 3y ago
      Good question - it's been almost two years since this project so I don't recall whether V.92 came up at all, even on a pure T-1 setup. I still have all the gear, if I set it up and run it again I'll update this thread.
      - simfree 3y ago