3 ms·
> 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
by 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 agoWay 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 agoIf 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 agoFair enough. How about V.92? Have you had a setup where upstream PCM could be successfully established?
- FrankPetrilli 3y agoGood 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 agoWhat gear were you using?
- FrankPetrilli 3y agohttps://frank.petril.li/posts/dialup-adventures-1/ https://frank.petril.li/posts/dialup-adventures-1/ explains it all!
- simfree 3y agoAny chance you would be bringing that modem bank to ToorCamp on Orcas Island? The ShadyTel network could always use a better, faster dialup ISP!
- RuggedPineapple 3y agoI think this is exactly right for reasons that weren't important to the base story. The year after the story they moved to these Cisco VoIP phones that plugged into Ethernet directly, which seems odd considering they had just recently moved onto what was supposedly a Telco provided IP phone service. I suspect someone either misunderstood what they were buying or someone misrepresented what they were selling. They moved to what they thought they were getting in the first place and dropped the telco entirely. For schools of that time, there was an incredible amount of money flowing around in the dot-com era. At least for a high school in a state capital. Both Microsoft and Cisco sponsored classes and teachers for them at my school. I got semesters A and B of my CCNA as a credit giving elective. My first time using Linux was (ironically) in the Microsoft sponsored class.