dropping a few bits (was modem error correction)
re: John Heritage Subject: Modem Error Correction - when/what baud/bps?
My questions are -- is this a correct recollection? and when did error correction become pretty standard ...
My memory may have dropped a few bits but I remember that error correction was first in the protocol the programs implemented, not the modem. Early modems had very little signal processing. The first ones didn't even auto-dial! "smart modems" added a microcontroller mostly for autodialing. That's why xmodem, uucp and such were developed. A checksum or CRC (cyclic redundency check) was added to each block. Any error caused the entire block to be sent again. Once micro-controllers were cheap enough, error correcting was moved into the modem hardware (so long as both modems supported the same "standard"). There were several totally incompatible families of modems - async (home users, bbs) - synchronous (mainframe links) - PEP (Telebit trailblazers, favored by Unix/uucp)
i.e. An early 300 or 1200 baud "client and BBS" (or server) would typically send the 8 bits for an ASCII byte and an additional 2 bits -- a start and stop bit ...
Once modems gained DSP (digital signal processors), the data format you "fed it" externally bore little relationship to the symbols used among the modems. External modems usually use asynchronous serial via RS232 to a UART. Internal modems are parallel, so there's no stop/start bits. Early modems were FSK: frequency shift keying. One tone for a '1' or mark, another tone for '0' or space (thus the museum's all discrete-part acoustic-coupled modem in the wood box). Faster modems use frequency and phase shifting together. See: https://en.wikipedia.org/wiki/Phase-shift_keying https://en.wikipedia.org/wiki/Constellation_diagram enough? -- Jeff Jonas
On Mon, Jan 22, 2018 at 12:54 AM, Jeffrey Jonas via vcf-midatlantic <vcf-midatlantic@lists.vintagecomputerfederation.org> wrote:
Once modems gained DSP (digital signal processors), the data format you "fed it" externally bore little relationship to the symbols used among the modems.
It's worth noting that there was a third generation of modems where they actually removed the DSP and did everything entirely in software (to reduce cost). These "winmodems" did all the signal processing on the host CPU (including error correction) and then the modem hardware itself was just a simple D/A. These were the norm in the mid to late 1990's unless you bought a more expensive modem such as the US Robotics Sportster.
External modems usually use asynchronous serial via RS232 to a UART. Internal modems are parallel, so there's no stop/start bits.
This is generally true, although many internal modems actually had an onboard 8250/16550 UART. Hence from a software/application perspective, they were indistinguishable from external modems. Devin -- Devin J. Heitmueller http://www.devinheitmueller.com
On 01/22/2018 09:37 AM, Devin Heitmueller via vcf-midatlantic wrote:
It's worth noting that there was a third generation of modems where they actually removed the DSP and did everything entirely in software (to reduce cost). These "winmodems" did all the signal processing on the host CPU (including error correction) and then the modem hardware itself was just a simple D/A. These were the norm in the mid to late 1990's unless you bought a more expensive modem such as the US Robotics Sportster.
Fortunately, these ill-advised products were short-lived, in the grand scheme of things. -Dave -- Dave McGuire, AK4HZ New Kensington, PA
The winmodems were the worst. That’s why I ended up recently getting a USR USB modem, and made sure it was the full modem, not the winmodems. On Mon, Jan 22, 2018 at 9:46 AM Dave McGuire via vcf-midatlantic < vcf-midatlantic@lists.vintagecomputerfederation.org> wrote:
On 01/22/2018 09:37 AM, Devin Heitmueller via vcf-midatlantic wrote:
It's worth noting that there was a third generation of modems where they actually removed the DSP and did everything entirely in software (to reduce cost). These "winmodems" did all the signal processing on the host CPU (including error correction) and then the modem hardware itself was just a simple D/A. These were the norm in the mid to late 1990's unless you bought a more expensive modem such as the US Robotics Sportster.
Fortunately, these ill-advised products were short-lived, in the grand scheme of things.
-Dave
-- Dave McGuire, AK4HZ New Kensington, PA
-- Normal Person: Hey, it seems that you know a lot. Geek: To be honest, it's due to all the surfing I do. Normal Person: So you go surfing? Normal Person: But I don't think that has anything to do with knowing a lot... Geek: I think that's wrong on a fundamental level. Normal Person: Huh? Huh? What?
On Mon, Jan 22, 2018 at 12:26 PM, Joseph Oprysko via vcf-midatlantic <vcf-midatlantic@lists.vintagecomputerfederation.org> wrote:
The winmodems were the worst. That’s why I ended up recently getting a USR USB modem, and made sure it was the full modem, not the winmodems.
Oh yes, I couldn't agree more, and I'm speaking as someone who worked for the company that was one of the largest manufacturer of winmodems (3Com, which owned USR at the time). Devin -- Devin J. Heitmueller http://www.devinheitmueller.com
On 01/22/2018 12:39 PM, Devin Heitmueller via vcf-midatlantic wrote:
On Mon, Jan 22, 2018 at 12:26 PM, Joseph Oprysko via vcf-midatlantic <vcf-midatlantic@lists.vintagecomputerfederation.org> wrote:
The winmodems were the worst. That’s why I ended up recently getting a USR USB modem, and made sure it was the full modem, not the winmodems.
Oh yes, I couldn't agree more, and I'm speaking as someone who worked for the company that was one of the largest manufacturer of winmodems (3Com, which owned USR at the time).
The same modems that were used for VoIP cards (at least some where). That interrupted the main CPU at least 64k times per second. Turning a server to mush. -- Linux Home Automation Neil Cherry ncherry@linuxha.com http://www.linuxha.com/ Main site http://linuxha.blogspot.com/ My HA Blog Author of: Linux Smart Homes For Dummies
participants (5)
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Dave McGuire -
Devin Heitmueller -
Jeffrey Jonas -
Joseph Oprysko -
Neil Cherry