4 ms·

> The sound of a modem handshake Apropos of nothing, but one of the stories I love to tell during interviews was the discovery that a 2,400bps modem was faster

> The sound of a modem handshake Apropos of nothing, but one of the stories I love to tell during interviews was the discovery that a 2,400bps modem was faster than the 56kbps one. I was designing a "box" to read a group of switches and transfer that switch state information via modem to another "box" at the end of the phone line that replicated the switch on/off positions. During initial testing, I found out that with a 2400bps modem, my system could read the switches, dial the downstream modem and transfer the data and hang up before the 56k modems had even finished negotiating their speeds. Latency often trumps bandwidth!

mey 5y ago
Something I had completely forgotten. I was writing a modem pool software server for a custom payment gateway back around 2004. Point of sale terminals would normally be configured for around 9600 baud for this reason. Payloads were typically small and link establishment was the priority.
JoeAltmaier 5y ago
Ha! I rewrote the CTOS serial protocol for the OS message passing link. It had been severely limited by fixed timers between polls, so going faster than 9600 didn't matter. I changed it to 'if you have a packet, send it. If you can't buffer a packet, NACK it'. Now we could run full speed (and full duplex!) It instantly became practical to log in remotely and use source control on our server. Which mattered to me, because I was remote-working. Anyway, nostalgia
zinckiwi 5y ago
Very neat. I recall in the dusty recesses of my mind various settings that you could pass to a modem at call initialisation to force certain speeds, modes, and other parameters. Did it have to be an actual 2400 baud modem or could you just "step down" the 56k for the same effect?
- nzmsv 5y ago
  Usually yes, with an AT command: http://www.messagestick.net/modem/Hayes_Ch1-1.html http://www.messagestick.net/modem/Hayes_Ch1-1.html Knowledge of AT commands is still useful today, but the modems are cellular: https://www.twilio.com/docs/iot/supersim/introduction-at-commands https://www.twilio.com/docs/iot/supersim/introduction-at-com...
  - HeyLaughingBoy 5y ago
    Just finished a project last summer with a cell modem, actually. Felt weird using AT commands to make an http connection.
- HeyLaughingBoy 5y ago
  It was a one-off I built for a water utility in Connecticut. I'm pretty sure it was an embedded XECOM 2,400bps modem, but I don't remember how I figured out it was faster to do that than to use a 56.6k modem which I would have expected to be faster. Since at 28.8k and higher the modems are negotiating more than just frequencies (phase shifts and bit compression are also negotiated, IIRC), telling a 56k to run at 2.4k should have the same effect.

mey 5y ago

Something I had completely forgotten. I was writing a modem pool software server for a custom payment gateway back around 2004. Point of sale terminals would normally be configured for around 9600 baud for this reason. Payloads were typically small and link establishment was the priority.

JoeAltmaier 5y ago

Ha! I rewrote the CTOS serial protocol for the OS message passing link. It had been severely limited by fixed timers between polls, so going faster than 9600 didn't matter. I changed it to 'if you have a packet, send it. If you can't buffer a packet, NACK it'. Now we could run full speed (and full duplex!) It instantly became practical to log in remotely and use source control on our server. Which mattered to me, because I was remote-working. Anyway, nostalgia

zinckiwi 5y ago

Very neat. I recall in the dusty recesses of my mind various settings that you could pass to a modem at call initialisation to force certain speeds, modes, and other parameters. Did it have to be an actual 2400 baud modem or could you just "step down" the 56k for the same effect?

nzmsv 5y ago
Usually yes, with an AT command: http://www.messagestick.net/modem/Hayes_Ch1-1.html http://www.messagestick.net/modem/Hayes_Ch1-1.html Knowledge of AT commands is still useful today, but the modems are cellular: https://www.twilio.com/docs/iot/supersim/introduction-at-commands https://www.twilio.com/docs/iot/supersim/introduction-at-com...
- HeyLaughingBoy 5y ago
  Just finished a project last summer with a cell modem, actually. Felt weird using AT commands to make an http connection.
HeyLaughingBoy 5y ago
It was a one-off I built for a water utility in Connecticut. I'm pretty sure it was an embedded XECOM 2,400bps modem, but I don't remember how I figured out it was faster to do that than to use a 56.6k modem which I would have expected to be faster. Since at 28.8k and higher the modems are negotiating more than just frequencies (phase shifts and bit compression are also negotiated, IIRC), telling a 56k to run at 2.4k should have the same effect.