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How does dialup work? Answered

Hello. I am currently working on making my own dialup BBS, but seem to have run into a problem. See, I understand how the raw data is sent over the phone line (FSK, and how 56k basically uses an ADC with a DAC together, and outputs according to the PSTN sample rate). However, there are some things which I have specific questions about.

First, when data is communicated between a BBS host and a BBS user, how is the text sent? If I recall correctly it just uses ASCII characters. Is this correct? How does it handle the login input and similar functions?

Second, could someone please explain how the modem handshake process works? I have seen a photo explaining the various parts, but I don't completely understand it. See this photo to see the photo I am referencing: https://i.imgur.com/Q3lKIr1.jpg

Lastly, could someone please explain trellis modulation (TCM) for dummies?

I was lead to believe that a BBS does use ASCII characters to transmit data, due to a video where someone used a text terminal, along with a mode, to connect to his BBS.



5 months ago

ASCII is just one clumsy single character code of the codes in use today...
When two computers recognize a carrier they will first agree on the highest suitable (error free code),
Hand shaking is performed with Xon /Xoff like codes.... Then a communication code and in a busy BBS the packet assignment then master will quiz the logger for the id code = preamble to ascertain permission status before actually exchanging meaningful messages after that there is agreement on preamble, data length, post error checks and lots more... You can get a good idea on an understandable communication tech if you study the hex or Intel program code for a micro computer like the Pi, Arduino or the Stamp format and remember google is your friend.....

May I ask you another question? I have been looking at a few single-chip-modem ICs, which are 600 bps (or was it 600 baud? I need to look). I was looking at the description of it, and I didn't see handshaking anywhere. Is this possible?

Baud infers speed... bps - bits per second..

The IC or modem only provides the base, the firmware or code running it makes it work.
See it as UHF radio: Whatever you get into the mic goes out the antenna.
How much of the handshake, protocols, error correction and so on happens only depends on what is used in the code and how it is done by the software addressing the system.
Back in the day our acoustic coupler was not much more than a speaker and mic plus the elctronis required to connect it to the computer and phone line.
Later the modems offered much more through the firmware but apart from using a direct use of the phone line it still is the same.