eli5: The “handshake” between dial-up modems and why it made those sounds
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Its just a more advanced version of morse code essentially.
Its just deciding how fast it can talk and send tones.
Your modem needs to know what device it’s listening to, what frequency it’s listening to and if it’s even compatible. There’s a lot of instructions that go through connecting to the internet and the only way to transmit this through phone is sending the bits as a frequency, i guess they kept the actual listening part in to debug and see if anything goes wrong. People had to design this stuff at some point
Modems (MOdulate/DEModulate) are a way of sending digital signals (a stepped system, e.g. ones and zeros) over an analog transmission carrier (continually varying power and/or frequency levels). Phone lines were originally designed to carry voice, a continually varying signal, so they required modems to send newer digital signals over them. The beeping noises that are the “handshake” are very simple, slow digital signals, that changes the frequency in steps, resulting in a sound at a specific frequency, so it sounds like a tone or note. The receiving side can detect these tones, and convert them back into ones and zeros. The sending side tells the receiving side what speeds it can do, and the receiving side picks a speed it is happy with, and tells the sending side what speed it’s picked. They then transmit at this speed, which is what sounds like static after the initial beeps, but it’s actually a similar sort of changing of frequency, just happening much faster.
The sound is not a byproduct, It is the direct transmission, but instead of being human-readable, its machine-readable. Its actually a clever hack to use the already existing Telephone infrastructure, in a time before fiber-optic cables everywhere.
Now for what’s actually being transmitted:
The first sounds were actually the dial tone and the phonenumber, to which the answering modem would respond with a dedicated Tone as well.
After that, both modems communicate their capabilities and settle on terms of the communication.
Now that the connection is established, data from the computer in the form of zeros and ones are sent to the modem, which translates it into sound, which can get translated back into binary by the other modem.
If you want to know what exactly these packets are, you can look at one example – the http handshake – which would be used to connect to a website.
https://developer.mozilla.org/en-US/docs/Web/HTTP/Session
basically a negotiation between two devices and exchanging of information about their capabilities. it allowed to use the optimum speed/settings that both had in common. this image will explain exacty what happened and when:
https://oona.windytan.com/posters/dialup-final.png
> why it made those sounds
you could actually silence your line to not hear it. but the default was sound on.
For more fun about sound and old landline telephone, [here](https://youtu.be/4tHyZdtXULw) is a fun little 30 minute video about “phone phreaking.”
So a modem is called a modulator/demodulator, because it encodes data from digital to analogue and back again. On your computer it is digital information, but if you want to talk to another computer you need to transform that data in a way it can be transmitted over a telephone line. The sounds you are hearing is analogue data being converted to digital data (1’s & 0’s). It’s similar to the old telegraph machines when they used mores code, but instead of a human converting the data you have a machine doing it faster and more accurately.
The phone lines could only transmit up to about 4Khz (human voice muffled). The dial up modems would try to establish a connection to the other side. Part of the process was to find out how good or bad was the phone line link. In this process tones of different frequencies are sent and received, and if the channel is muffling a particular part, then the modem would learn to increase the volume in that particular part (frequency). Later the modem would start trying to transmit data using sound and try to determine how many errors were detected on the other side. The modem would try to go to the highest data rate it could with limited errors, and once it found it, it would then start sending data, which to us sounded like white noise.
The dial up modems communicate over phone lines (hence dial up), and that sound is the data that they’re communicating (1s and 0s) They have to tell each other what they are capable of (speed, location in the network, etc) before any actual data can be exchanged. That’s what the handshake is for, and once it’s done, actual data can be transmitted.
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