Long before the vwip! of iMessage and the funky beats of the Skype startup song, digital communication had a strange soundtrack. Dial-up modems, which connected to servers and other computers via public telephone networks, bracketed the otherwise silent browsing experience with their inimitable song.

But what seemed to internet users to be a noisy side effect of the mystical mechanical process of “going online” actually was the process. Known to technicians as a “handshake,” the sound was a means of negotiating terms between remote machines, after which they could successfully exchange data during browsing—also with sound, though virtually unprocessable to the human ear.

The few breakdowns of the mysterious dial-up handshake that remain online invariably include the “example handshake” graphic by Finnish software developer Oona Räisänen below. Räisänen developed the graphic in 2012 to satisfy her own curiosity about the dial-up sound, which was then the subject of a wave of nostalgia for the early days of the commercial Internet.

Oona Räisänen, Creative Commons

“I had always been intrigued by the fact that computers can literally make phone calls to each other, and I wanted to know exactly how they do that,” Räisänen tells Popular Mechanics. “I found the information by reading through the ITU-T standards , the technical documents that define different parts of the handshake procedure.”

Räisänen discovered that every section of the dial-up song serves a specific purpose: to establish the terms of the coming data transfer for maximal security and minimal information loss. Here, with her help, we break down the iconic sound below, beat by beat.

Part 1: Exchanging Pleasantries

The earliest networks, like ARPANET, were limited in reach by their own infrastructure. Dial-up internet was introduced in the early 1990s as a way to work around that obstacle by taking over a signaling system that was already in place worldwide: the Public Switched Telephone Network, or PTSN. The first dial-up modems connected to the internet by using the phone much like humans do: by putting their speakers next to the mouthpiece. Later models could be connected more directly to a building’s phone line, though they still took over the phone line completely.

That channel monopoly was essential, because the dial-up sound, rather than being a side effect, was the actual conversation between modems; any non-modem audio would just interfere with the signal. “The handshake is not an artifact; it is the actual negotiation,” writes Räinänen. “All information that the computers exchange is audible in that sound.”

Thus the first sound of the handshake is the dial tone, followed by a phone number in the familiar key of Dual-Tone Multi-Frequency , a system that assigns a special tone to every digit, including a country code, area code, and seven-digit phone number. After a pause, the answering modem responds with a tone that initiates a protocol, or a special conversational routine, like V.8bis, another ITU recommendation. When the calling modem accepts the protocol, the real negotiations can begin.

Part 2: Setting the Terms

This part of the handshake, in which modems list their capabilities and settle on terms for the connection, really does sound like a conversation. The speakers overlap a little, as conversing humans do, but still leave the impression of a back-and-forth. There’s even a bit of a business vibe, albeit what a business would sound like if it were conducted between giant robot-bird-aliens in a screeching tonal shorthand over a busted CB radio.

In the original graphic, Räisanen captures that professional atmosphere by having the modems make requests by saying “please” and sprinkling the translated conversation with chipper exclamation points. “Computer protocols are like extremely rigid rules of etiquette,” Räisänen says. “I thought every rule of etiquette should mandate a level of politeness.”

Part 3: A Line of Their Own

Hijacking a phone system to connect to the internet requires a lot less hardware than, say, building an international fiber-optic cable network, but dial-up pays a steep price for using borrowed infrastructure. The PSTN offers a few technological features that strongly favor human callers over modems. One of these is echo suppression, which relies on the fact that humans generally take turns speaking on the phone; when one starts talking, the network silences the return channel, which prevents the speaker from being confused by hearing echoes of their own voice.

But modems are full-duplex , which means they have no problem talking and listening at the same time, says Räisänen, so echo suppression just halves the efficiency of the conversation. To eliminate it, the answering modem plays a special tone that disables echo suppression circuit and another circuit modems don’t need called an echo canceller.

Jill Ferry Photography Getty Images

Part 4: Modulation Station

Non-sound information, like bits, doesn’t travel happily over sound-only channels like phone lines. It’s a classic information theory problem and one of the first topics introduced in digital communications courses. The channel is analog (varying in pitch and volume along the sonic spectrum), but the data are digital (limited to a closed set of symbols like zeroes and ones).

A modem’s job is to put one form of information in terms that are legible in the other using one of several protocols that define the unit equivalents between analog and digital data. Turning binary code sequences into analog waveforms—think of the wiggly sound wave you see in audio editing platforms—is called modulation, and recreating a binary sequence out of a received waveform is demodulation. Modems must perform both processes simultaneously.

In order to understand one another, it’s essential that the modems use the same key to make their translations, which is why much of the handshake consists of modems chatting about which modes and protocols they prefer. Finding the right modulation mode requires both talking and testing; the modems list the modes they support until they find one they both know, and then emit a series of test tones to figure out which speed will work best over the phone line.

Part 5: Sound Check

At this point, the distinct tones and beeps yield to what sounds like an electric snowstorm that suddenly shifts to a lower pitch before falling silent. The modems have now switched to scrambled data in order to keep the power distribution even across the channel and to weed out any annoying patterns on the line, Räisänen says. More test tones follow in this format; the modems adjust their equalizers to make sure they’re not missing any important sounds.

The end of the song comes when modem speakers go silent, allowing the computers themselves to start transferring data according to the terms set during the handshake.

The Evolution of Song

The post dial-up generation of computers relies on its own network infrastructure, satellites, or other wireless transmitters rather than phone line audio to connect to the internet, all of which are “hugely faster methods that can’t even be expressed as sounds.” The most modern routers transmit over specific radio frequencies , bypassing hardware altogether, though they still have to negotiate the way they’ll communicate.

“Almost all connections between computers involve some kind of handshake,” says Räisänen, but the modern versions “vastly exceed the information capacity of human hearing, or of any sense of biological creature, for that matter.”

Robotic communication is now frequently too complex to be understood by humans, including the engineers who design it. Nostalgia for the days of dial-up may be a sign that transparency is almost worth the slowest connection imaginable.