The Noisy Language of Modems

By Tim Slavin / History / February 2022 Issue

Do you think your internet is slow? Wait till you learn about dial-up!

Believe it or not, computers actually used to talk out loud to each other. Not with words, of course, but with beeps, boops, screeches, and other odd noises. Just like humans use language, computers, before wireless technology, used modems to introduce themselves, explain what they can do with each other, pass data back and forth while checking for accuracy, and then say goodbye…all with sounds humans could hear but not interpret.

The word modem is a contraction of two words that describe its function: modulation and demodulation, transmitting with varying sound tones (modulation) and listening then separating tones into digital data (demodulation). Modems translate computer digital data into analog sounds and back again to digital data using analog telephone wires to pass this data from computer to computer. Analog telephone lines are designed to transmit human voices through sound vibrations.

Once the modem hears a dial tone on a telephone line, it dials a number using the same sounds we hear when we type numbers on an old landline phone. The remote modem picks up the call from a modem calling them and emits a distinct tone recognized by the calling modem. They trade short bursts of binary data converted to sound in order to agree on a protocol to talk with each other. It’s like a conversation between a person who speaks Chinese talking with someone who speaks Welsh:

“I speak Chinese. Do you speak Chinese?”
“No. I speak Welsh.”
“Do you speak English?”
“Yes.”
“I can speak English. We’ll talk in English.”
“Agreed. We’ll talk in English.”

This process has a fun name, a handshake. Modems use sounds instead of words to agree and shake hands.

Next, the remote modem hacks the telephone line. Humans don’t like to hear echoes when talking, so analog phone lines suppress the return channel from the listener to the person speaking. The remote modem that picks up the call emits an answer tone to disable the ability of the phone line to temporarily silence the return channel. Unlike humans, computers and modems can pass data at the same time without any problems.

The next setup task is finding a mode both modems can handle by trying different modes and agreeing or disagreeing. They also send test tones to see how the telephone line responds to different tone frequencies.

Finally the fun begins: a loud crashing sound as the calling modem and remote modem send the scrambled data to make distribution more even and to check for any problems when sending. They also shape their incoming signals from each other by sending binary 1s. If data errors happen, the calling modem resends data. After the loud crashing sound, most modems turn silent as data is passed.

Last, the calling and remote modems confirm all data has been sent and received. Both disconnect from the shared telephone line. Any human listening would hear a click followed by a dial tone, then silence as their modem shuts off.

Most modems disappeared in the mid to late 2000s when wired connections like Ethernet appeared, followed by wifi and the public internet. These digital technologies allow computers to talk to each other faster than modems. Digital data is like a light bulb that turns on or off while analog is like a lightbulb on a dimmer switch, which can be turned on, off, and has many states of brightness in between those states. Most of us don’t need the level of perfection
modems and telephone lines provided to send email or visit websites.

For those of us who used modems to connect to the early internet, the weird sounds made the experience much more fun and mysterious, (especially the loud crashing sound) unlike today, when we silently log into our computers and open a web browser.

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Tim Slavin

Tim is an award-winning writer and technologist who enjoys teaching tech to non-technical people. He has many years experience with web sites and applications in business, technical, and creative roles. He and his wife have two kids, now teenagers, who are mad about video games.

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