Dial-up handshaking illustrated

Oona Räisänen has written a thorough and engrossing article about the noises emitted by dial-up modems while they connect and handshake, and the accompanying graphic (ZOMG HUGE) is nothing short of spectacular. It would make a great full-size poster -- maybe a framed art-print.

Now the modems must address the problem of echo suppression. When humans talk, only one of them is usually talking while the other one listens. The telephone network exploits this fact and temporarily silences the return channel to suppress any confusing echoes of the talker's own voice.

Modems don't like this at all, as they can very well talk at the same time (it's called full-duplex). The answering modem now puts on a special answer tone that will disable any echo suppression circuits on the line. The tone also has periodic "snaps" (180° phase transitions) that aim to disable yet another type of circuit called echo canceller.

Now the modems will list their supported modulation modes and try to find one that both know. They also probe the line with test tones to see how it responds to tones of different frequencies, and how much it attenuates the signal. They exchange their test results and decide a speed that is suitable for the line.

After this, the modems will go to scrambled data. They put their data through a special scrambling formula before transmission to make its power distribution more even and to make sure there are no patterns that are suboptimal for transfer. They listen to each other sending a series of binary 1's and adjust their equalizers to optimally shape the incoming signal.

The sound of the dialup, pictured (via JWZ)



  1. This would have been very nice to have 20 years ago when I heard that sound all the time. 

    We had a pair of Telebit Trailblazer modems that, while not standard, could move data many times faster than the generic 9600 baud modems of the day. They could even figure out what Unix file transfer commands were being sent, and optimize accordingly.

  2. And people used to think you were weird if you knew what speed your modem negotiated before the speaker turned off….

  3. I can honestly say that this is the first time I’ve looked at an image and heard sound in my head. it’s a nerd-synesthesia simulator!

  4. My favourite nerd superpower was my ability to whistle the “300 baud init tone” – the v.8 bis ACK/ESC tone sequence, which prompts many answering modems to send /their/ FSK data.

    1.  When I was a kid in High School in Cali, AOL went down, so, bored out of my skull, I picked up the phone and dialled the number anyway, whistling the tones that I’d learned from my 56k modem. It was an awesome moment when I heard the other modem start sending me data :)

  5. US Robotics had their own high-speed protocol, HST, which handled the negociation in a manner more like the way Internet protocols handle handshacking.  Whereas, essentially, V.32 tried all various speed/protocols until the modems could agree on one, the HST modems would just say “Hi, I’m a HST modem, do you speak HST?”.  Their handshaking was super-quick.

    I still have my big, black USR Dual Standard.

  6. This would have been handy 15 years ago, of course then it would have taken half an hour to download ;)

  7. The artist wasn’t very talented 15 years ago. :) But, referring to the article – and pardon the spam – RedBubble is selling this as a poster.

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