The sound of two modems connecting was the sound of the Internet for many years. For those who don't remember this, when you used a phone and a modem to dial in, the modem would play the sounds of the connection through the speaker until the connection was fully established. Weird as this may sound, with a bit of experience you got to know the sounds well, and could quickly tell if something was wrong by listening.
This beautiful illustration by +Oona Räisänen
walks you through the entire dialogue that the two modems are having: in this case, a pair of 19,200 baud modems* talking to each other and establishing how they can talk.
* One baud is one line level transition per second -- that is, the signal flips from 0 to 1 or vice-versa. That's almost but not quite the same as bits per second, because some of those transitions are used to ensure that the data made it across the (noisy) phone lines clearly. The most common configuration was "8-N-1," meaning that it would send eight bits of data, no parity bits, and one "stop" bit to indicate the end of a byte. (A parity bit is a type of checksum that the receiver can use to make sure no data was corrupted during transmission) So nine level transitions would be used to send eight bits of data, and so a 19,200-baud modem would send about 17,000 bits per second.
Compare that to your cable modem, whose speed is measured in megabits
per second, and you realize how much has changed. But 19,200 was shockingly fast in its day: it meant that you could download a jpg in half a minute.
(Side note: When people use "mega" and "giga" for network speeds, they mean decimal
mega and giga, i.e. one million and one billion. When they use it for RAM amounts, they mean binary
mega and giga, i.e. 2^20 (1,048,576) and 2^30 (1,073,741,824). And when they use it for hard disk sizes, they mean "somewhere roughly in this region, because hard disk manufacturers lie like dogs about their capacities anyway." Confusion between these things has led to some pretty spectacular engineering problems over the years.)
h/t to +Ilya Grigorik
, +Lauren Weinstein
, and the various other people that have pointed me at this figure over the years.