Making the rounds today, this excellent write-up (and informative graphic) from Oona Räisänen about the once-familiar ‘handshake’ sound modems make when they first connect. She writes:

If you ever connected to the Internet before the 2000s, you probably remember that it made a peculiar sound. But despite becoming so familiar, it remained a mystery for most of us. What do these sounds mean?

As many already know, what you’re hearing is often called a handshake, the start of a telephone conversation between two modems. The modems are trying to find a common language and determine the weaknesses of the telephone channel originally meant for human speech.

…

The first thing we hear in this example is a dial tone, the same tone you would hear when picking up your landline phone. The modem now knows it’s connected to a phone line and can dial a number. The number is signaled to the network using Dual-Tone Multi-Frequency signaling, or DTMF, the same sounds a telephone makes when dialing a number.

The remote modem answers with a distinct tone that our calling modem can recognize. They then exchange short bursts of binary data to assess what kind of protocol is appropriate. This is called a V.8 bis transaction.

Read the whole thing — it’s good stuff!