Why Are Dial-up Modems so Noisy?

Throughout the 1990s the majority of internet users began their session with the noisy handshake of a dial-up modem, but what exactly was all that electronic chatter about? Read on as we investigate one of the more iconic sounds of the burgeoning Internet age.

Today’s Question & Answer session comes to us courtesy of SuperUser—a subdivision of Stack Exchange, a community-drive grouping of Q&A web sites.

While dial-up modem use might be down from the nearly 100% market saturation in the 1990s to only 10% of current US internet users, the sound of a dial-up modem connecting lives on in the memories of geeks everywhere. This week we’re taking a look at the technology behind the noisy process and what exactly was going on when you dialed in for your internet session.

The Question

SuperUser reader Celeritas poses the question surely millions of people have asked themselves over the years:

I know that the signal was just tone pulses but why was it when (back in the 90s) when you first connected to the internet you heard a bunch of funny noises. After that if you were to use the internet, it still was using the telephone line, why no funny noises then?

Why indeed? What was going on during the noisy part and why the silence afterwards?

The Answers

Several SuperUser contributors fleshed out an answer for us. Scott Chamberlain writes:

Modems originally allowed you to send data over a network that was designed to only carry voice. Because of that, the communication method between two modems had to be in the audible hearing range (or it would not get carried on the phone line). This is no longer needed because the phone system can now carry both voice and data at the same time (DSL).

The sounds were there all the time, you just needed to pick up the phone to hear it. The reason they played it over a loudspeaker to start with is so you could hear if somthing went wrong with the connection (busy signal, wrong number, a person picked up instead of a modem on the other end, etc).

Tylerl expands on that and explains how you could manipulate your modem to pipe down:

The whistles and chirps and buzzes that you hear when a modem is going through its initial handshake process is a test of the telephone line quality. A modem send precisely specified sounds and the other listens see what it actually hears on the other end. This way the modems know how clear the line is between them and what sort of frequencies they can use to communicate with each other. The more frequencies they can use and the lower the noise, the higher the speed they’ll be able to communicate at.

If a connection ever failed due to connection quality, it would generally fail during this initial handshake process. And if you were listening, you could usually tell why (e.g. you got an answering machine on the other end instead of a modem).

As such, modems were usually configured to play this handshake sequence out loud. This was configured by sending AT M1 to the modem during setup. Alternately, AT M2 means to leave the speaker on all the time, while AT M0 means don’t turn the speaker on at all. See the AT command set for more information.

The actual transmission noise that you would hear if you picked up the phone during an active session (as opposed to during this handshake procedure) just sounds like static.

Oh the magic of AT M0; discovering that command was like being given an invisibility cloak-stealthy late night browsing for everyone. While Tylerl notes that high-baud traffic sound just like static, contributor Supercat notes that very low-baud modems were a different story:

At 300 baud, it’s possible to audibly hear incoming data. On occasions, I’ve turned on the modem speaker if I wanted to hear when characters arrived on a generally-idle line. Higher baud rates use a “data-scrambler” circuit so that most patterns of data are no longer audibly distinguishable.

Have something to add to the explanation? Sound off in the the comments. Want to read more answers from other tech-savvy Stack Exchange users? Check out the full discussion thread here.