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.

今天的問答環節是由SuperUser提供的，SuperUser是Stack Exchange的一個分支，它是一個由Q&a網站組成的社群驅動分組。

雖然撥號調變解調器的使用率可能會從上世紀90年代接近100%的市場飽和率下降到目前美國網際網路使用者的10%，但撥號調變解調器連線的聲音仍然留在各地極客的記憶中。本週，我們將瞭解這一嘈雜過程背後的技術，以及您撥入internet會話時到底發生了什麼。

問題

超級使用者讀者Celeritas提出了一個問題，多年來，數百萬人肯定都在問自己：

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?

為什麼呢？吵鬧的那部分發生了什麼？為什麼之後會保持沉默？

答案

幾個超級使用者貢獻者為我們提供了一個具體的答案。斯科特·張伯倫寫道：

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對此進行了擴充套件，並解釋瞭如何操縱調變解調器向下傳輸：

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 tran**ission 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.

哦，在M0的魔力；發現這個命令就像被賦予了隱形斗篷，每個人都在深夜偷偷瀏覽。Tylerl指出，高波特率的流量聽起來就像靜態的，而Supercat指出，非常低的波特率調變解調器則是另一回事：

At 300 baud, it’s possible to audibly hear incoming data. On occasi***, 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.

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