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A standard modem of today contains two functional parts: an analog section for generating the signals and operating the phone, and a digital section for setup and control. This functionality is actually incorporated into a single chip, but the division remains in theory. In operation the modem can be in one of two modes, data mode in which data is sent to and from the computer over the phone lines, and command mode in which the modem listens to the data from the computer for commands, and carries them out. A typical session consists of powering up the modem (often inside the computer itself) which automatically assumes command mode, then sending it the command for dialing a number. After the connection is established to the remote modem, the modem automatically goes into data mode, and the user can send and receive data. When the user is finished, the escape sequence, "+++" followed by a pause of about a second, is sent to the modem to return it to command mode, and the command ATH to hang up the phone is sent.

The commands themselves are typically from the Hayes command set, although that term is somewhat misleading. The original Hayes commands were useful for 300 bit/s operation only, and then extended for their 1,200 bit/s modems. Faster speeds required new commands, leading to a proliferation of command sets in the early 1990s. Things became considerably more standardized in the second half of the 1990s, when most modems were built from one of a very small number of chipsets. We call this the Hayes command set even today, although it has three or four times the numbers of commands as the actual standard.

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