Modems are the unsung heroes of early internet connectivity. Starting with the 300-bps modem makes sense. These devices are straightforward. They use frequency shift keying (FSK) to send digital data over phone lines. FSK works by assigning a specific tone to a bit.
When two modems connect, they use different frequencies to talk simultaneously. This is full-duplex operation. The originating modem sends a 1,070-hertz tone for a 0. It uses a 1,270-hertz tone for a 1. The answering modem does the opposite. It transmits a 2,025-hertz tone for a 0. A 2,225-hertz tone represents a 1.
This separation allows both modems to send data at the same time. Half-duplex modems can only transmit in one direction. Those are rare.
Consider the letter “a”. Its ASCII code is 97 in decimal. In binary, that is 01100001. A UART inside the terminal converts this byte into bits. It sends them one by one through the RS-232 port, also known as the serial port. The modem receives these bits and sends them over the phone line.
