Understanding 300 BPS Modems

300 BPS modems are extremely easy to understand, so let's use them as a starting point. A 300 BPS modem is a device that uses Frequency Shift Keying (FSK) to transmit digital information over a telephone line. In frequency shift keying, a different tone (frequency) is used for the different bits.

When a terminal's modem dials a computer's modem, the terminal's modem is called the originate modem. It transmits a 1,070 hertz tone for a zero and a 1,270 hertz tone for a 1. The computer's modem is called the answer modem, and it transmits a 2,025 hertz tone for a 0 and a 2,225 hertz tone for a 1. Because the originate and answer modems transmit different tones, they can both use the line simultaneously. This is known as full-duplex operation. Modems that can transmit in only one direction at a time are known as half-duplex modems, and they are rare.

Let's say that two 300 BPS modems are connected, and the user at the terminal types the letter "a". The ASCII code for this letter is 97 decimal or 01100001 binary. A device inside the terminal called a UART (Universal Asynchronous Receiver/Transmitter) would convert the byte into its bits and send them out one at a time through the terminal's RS-232 port (also know as a serial port). The terminal's modem is connected to the RS-232 port, so it receives the bits one at a time and its job is to send them over the phone line.

Faster Modems

In order to create faster modems, modems designers must use techniques far more sophisticated than Frequency Shift Keying. First they moved to Phase Shift Keying (PSK), and then Quadrature Amplitude Modulation (QAM). These techniques allow an incredible amount of information to be crammed into the 3,000 hertz of bandwidth available on a normal voice-grade phone line. 56K-bit modems, which actually connect at something like 48K-bits on anything but absolutely perfect lines, are about the limit of these techniques. See the links section for details if you are interested.

All of these high-speed modems incorporate a concept of gradual degradation, meaning they can test the phone line and fall back to slower speeds if the line cannot handle the modem's fastest speed.

The latest step in the evolution of the modem is Asymmetric Digital Subscriber Line, or ADSL, modems. The word Asymmetric is used because these modems send data faster in one direction than they do in another. An ADSL modem takes advantage of the fact that any normal home, apartment or office has a dedicated copper wire running between it and phone company's nearest mux or central office. This dedicated copper wire can carry far more data than the 3,000 hertz signal needed for your phone's voice channel. If both the phone company's mux or central office is equipped with an ADSL modem on your line and so is your house, then the section of copper wire between your house and the phone company can act as a purely digital high-speed transmission channel. The capacity is something like one million bits per second between the home and the phone company (upstream) and eight million bits per second between the phone company and the home (downstream) under ideal conditions. The same line can transmit both a phone conversation and the digital data.

The approach an ADSL modem takes is very simple in principle. The phone line's bandwidth between 24,000 hertz and 1,100,000 hertz is divided into 4,000 hertz bands, and a virtual modem is assigned to each band. Each of these 249 virtual modems tests its band and does the best it can with the slice of bandwidth it is allocated. The aggregate of the 249 virtual modems is the total speed of the pipe.