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How Modems Work

 

If you are reading this article on your computer at home, chances are that it arrived via a modem. This edition of How Stuff Works will show you how a modem works by starting with the original 300 baud modems and progressing all the way through to the latest ADSL configurations!

[Note: If you are unfamiliar with bits, bytes and the ASCII character codes, the HSW article entitled How Bits and Bytes Work will help make this article much clearer.]

The Origin of Modems
The word modem is a contraction of the words modulator-demodulator. A modem is typically used to send digital data over a phone line. The sending modem modulates the data into a signal that is compatible with the phone line, and the receiving modem demodulates the signal back into digital data. Wireless modems are also frequently seen converting data into radio signals and back.

Modems came into existence in the 1960s as a way to allow terminals to connect to computers over the phone lines. A typical arrangement is shown below:

In a configuration like this, a dumb terminal at an off-site office or store could "dial in" to a large, central computer. The 1960s were the age of time-shared computers, so a business would often buy computer time from a time-share facility and connect to it via a 300 bit-per-second (BPS) modem.

A dumb terminal is simply a keyboard and a screen. A very common dumb terminal at the time was called the DEC VT-100, and it became a standard of the day (now memorialized in terminal emulators worldwide). The VT-100 could display 25 lines of 80 characters each. When the user typed a character on the terminal, the modem sent the ASCII code for the character to the computer. The computer would then send the character back to the computer so it would appear on the screen.

When personal computers started appearing in the late 1970s, bulletin board systems became the rage. A person would set up a computer with a modem or two and some BBS software, and other people would dial in to connect to the bulletin board. The users would run terminal emulators on their computers to emulate a dumb terminal.

People got along at 300 BPS for quite awhile. The reason this speed was tolerable was because 300 BPS represents about 30 characters per second, and that is much faster than a person can type characters or read. Once people started transferring large programs and images to and from bulletin board systems, however, 300 BPS became intolerable. Modem speeds went through a series of steps at two year or so intervals:

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 (see How Guitars Work for a discussion of tones and frequencies).

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 (see the HSW article entitled How Bits and Bytes Work for details). 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.

Point-to-Point Protocol
Today no one uses dumb terminals or terminal emulators to connect to an individual computer. Instead we all use our modems to connect to an Internet Service Provider (ISP), and the ISP connects us into the Internet. The Internet lets us connect to any machine in the world (see the HSW article entitled How Web Servers and the Internet Work for details). Because of the relationship between your computer, the ISP and the Internet, it is no longer appropriate to send individual characters. Instead, your modem is routing TCP/IP packets between you and your ISP.

The standard technique for routing these packets through your modem is called PPP or the Point-to-Point Protocol. The basic idea is simple - your computer's TCP/IP stack forms its TCP/IP datagrams normally, but then the datagrams are handed to the modem for transmission. The ISP receives each datagram and routes it appropriately onto the Internet. The same thing happens to get data from the ISP to your computer. See this page and this page for additional information.

Links
If you want to know more about modems, and especially if you wish to delve into things like PSK and QAM in more detail, the following links are extremely helpful.

 

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