If you are reading this article on your computer at home, it probably
arrived via modem.
In this edition of
HowStuffWorks, we'll show you how a modem brings you Web pages.
We'll start with the original 300-baud modems and progress all the way
through to the ADSL
(Note: If you are unfamiliar with bits, bytes and the ASCII character
codes, reading How
Bits and Bytes Work will help make this article much clearer.)
Let's get started with a short recap of how the modem came to be.
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 convert digital
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
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 then sent the character back to the
computer so it would appear on the screen.
computers started appearing in the late 1970s, bulletin board systems
(BBS) 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
board. The users would run terminal emulators on their computers
to emulate a dumb terminal.
People got along at 300 bps for quite a while. The reason this speed was
tolerable was because 300 bps represents about 30 characters per second,
which is a lot more characters per second than a person can type 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 approximately two-year intervals:
- 300 bps - 1960s through 1983 or so
- 1200 bps - Gained popularity in 1984 and 1985
- 2400 bps
- 9600 bps - First appeared in late 1990 and early 1991
- 19.2 kilobits per second (Kbps)
- 28.8 Kbps
- 33.6 Kbps
- 56 Kbps - Became the standard in 1998
- ADSL, with theoretical maximum of up to 8 megabits per second (Mbps) -
Gained popularity in 1999
(Check out How DSL
How Cable Modems Work for more information on the progression of modem
technology and current speeds.)
We'll use 300-bps modems as a starting point because they are extremely easy
to understand. 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 0
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 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
How Bits and Bytes
Work for details on binary). A device inside the terminal called a UART
(universal asynchronous receiver/transmitter) converts the byte into its
bits and sends them out one at a time through the terminal's RS-232 port
(also known as a
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.
In order to create faster modems, modem designers had to 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 modems, which actually connect at something like 48 Kbps on
anything but absolutely perfect lines, are about the limit of these
techniques (see the links at the end of this article for more information).
Here's a look inside a typical 56K modem:
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 next step in the evolution of the modem was asymmetric digital
subscriber line (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 central office and your house are equipped with an ADSL
modem on your line, 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 1 million bits per second (Mbps)
between the home and the phone company (upstream) and 8 Mbps between
the phone company and the home (downstream) under ideal conditions.
The same line can transmit both a phone conversation and the digital
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.
(For information on the latest DSL technology, see
How DSL Works.)
Today, no one uses dumb terminals or terminal emulators to connect to an
individual computer. Instead, we 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
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
packets through your modem is called the Point-to-Point Protocol (PPP).
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 process occurs to get data from the ISP to your
this page for additional information on PPP.