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
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
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 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
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
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 bulletin
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
- 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 Works
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 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.
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
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
The standard technique for routing
these 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 computer. See this
page for additional information on PPP.
If you want to know more about modems, protocols, and
especially if you wish to delve into things like PSK and QAM
in more detail, check out the links on the next page!
Lots More Information!
Where to Buy
More Great Links!