FREE ESSAY ON HOW A MODEM WORKS

HOW A MODEM WORKS

Abstract
This paper has explained in some detail what a modem is and how a modem works. The
evolution of a modem, transfer of speeds through various sized modems, and how data
transferred has also been discussed. 
The word modem is an acronym for Modulator Demodulator. Modems accept digital data
supplied by the personal computer, and convert it to a modulated analog waveform that can
be transmitted over an analog telephone line. Modems also work in reverse. They take a
modulated analog wave from the phone line, then convert it to a digital form, then pass
it onto the personal computer (PC). Modems, at connection, send tones to each other to
negotiate the fastest mutually supported modulation method that will work over whatever
quality of line has been established for that call. There are two main differences in the
types of modems for the PC, internal and external.
Modem data transfer rates have gone from 300bps to 1,200bps to 9,600bps to 14.4kbps to
28.8kbps and to 33.6kbps. Now new modem standards are emerging, reaching speeds of
56kbps. Viewing graphics or downloading sound files improves dramatically with 56kbps.
The experts say that the next major improvement will have to come from the telephone
companies, with the fiber optic cables so we can have an integrated services digital
network (ISDN). Digital connections are more efficient because of the frequent
disconnects and modem freezing of the analog modem. These errors can be caused by line
noise, which can come from many sources such as lightning storms, sunspots, and other
electromagnetic phenomena. Modems will automatically reduce their speed to avoid
introducing data errors.
The transmission of speed sometimes causes confusion. The root of this problem seems to
stem from the term's baud and bits are used interchangeably. A baud is a change in a
signal from positive to negative or vice-versa that is used as a measure of transmission
speed and bits per second is a measure of the number of data bits (digital 0's and 1's)
transmitted each second in a communications channel. 
Synchronous and Asynchronous data transfer are two methods of sending data over the
telephone line. In synchronous data transmission, data is sent via a bit stream, which
sends a group characters in a single stream. In order to do this, modems gather groups of
characters into a buffer, where they are prepared to be sent as such a stream. In order
for the stream to be sent, synchronous modems must be in perfect synchronization with
each other. When the clocks of each modem are in synchronization, the data stream is
sent. In asynchronous transmission, data is coded into a series of pulses, including a
start bit and a stop bit. A start bit is sent by the sending modem to inform the
receiving modem that a character is to be sent. The character is then sent, followed by a
stop bit designating that the transfer of a bit is complete.
There are about 16,000 bits in a full page of English text. In order to view a full
motion full screen video it would require approximately 10,000,000 bits per second,
depending on the data compression. 
In some cases, the PC equipped with a 28.8kbps modem would be faster than a 33.6kbps or
even a 56k modem, especially with sites that do not have a great deal of graphics. There
are several factors in determining the time it takes to connect and display a Web site.
These include the speed of your computer, your connection to your Internet service
provider (ISP), your ISP's connection to the Internet itself, traffic on the Internet and
the speed and current traffic conditions of the site your visiting. 
U.S. Robotics, Cardinal, Rockwell, and other manufacturers have developed modems capable
of 56k speeds over standard telephone lines. U.S. Robotics line of modems called X2 uses
an "asymmetric" scheme. This lets you download at speeds up to 56kbps from any on-line
service or ISP using matching U.S. Robotics modems. One catch with the data you upload to
the provider is still limited to 33.6kbps or 28.8kbps. 
There are no set standards for the 56kbps modems as of yet, not all vendors support the
same specifications. This means that the Rockwell based modem will not work with the U.S.
Robotics modems.
ISDN (Integrated Services Digital Network) is a way to move more data over existing
telephone lines. ISDN cards are like modems, but are approximately 5 times faster then
regular 28.8 modems. It can provide speeds of up to 128,000 bits per second. ISDN has a
couple of advantages. It can use the same existing phone line, so the telephone company
may not have to install new line. A physical ISDN line provides two 64kbps phone lines
called channels that can be used for voice and data. The cost on ISDN is substantial,
installation, setup fees and the cost of a special digital adapter. An ISDN line goes
through a converter powered by AC current and if the power fails, so does the telephone
line.
Cable modems offer a very fast connection and are capable of uploading and downloading
approximately 10 to 27 megabits per second. A 500k file that would take 1.5 minutes
download over ISDN would take about one second using cable. Cable modems use the same
coaxial cable that connects televisions. 
Bibliography
References
56k Modem Info Center
www.sirus.com/~rmoss/
Cable-Modem.net
www.cable-modem.net/
Modem Central
www.56k.com/trouble/connect.shtml
Modem- HOWTO
David S. Lawyer
www.linuxdoc.org/HOWTO/Modem.html
Modems Rosenet
www.rosenet.net/~costmo/
Teleport. Com
www.teleport.com/~curt/modems.html
Yahoo.com
http://google.yahoo.com/bin/query?p=what+is+a+modem&hc=0&hs=2