Modems


Modems are used to connect two computers over a phone line. Modem is
short for Modulator Demodulator. It\'s a device that converts data from digital
computer signals to analog signals that can be sent over a phone line. This is
called modulation. The analog signals are then converted back into digital data
by the receiving modem. This is called demodulation. A modem is fed digital
information, in the form of ones and zeros, from the CPU. The modem then
analyzes this information and converts it to analog signals, that can be sent
over a phone line. Another modem then receives these signals, converts them back
into digital data, and sends the data to the receiving CPU. At connection time,
modems send tones to each other to negotiate the fastest mutually supported
modulation method that will work over whatever quality line has been established
for that call. There are two main differences in the types of modems for PC,
internal and external modems.

Evolution of Modems

In the last 10 years, modem users have gone from data transfer rates of
300bps to 1,200 bps to 2,400 bps to 9,600 bps to 14.4Kbps to 28.8Kbps to, and to
33.6Kbps. Now new modem standards are emerging, reaching speeds of up to
56Kbps. Unlike the 33.6Kbps modems being sold today, 56Kbps is a significant
improvement over 28.8Kbps modems. Viewing complex graphics or downloading sound
files improves significantly with 56Kbps. The modem experts keep telling us that
we are about maxed out. For instance when the 28.8 modems where first introduced
they said that we\'ve reached our maximum speed, and the same thing was said
about the 33.6 and now again for the 56K, but how true is this? The experts say
that the next major improvement will have to come from the telephone companies,
when they start laying down fibber-optic cables so we can have integrated
services digital network (ISDN) . The thing that makes digital modems better
than analog is because with analog modem transmission errors are very frequent
which results in your modem freezing or just freaking out. These errors are
caused mainly by some kind of noise on the line due to lightning storms,
sunspots, and other fascinating electromagnetic phenomena, noise occurs anywhere
on the line between your PC and the computer you\'re communicating with 2,000
miles away. Even if line noise is minimal, most modems will automatically reduce
it\'s speed to avoid introducing data errors.

Baud vs bps

While taking about modems, the transmission speed is the source of a lot
of confusion. The root of the problem is the fact that the terms "baud" and
"bits per second" are used interchangeably. This is a result of the fact that
it\'s easier to say "baud" than "bits per second," though misinformation has a
hand in it, too. A baud is "A change in 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. This is sometimes referred to as "bit rate."
Individual characters (letters, numbers, spaces, etc.), also referred to as
bytes, are composed of 8 bits. Technically, baud is the number of times per
second that the carrier signal shifts value, for example a 1200 bit-per-second
modem actually runs at 300 baud, but it moves 4 bits per baud (4 x 300 = 1200
bits per second).

Synchronous vs. Asynchronous Data Transfer

Synchronous and Asynchronous data transfer are two methods of sending
data over a phone line. In synchronous data transmission, data is sent via a
bit-stream, which sends a group of 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. They accomplish this
by sending special characters, called synchronization, or syn, characters. 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 that bit is
complete.

Modems Speeds

A full page of English text is about 16,000 bits. And in order to view
full-motion full-screen video it would require roughly 10,000,000 bits-per-
-second, depending on data compression.