The modern telephone system is designed to carry the
sound of a human voice talking normally. The engineers save money by cutting out
the high a low pitch sounds that occur only in music, or sounds that last for
a very short period of time. Modems convert data into sound so that it can be
sent through the phone system. Just as the telegraph sent data by alternating
dots and dashes, the earliest modems sent data by alternating the sound on a phone
between two tones. However, modern modems move much larger amounts of data by
sending combinations of tones at different frequencies. If you have access to
all possible frequencies, there is an enormous amount of data you can carry. Each
TV channel is about 3 Megabits of data per second, and a cable system carries
nearly 100 channels. However, the telephone system is designed to only handle
the frequencies of the human voice, and the maximum amount of data it can theoretically
carry is 4000 bytes per second per phone line. Trying to squeeze as much data
as possible into the available frequencies makes the signal vulnerable to "noise"
that is introduced from a bad connection, "crosstalk" from other phone
conversations in the same bundle of wires, and external electrical sources.
Computer
chips made it possible to build a "smart" modem that transmits a block
of data, waits for an acknowledgment, and retransmits the block if it something
went wrong the first time. Then data compression was added. In current use, the
information actually exchanged between the two modems may be much more complicated
and sophisticated than the data that either computer sees.Digital
phone equipment is used everywhere except at your home or office. Since the limit
on data transmission is caused by the conversion of sound to a digital signal,
the obvious direct solution to the problem is to extend digital signaling all
the way to the home. A T1 line would provide this type of connection. It carries
24 phone circuits each with 8000 bytes per second, or a single circuit of around
1.5 Million bits per second.
