Information Technology Reference
In-Depth Information
15
Modems
15.1 Introduction
Modems (MOdulator/DEModulator) connect digital equipment to a telephone line. It con-
nects digital equipment to a speech bandwidth-limited communications channel. Typically,
modems are used on telephone lines, which have a bandwidth of between 400 Hz and
3.4 kHz. If digital pulses were applied directly to these lines, they would end up severely
distorted.
Modem speeds range from 300 bps to 56 kbps. A modem normally transmits about 10 bits
per character (each character has 8 bits). Thus, the maximum rate of characters for a high-
speed modem is 2880 characters per second. This chapter contains approximately 15 000
characters and thus to transmit the text in this chapter would take approximately 5 seconds.
Text, itself, is relatively fast transfer; unfortunately, even compressed graphics can take some
time to be transmitted. A compressed image of 20 KB (equivalent to 20 000 characters) will
take nearly 6 seconds to load on the fastest modem.
The document that was used to store this chapter occupies, in an uncompressed form,
360 KB. Thus to download this document over a modem, on the fastest modem, would take
Total
file
size
360
000
Time
taken
=
=
=
125
s
Characters
per
second
2
800
A 14.4 kbps modem would take 250 seconds. Typically home users connect to the Internet
and WWW through a modem (although increasingly ISDN is being used). The example
above shows the need to compress files when transferring them over a modem. On the
WWW, documents and large files are normally compressed into a ZIP file and images and
video compressed in GIF and JPG.
Most modems are able to do the following:
Automatically dial (known as auto-dial) another modem using either touch-tone or pulse
dialing.
Automatically answer (known as auto-answer) calls and make a connection with another
modem.
Disconnect a telephone connection when data transfer has completed or if an error oc-
curs.
Automatic speed negotiation between the two modems.
Convert bits into a form suitable for the line (modulator).
Convert received signals back into bits (demodulator).
Transfer data reliably with the correct type of handshaking.
Search WWH ::




Custom Search