Data transmission
Modem is an electronic device that converts digital data into analog (modulated-wave)
signals suitable for transmission over analog telecommunications circuits (e.g.,
traditional phone lines) and demodulates received analog signals to recover
the digital data transmitted. The modulator/demodulator thus makes it
possible for existing communications channels to support a variety of
digital communications, including e-mail, Internet access, and fax
transmissions. An ordinary modem, operating over traditional phone lines,
has a data transmission speed limit of about 56 kilobits per second. ISDN
lines allow communications at over twice that rate, and cable modems and DSL
lines have transmission rates of over a million bits per second.
Data transmission is sending and receiving data via cables (e.g., telephone
lines or fibre optics) or wireless relay systems. Because ordinary telephone
circuits pass signals that fall within the frequency range of voice
communication (about 3003,500 hertz), the high frequencies associated with
data transmission suffer a loss of amplitude and transmission speed. Data
signals must therefore be translated into a format compatible with the
signals used in telephone lines. Digital computers use a modem to transform
outgoing digital electronic data; a similar system at the receiving end
translates the incoming signal back to the original electronic data.
Specialized data-transmission links carry signals at frequencies higher than
those used by the public telephone network.
Bulletin-board system is a computerized system used to exchange public
messages or files. A BBS is typically reached by using a dial-up modem. Most
are dedicated to a special interest, which may be an extremely narrow topic.
Any user may post his or her own message (so that they appear on the site
for all to read). Bulletin boards produce conversations between interested
participants, who may download or print out messages they desire to keep or
pass on to others. BBS sites today number in the tens of thousands.
Streaming is a method of transmitting a media file in a continuous stream of
data that can be processed by the receiving computer before the entire file
has been completely sent. Streaming, which typically uses data compression,
is especially effective for downloading large multimedia files from the
Internet; it permits, for example, a video clip to begin playing on a user's
computer as soon as it begins to be downloaded from a Web site. Even with
improved modems and connection speeds, downloading and playing large audio
and video files without the use of streaming techniques still takes an
inconveniently long time. To accept streaming data, the receiving computer
needs to be running a player, a program that decompresses the incoming data
and sends the resulting signals to the display and speakers. The audio and
video files may be prerecorded, but streaming can also accommodate a live
feed over the Internet.
Data compression is the process of reducing the amount of data needed for
storage or transmission of a given piece of information (text, graphics,
video, sound, etc.), typically by use of encoding techniques. Data
compression is characterized as either lossy or lossless depending on
whether some data is discarded or not, respectively. Lossless compression
scans the data for repetitive sequences or regions and replaces them with a
single token. For example, every occurrence of the word the or region with
the colour red might be converted to $. ZIP and GIF are the most common
lossless formats for text and graphics, respectively. Lossy compression is
frequently used for photographs, video, and sound files where the loss of
some detail is generally unnoticeable. JPEG and MPEG are the most common
lossy formats.