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for the variations, we need to look back a little into history.

The Growth of Telegraphy

The essential techniques of sending data along wires has a history
of 150 years, and some of the common terminology of modern data
transmission goes right back to the first experiments.
The earliest form of telegraphy, itself the earliest form of
electrical message sending, used the remote actuation of electrical
relays to leave marks on a strip of paper. The letters of the
alphabet were defined by the patterns of 'mark' and 'space'.
The terms have come through to the present, to signify binary
conditions of '1' and '0' respectively. The first reliable machine
for sending letters and figures by this method dates from 1840; the
direct successor of that machine, using remarkably unchanged
electromechanical technology and a 5-bit alphabetic code, is still
widely used today, as the telex/teleprinter/teletype. The mark and
space have been replaced by holes punched in paper-tape: larger holes
for mark, smaller ones for space. Synchronisation between sending and
receiving stations is carried out by beginning each letter with a
'start' bit (a space) and concluding it with a 'stop' bit (mark). The
'idle' state of a circuit is thus 'mark'. In effect, therefore, each
letter requires the transmission of 7 bits:

. * * . . . * (letter A: . = space; * = mark)

of which the first . is the start bit, the last * is the stop bit and

* * . .. is the code for A.

This is the principle means for sending text messages around the
world, and the way in which news reports are distributed globally.
And, until third-world countries are rich enough to afford more
advanced devices, the technology will survive.
Early computer communications

When, 110 years after the first such machines came on line, the
need arose to address computers remotely, telegraphy was the obvious
way to do so. No one expected computers in the early 1950s to give
instant results; jobs were assembled in batches, often fed in by
means of paper-tape (another borrowing from telex, still in use) and
then run. The instant calculation and collation of data was then
considered quite miraculous. So the first use of data communications
was almost exclusively to ensure that the machine was fed with
up-to-date information, not for the machine to send the results out
to those who might want it; they could wait for the 'print-out' in
due course, borne to them with considerable solemnity by the computer
experts. Typical communications speeds were 50 or 75 baud. (The baud
is the measure of speed of data transmission: specifically, it refers
to the number of signal level changes per second and is thus not the