"Hugo Cornwall "The Hacker's handbook"" - читать интересную книгу автора

same as bits-per-second.)
These early computers were, of course, in today's jargon,
single-user/single-task; programs were fed by direct machine coding.
Gradually, over the next 15 years, computers spawned multi-user
capabilities by means of time-sharing techniques, and their human
interface became more 'user-friendly'.
With these facilities grew the demand for remote access to
computers, and modern data communications began.
Even at the very end of the 1960s when I had my own very first
encounter with a computer, the links with telegraphy were still
obvious. As a result of happenstance, I was in a Government-run
research facility to the south-west of London, and the program I was
to use was located on a computer just to the north of Central London;
I was sat down in front of a battered teletype--capitals and figures
only, and requiring not inconsiderable physical force from my
smallish fingers to actuate the keys of my choice. As it was a
teletype outputting on to a paper roll, mistakes could not as readily
be erased as on a VDU, and since the sole form of error reporting
consisted of a solitary ?, the episode was more frustrating than
thrilling. VDUs and good keyboards were then far too expensive for
'ordinary' use.

The telephone network


But by that time all sorts of changes in datacomms were taking
place. The telex and telegraphy network, originally so important, had
long been overtaken by voice-grade telephone circuits (Bell's
invention dates from 1876). For computer communication, mark and
space could be indicated by different audio tones, rather than by
different voltage conditions. Data traffic on a telex line can
operate in only one direction at a time, but, by selecting different
pairs of tones, both 'transmitter' and 'receiver' could speak
simultaneously--so that in fact, one has to talk about 'originate'
and 'answer' instead.
Improved electrical circuit design meant that higher speeds than
50 or 75 baud became possible; there was a move to 110 baud, then 300
and, so far as ordinary telephone circuits are concerned, 1200 baud
is now regarded as the top limit.
The 'start' and 'stop' method of synchronising the near and far
end of a communications circuit at the beginning of each individual
letter has been retained, but the common use of the 5-bit Baudot code
has been replaced by a 7-bit extended code which allows for many more
characters, 128 in fact.
Lastly, to reduce errors in transmission due to noise in the
telephone line and circuitry, each letter can be checked by the use
of a further bit (the parity bit), which adds up all the bits in the
main character and then, depending on whether the result is odd or
even, adds a binary 0 or binary 1.
The full modern transmission of a letter in this system, in this