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

expect any hacking anecdote to be completely truthful.

CHAPTER 2
Computer-to-Computer Communications

Services intended for access by microcomputers are nowadays
usually presented in a very user-friendly fashion: pop in your
software disc or firmware, check the connections, dial the telephone
number, listen for the tone...and there you are. Hackers, interested
in venturing where they are not invited, enjoy no such luxury. They
may want to access older services which preceded the modern 'human
interface'; they are very likely to travel along paths intended, not for ordinary
customers, but for engineers or salesmen; they could be utilising facilities that
were part of a computer's commissioning process and have been hardly used
since.
So the hacker needs a greater knowledge of datacomms technology than
does a more passive computer user, and some feeling for the history of the
technology is pretty essential, because of its growth pattern and because of the
fact that many interesting installations still use yesterday's solutions.
Getting one computer to talk to another some distance away means
accepting a number of limiting factors:
( Although computers can send out several bits of information at
once, the ribbon cable necessary to do this is not economical at any
great length, particularly if the information is to be sent out over
a network--each wire in the ribbon would need switching separately,
thus making ex- changes prohibitively expensive. So bits must be
transmitted one at a time, or serially.
( Since you will be using, in the first instance, wires and networks
already installed--in the form of the telephone and telex
networks--you must accept that the limited bandwidth of these
facilities will restrict the rate at which data can be sent. The data
will pass through long lengths of wire, frequently being
re-amplified, and undergoing de- gradation as it passes through dirty
switches and relays in a multiplicity of exchanges.
( Data must be easily capable of accurate recovery at the far end.
( Sending and receiving computers must be synchronised in their working.
( The mode in which data is transmitted must be one understood by all
computers; accepting a standard protocol may mean adopting the
speed and efficiency of the slowest.
( The present 'universal' standard for data transmission used by
microcomputers and many other services uses agreed tones to signify
binary 0 and binary 1, the ASCII character set (also known as
International Alphabet No 5), and an asynchronous protocol, whereby
the transmitting and receiving computers are locked in step every
time a character is sent, not just at the beginning of a transmission
stream. Like nearly all standards, it is highly arbitrary in its
decisions and derives its importance simply from the fact of being
generally accepted. Like many standards, too, there are a number of
subtle and important variations.
To see how the standard works, how it came about and the reasons