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

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