Computer to Computer Communication
 

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.





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

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

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

With these facilities grew the demand for remote access to

computers, and modern data communications began.