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.