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 alphabet were defined by the patterns of 'mark' and 'space'. ** Page 8 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 The mark and space have been replaced by holes punched in paper-tape: larger holes for mark, smaller ones for space.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 code for A.
* is the stop bit and *
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