The telephone network

But by that time all sorts of changes in datacomms were taking

place. The telex and telegraphy network, originally so important, had

long been overtaken by voice-grade telephone circuits (Bell's

invention dates from 1876). For computer communication, mark and

space could be indicated by different audio tones, rather than by

different voltage conditions. Data traffic on a telex line can

operate in only one direction at a time, but, by selecting different

pairs of tones, both 'transmitter' and 'receiver' could speak

simultaneously--so that in fact, one has to talk about 'originate'

and 'answer' instead.

    Improved electrical circuit design meant that higher speeds than

50 or 75 baud became possible; there was a move to 110 baud, then 300

and, so far as ordinary telephone circuits are concerned, 1200 baud

is now regarded as the top limit.

    The 'start' and 'stop' method of synchronising the near and far

end of a communications circuit at the beginning of each individual

letter has been retained, but the common use of the 5-bit Baudot code

has been replaced by a 7-bit extended code which allows for many more

characters, 128 in fact.

    Lastly, to reduce errors in transmission due to noise in the

telephone line and circuitry, each letter can be checked by the use

of a further bit (the parity bit), which adds up all the bits in the

main character and then, depending on whether the result is odd or

even, adds a binary 0 or binary 1.

The full modern transmission of a letter in this system, in this

case, K, therefore, looks like this:

START-STOP TRANSMISSION OF A DATA CHARACTER

TIME

INTERVAL_____________9___0___1___2___3___4___5___6___7___8___9___

NUMBER

                     1       1           1       1   1       1

             Mark  +---+   +---+       +---+   +---+---+   +---+

LINE               |   | 0 |   | 0   0 |   | 0 |       | 0 |   |

CONDITION    Space-+   +---+   +---+---+   +---+       +---+   +-



                   ^   ^

                   |   |

BINARY        STOP-+  START  1   0   0   1   0   1   1   0

DIGIT


    The first 0 is the start bit; then follows 7 bits of the actual

letter code (1001011); then the parity bit; then the final 1 is the

stop code.

    This system, asynchronous start-stop ASCII (the common name for

the alphabetic code), is the basis for nearly all micro-based

communications. The key variations relate to:

bit-length; you can have 7 or 8 databits (*) parity; (it can be even or odd, or

entirely absent),

Tones - The tones used to signify binary 0 and binary 1, and which

computer is in 'originate' and which in 'answer', can vary according

to the speed of the transmission and also to whether the service is

used in North America or the rest of the world.  (Briefly, most of

the world uses tones and standards laid down by the Geneva-based

organisation, CCITT, a specialised agency of the International

Telecommunications Union; whereas in the United States and most parts

of Canada, tones determined by the telephone utility, colloquially

known as Ma Bell, are adopted.) The following table gives the

standards and tones in common use.