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has been replaced by a 7-bit extended code which allows for many more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

(*) There are no 'obvious explanations' for the variations commonly
found: most electronic mail services and viewdata transmit 7 data
bits, even parity and I stop Bit; Telecom Gold and most hobbyist
bulletin boards transmit 8 data bits, odd parity and 1 stop bit.
Terminal emulator software--see chapter 3--allows users to adjust for
these differing requirements.

Service Speed Duplex Transmit Receive Answer
Designator 0 1 0 1
V21 orig 300(*) full 1180 980 1850 1650 -
V21 ans 300(*) full 1850 1650 1180 980 2100
V23 (1) 600 half 1700 1300 1700 1300 2100
V23 (2) 1200 f/h(**) 2100 1300 2100 1300 2100
V23 back 75 f/h(**) 450 390 450 390 -
Bell 103 orig 300(*) full 1070 1270 2025 2225 -
Bell 103 ans 300(*) full 2025 2225 1070 1270 2225
Bell 202 1200 half 2200 1200 2200 1200 2025

(*)any speed up to 300 baud, can also include 75 and 110 baud
services
(**)service can either be half-duplex at 1200 baud or asymmetrical
full duplex, with 75 baud originate and 1200 baud receive (commonly
used as viewdata user) or 1200 transmit and 75 receive (viewdata
host)
Higher Speeds
1200 baud is usually regarded as the fastest speed possible on an
ordinary voice-grade telephone line. Beyond this, noise on the line
due to the switching circuits at the various telephone exchanges,
poor cabling, etc. make accurate transmission difficult. Indeed, at
higher speeds it becomes increasingly important to use transmission
protocols that include error correction.
Error correction techniques usually consist of dividing the
transmission stream into a series of blocks which can be checked, one
at a time, by the receiving computer. The 'parity' system mentioned
above is one example, but obviously a crude one. The difficulty is
that the more secure an error-correction protocol becomes, the
greater becomes the overhead in terms of numbers of bits transmitted
to send just one character from one computer to another. Thus, in the
typical 300 bit situation, the actual letter is defined by 7 bits,
'start' and 'stop' account for another two, and the check takes a
further one--ten in all. After a while, what you gain in the speed
with which each actual bit is transmitted, you lose, because so many
bits have to be sent to ensure that a single character is accurately
received!

Although some people risk using 2400 baud on ordinary telephone
lines--the jargon is the PTSN (Public Telephone Switched
Network)--this means using expensive modems. Where higher speeds are
essential, leased circuits, not available via dial-up. become
essential. The leased circuit is paid for on a fixed charge, not a
charge based on time-connected. Such circuits can be conditioned',
for example by using special amplifiers, to support the higher data
rate.
For really high speed transmissions, however, pairs of copper
cable are inadequate. Medium speed is obtainable by the use of
coaxial cable (a little like that used for tv antenna hook-ups) which
have a very broad bandwidth. Imposing several different channels on
one cable-length is called multiplexing and, depending on the
application, the various channels can either carry several different
computer conversations simultaneously or can send several bits of one
computer conversation in parallel, just as though there were a ribbon
cable between the two participating computers. Either way, what
happens is that each binary 0 or binary 1 is given, not an audio
tone, but a radio frequency tone.