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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 -